Thursday, February 11, 2010

Fetal development - 3

Fetal period

The fetal period begins at the end of the 10th week of gestation (8th week of development). Since the precursors of all the major organs are created by this time, the fetal period is described both by organ and by a list of changes by weeks of gestational age.

Because the precursors of the organs are formed, fetus also is not as sensitive to damage from environmental exposures as the embryo. Instead, toxic exposures often cause physiological abnormalities or minor congenital malformation.

Changes by organ

Each organ has its own development.

Development of circulatory system

Heart development

Development of digestive system

Tooth development

Development of endocrine system

Development of integumentary system

Development of lymphatic system

Development of muscular system

Development of nervous system

Development of the urinary and reproductive system

Development of the reproductive system

Development of the gonads

Development of respiratory system

Changes by weeks of gestation

From the 8th week until birth (around 38 weeks), the developing organism is called a fetus. The fetus is not as sensitive to damage from environmental exposures as the embryo, and toxic exposures often cause physiological abnormalities or minor congenital malformation. All major structures are already formed in the fetus, but they continue to grow and develop.

Fetus at 8 weeks after fertilization. Weeks 10-12

Gestational age: 9–11 weeks old.

Embryonic age: Weeks nr 8-10. 7–9 weeks old.

Embryo measures 30 mm-8 cm (1.2-3.2 inches) in length.

Intestines rotate.

Facial features continue to develop.

The eyelids are more developed.

The external features of the ear begin to take their final shape.

The head comprises nearly half of the fetus' size.

The face is well formed

The eyelids close and will not reopen until about the 28th week.

Tooth buds, which will form the baby teeth, appear.

The limbs are long and thin.

The fetus can make a fist with its fingers.

Genitals appear well differentiated.

Red blood cells are produced in the liver.

Weeks 13 to 16

Gestational age: 12–15 weeks old.

Embryonic age: Weeks nr 11-14. 10–13 weeks old.

The fetus reaches a length of about 15 cm (6 inches).

A fine hair called lanugo develops on the head.

Fetal skin is almost transparent.

More muscle tissue and bones have developed, and the bones become harder.

The fetus makes active movements.

Sucking motions are made with the mouth.

Meconium is made in the intestinal tract.

The liver and pancreas produce fluid secretions.

Fetus at 18 weeks after fertilization. Week 19

Gestational age: 18 weeks old.

Embryonic age: Week nr 17. 16 weeks old.

The fetus reaches a length of 20 cm (8 inches).

Lanugo covers the entire body.

Eyebrows and eyelashes appear.

Nails appear on fingers and toes.

The fetus is more active with increased muscle development.

"Quickening" usually occurs (the mother and others can feel the fetus moving).

The fetal heartbeat can be heard with a stethoscope.

Week 23

Gestational age: 22 weeks old.

Embryonic age: Week nr 21. 20 weeks old.

The fetus reaches a length of 28 cm (11.2 inches).

The fetus weighs about 725 g (1 lb 10 oz).

Eyebrows and eyelashes are well formed.

All of the eye components are developed.

The fetus has a hand and startle reflex.

Footprints and fingerprints continue forming.

Alveoli (air sacs) are forming in lungs.

Week 27

Gestational age: 26 weeks old.

Embryonic age: Week nr 25. 24 weeks old.

The fetus reaches a length of 38 cm (15 inches).

The fetus weighs about 1.2 kg (2 lb 11 oz).

The brain develops rapidly.

The nervous system develops enough to control some body functions.

The eyelids open and close.

The cochleae are now developed, though the myelin sheaths in neural portion of the auditory system will continue to develop until 18 months after birth.

The respiratory system, while immature, has developed to the point where gas exchange is possible.

Week 31

Gestational age: 30 weeks old.

Embryonic age: Week nr 29. 28 weeks old.

The fetus reaches a length of about 38–43 cm (15-17 inches).

The fetus weighs about 2 kg (3 lb 0 oz).

The amount of body fat rapidly increases.

Rhythmic breathing movements occur, but lungs are not fully mature.

Thalamic brain connections, which mediate sensory input, form.

Bones are fully developed, but are still soft and pliable.

The fetus begins storing iron, calcium, and phosphorus.

Week 35

Gestational age: 34 weeks old.

Embryonic age: Week nr 33. 32 weeks old.

The fetus reaches a length of about 40–48 cm (16-19 inches).

The fetus weighs about 2.5 to 3 kg (5 lb 12 oz to 6 lb 12 oz).

Lanugo begins to disappear.

Body fat increases.

Fingernails reach the end of the fingertips.

A baby born at 36 weeks has a high chance of survival, but may require medical interventions.

Fetus at 38 weeks after fertilization. Weeks 36 to 39

Gestational age: 35–38 weeks old.

Embryonic age: Weeks nr 34-37. 33–36 weeks old.

The fetus is considered full-term at the end of the 37th week of gestational age.

It may be 48 to 53 cm (19 to 21 inches) in length.

The lanugo is gone except on the upper arms and shoulders.

Fingernails extend beyond fingertips.

Small breast buds are present on both sexes.

Head hair is now coarse and thickest.

The development is continued postnatally with child development stages

Improper prenatal methods will hurt the fetus

As we all know, scientific prenatal education can improve the intellectual development of the fetus. If prenatal education methods are not appropriate, however, it will hurt the fetus. If prenatal education with unreasonable language, activities, then negative feelings will affect the fetus. Therefore, it is very important for mothers to pay attention!

1, unreasonable language education:

Pregnant women better use voice with a moderate volume to talk to fetus, such as reading poetry, or singing a ballad, or counting numbers. This would give baby good memories. Avoid a loudly and brutally arguing, this will result in fetal irritability. After the baby was born, etc., baby will become very nervous, and even there is a resentment and hostility to language.

2, bad mood:

The emotional state of pregnant women is very important to fetal development. Bad emotional state stimulates a maternal hormone abnormal, which result in fetal harm. Therefore, pregnant women should pay extra attention to mental health, to make themselves pleasant, feeling happy and being full of hope for life.

3, Noise:

Noise can lead endocrine gland dysfunction, an excess oxytocin secreted by the pituitary gland, causing strong uterine contraction, leading to miscarriage, premature birth. Noise on the fetus has such serious implications, so pregnant women must be vigilant the noise, not affected by noise, not irritating to listen to loud sound.

4, unreasonable Sport Education:

Sport is very effective way and mean of a prenatal education, but an unreasonable sport is not proper for prenatal education. When you contact with the fetus during exercise, to gently stroke the fetus, 2 ~ 4 times a day is appropriate, and sometimes the fetus would not comply with the mother, at this time we must be patient, do not be impatient for success.

What impact on the babies of pregnant women sleeping position?

An early stages of pregnancy (1 ~ 3 months)

At early stages of pregnancy the womb is still in the maternal pelvis. Due to the oppression directly from the external, or its own will not be heavy, so the mother's sleeping posture can be free, only if the position is comfortable, such as the supine position, lateral position. Tummy sleeping should be avoided.

Second trimester (4 to 7 months)

In this stage, pregnant women should pay attention for protection of the abdomen, to avoid the direct external forces. If the pregnant women have polyhydramnios, twin or multiple pregnancies, it is necessary to take lateral sleeping position. If the pregnant women feel the leg are so heavy, it is necessary to take supine position, can slightly raise the legs with soft pillow.

Three late pregnancy (8 ~ 10 months)

In late pregnancy, sleeping position is very important to the safety of fetus in the womb. It is better to take left supine. It can correct uterus dextral. It also can reduce the pressure of the uterus on the abdominal aorta, improve blood circulation, increase blood supply to the baby will help your baby's growth and development. It is suggested do not take supine during this period. Because the uterus would put a huge pressure on inferior vena cava, result in Rhodobryum roseum blood volume and cardiac output reduction, leading to low blood pressure. Pregnant women will feel dizziness, palpitation, nausea, suffocation and other symptoms, sometimes pale, limb weakness , a cold sweat and so on. If the above symptoms appeared, you should immediately take the left supine, blood pressure can gradually return to normal, the symptoms will disappear.

Pregnant mother should pay attention to shoes

During pregnancy, particularly in the late three months of pregnancy, pregnant women need special shoes, it is very important.

The general situation, the size and shape of human feet in a single day are different, and this change in the human foot was considered to be acceptable. The tolerable range is time-related, the shorter the time, the larger tolerable range. Average the best tolerable rage is 3-5mm.

Pregnant women's feet is a more complicated situation. There are many physiological changes. The foot change (swelling) the volume of between 10-25mm, far exceeding the tolerable range of human feet. The type of changes in pregnant women is only limited to foot circumference (fertility) changes. Therefore, pregnant women during pregnancy, particularly in the late three months of pregnancy, pregnant women need special shoes, it is very important.

Following are the principals for pregnant women to choose the shoes:

(1) Footwear Size: size is determined by feet length, we must note that sitting, standing and walking posture can extend the volume, therefore, allow a 10mm extra.

(2) Select shoes with round head and with wide fat-degree, the surface material should be soft.

(3) To choose open-type shoes, or lace-up, or velcro. You can also choose shoes with adjustable elastic band and so on.

(4) The whole journey of pregnancy can be divided into early (0-6 months) and late (6-10 months). You can choose different shoes for each stage. IN second half pregnancy, pregnant women can choose the shoes with heel cup.

(5) When purchase shoes for pregnant women, you must pay attention to heel height. The ideal heel height :15-30mm. Flat shoes can be acceptable, however, since the weight increasing, it brings a discomfort at heel.

(6) To choose shoes with a good durability and anti-slip functions.

Health and safety issues at early pregnancy

What conditions need to see doctor

For pregnant women, in the early stages of pregnancy, the most dangerous thing is ectopic pregnancy or abortion. In these circumstances the pregnant woman can have abdominal pain, vaginal bleeding. Therefore, if the pregnant woman found to have abdominal pain or vaginal bleeding, it is necessary to obtain a medical assistance immediately. Women in their early pregnancy, even the ordinary diarrhea or abdominal pain can also cause a miscarriage. Do not ignore it and without medical treatment.

Prevention of fetal malformation

The fetal malformations come from the genetic and the environment. Genetic factors was pre-determined, which can not be changed, but environmental factors can be changed, which including infections, environmental pollution, drug intake and other factors.

First trimester of pregnancy is the critical period for fetal neural tube development, therefore, in order to avoid the abnormal fetus, expectant mothers should avoid all kinds of negative factors such as tobacco and alcohol, environmental, hazardous chemical substances, newly renovated houses, the ventilation the environment, etc., and to improve their immunity against viral infection, do not take drugs privately.

In order to prevent fetal malformation, pregnant women, starting from the first trimester (preferably starting from a planned pregnancy), need to take folic acid. Folate-rich foods are red amaranth, spinach, lettuce, asparagus, beans, yeast, animal liver and apples, oranges, etc.. At the same time, have a regular prenatal care with the doctor. All these above are positive means to avoid the birth abnormalities.

During early pregnancy, is it necessary to supplement a lot of nutrition?

Generally, at the early stage of pregnancy, there is no need to give a lot of nutrition. Pregnant women can eat what they like and base on their preference. The women who have the severe vomiting should eat light, easily digestible food.

Taking some medicines before knowing pregnancy, how can I do?

Some pregnant women took some medicine before knowing pregnancy. They must see the doctor. In general, if the medication was taken at early embryos stage and there is no implantation, there a little effect on the fetus will be. And usually a pregnant woman takes only small dose of medicine, there will no impact at all. However, if a pregnant woman takes a kind of medicine for a long period and large amount of it, or takes antiviral drugs, the fetus will be affected and need a special consultation.

Is dangerous traveling by public transport?

Crowding has a little impact to pregnant women. However, in early pregnancy, pregnant women with pregnant symptoms must pay attention to the air circulation. Feeling depressed in the cars may have adverse effects on mother and child.

Beware of radiation

The radiation would affect the fetus in daily life still not be identified very clearly, therefore, it is recommended that pregnant women wearing anti-spoke clothing, further away from the television and microwave oven, do not hang cell phone in the chest.

Get enough rest

In the early stages of pregnancy, pregnant women easy to feel tired, so get enough rest is necessary. Moreover, over-tired easily lead to miscarriages for pregnant women, especially for those pregnant women, who are over best maternal age, who have had abortions history, who are suffering from certain chronic diseases, need extra attention to rest. During the earliest three months, pregnant women need to avoid over-exertion, avoid strenuous exercise, avoid emotional depression, to keep in a happy mood..


The fetus in early pregnancy is in the most volatile period, in order to "stabilize" the fetus, pregnant women should be advised to avoid to have sex . especially for those pregnant women, who are over best maternal age, who have had abortions history, it is suggested must temporarily stop having sex in order to avoid trouble baby.

The seven things in postpartum the new mother had to know-1

1, observe the amount of postpartum hemorrhage

Postpartum hemorrhage is first thing the new mother need to pay attention to. According to recent statistics, the post-partum haemorrhage is the first killer to new mothers. It should be given enough attention. According to doctors, the amount of bleeding is greater than 500 ml can be diagnosed as postpartum hemorrhage. There are about 6% new mothers have postpartum hemorrhage due to polyhydramnios, oversized baby, lacenta previa, prolonged. There are several more common reasons: weak contraction of the uterus; soft birth canal laceration; the residues o f placenta and fetal membranes. In general speaking, the placenta should be out after giving birth for about 30 minutes. Abortion and twin fetus can make the placenta to attached to the lower part of uterine. It causes retained placenta; clotting mechanism, combined with maternal blood system diseases, can not stop bleeding.

Mothers after giving birth most likely to have post-partum haemorrhage within two hours. Therefore, after giving birth the new mother are still required to be observed in production room. After two hours of observation in the delivery room, mother and baby can go to the baby-friendly area, should continue to observe by mothers themself. Because at this time can also cause uterine contraction fatigue postpartum hemorrhage.

The first hour after delivery I

Successful! Soft, warm, but also tidal little baby lay on your stomach, whose body also distributes a faint fragrance. And you may also be immersed in the relief after birth, and do not take note of your body is undergoing great changes, but also did not notice the little guy who actually harbors so many secrets!

Many women in their first hour after delivery still can’t let them out of the feeling of pregnancy, and the sense of being a mom is not that strong. It takes some tome for mommies to enjoy the happiness, because they consume too much energy in delivery, and need to recover. On contrary, when the young father sees their newborn baby, he often has tears in his eye. Therefore, some new mothers can not help but wonder why they themselves are so calm on the just-born baby? Don’t be shamed and blame yourself. Please enjoy this valuable post-natal first hour, carefully take a look of yourself and that little one, and you will have a lot of surprises and harvest!

The new mom's first hours

What changes will happen in you boby within the short span of the first hour after delivery?

1, After a child is born, the mother's weight alleviates about 5 kg. This 5 kg includes the child's weight and the weight of amniotic fluid.

2, 5 minutes to 2 hours after giving birth, the placenta will be shed, resulting in a palm-large wound in the womb. Soft tissue will bleed, but not so terrible as thought, about 300 milliliters. The blood was stored during pregnancy, so the body will not be hurt much and it will easily be recovered.

3, After delivery, the womb will quickly shrink from the size of a fitness ball into size of a melon. It is the contraction of the uterus wall muscles at work.

4, Uterine ligaments remain on stretched state. It will take some time to return to normal size. Therefore, for some time after delivery, the body will feel some weakness.

5, During pregnancy, your heart, liver, stomach and lungs were pressured upward by uterine. After giving birth, there has been more space. These organs gradually back to its normal place. This will make you feel relieved. However, at the beginning, the organs the downward sliding will also cause some uncomforts.

6, A kind of hormone called endorphin keeps in a very high level after delivery. Endorphin is an analgesics which produced by your own body. It can reduce pain of the process of childbirth.

7,Hormones promoted by throes still circulate in the blood, which will cause uterine contraction still exists after birth, leave the new mother feeling a pain like throes. In addition, it will help awaken a new sense of motherhood, prompting the young mothers pick up the child, and the care of him.

8, The most in need is quiet. When the child was born, what the new mother wants most is quiet rest. If people mill around, she would feel very uncomfortable. 90% of the households are willing to enjoy a bit of privacy in the delivery room. Doctors and nurses can quietly leave for a while.

9, Waiting for the placenta off

If there is no post-natal complications after giving birth, then few things are needed to do, only to give the baby a bath, weighing, measuring height and dressing up. After the placenta falling, midwives will carefully examine if the placenta is complete. If there is any remain in the womb, all would lead to uterine bleeding.

Many young parents are curious about how big the placenta is in deed? In fact, despite the placenta has contributed a lot, but it weighs much less than we imagined. Placenta is only about 500 grams, or about 1 / 6 of birth weight. It is soft and smooth. Under normal circumstances, it would fall from the vagina with no pain.

One in the 150 cases of childbirth, the placenta can’t automatically fall off. In such case, doctor will reach into the womb to help the placenta come off.

10,Start breast-feeding, the sooner the better

When the newborn touch the mother's breast, almost without any help, he can quickly learn how to suck and get what he wanted. Within 20 to 30 minutes after birth, the newborn is with the strongest intense of sucking. Most of the babies are ready, and eager for mommy’s nipple. If you miss this prime time, baby's sucking reflex will be weakened in a day and a half, it may affect the breast-feeding. Baby's sucking also can stimulate uterine contractions and reduce bleeding. Therefore, if conditions permit, mothers must start breast feeding as soon as possible.

Get ready for the big time

Pre-pregnancy nutritional status has a very close relationship with the health of the newborn. Good pre-pregnancy health condition brings healthy and lively newborns, with very few perinatal illness, and even has positive impact on babies’ intelligence development.

During pregnancy, baby lives in the mother's womb for about 40 weeks. The fertilized egg grows and develops finally into a handsome lovely baby who has independent organs and weights about 3200 grams.

Fetus stage is the fastest growing and developing period in a people’s whole life. Of course, this stage needs a lot of nutrients which directly come from the mother. In order to ensure fetus's healthy growth, new mommies must ensure that the uterus, placenta, amniotic fluid and breast areas are all healthy and ready for the new arrivals. Therefore, pregnant women, from preparation of pregnancy, you need to add extra nutrition. If the nutritional intake of pregnant women is inadequate, fetus won’t have adequate nutrition as well.

If you want to get pregnant, according to your personal physical situation, you need to have pre-pregnancy diet as soon as possible. Don’t wait until it is too late to all. Everyone is different. Therefore, before you take any supplements, do consult your GP or nutrition consultants. Choose the best method and have the appropriate dose based on your own condition.

In general, ordinary women need to start special diet and have supplements 3 months to half a year before pregnancy. Their daily meal need to content quality protein, vitamins, minerals, trace elements and certain amount of fat. These nutrients are the material basis of fetal growth and development.

High quality protein is easily digested and absorbed, such as chickens, ducks, fish, lean meat, shrimp, eggs, tofu, soy products, etc. Vitamin with A, D, C, B are essential. Fresh vegetables and fruits are rich in vitamins, minerals and trace elements, including calcium, iron, phosphorus, zinc, iodine. Calcium and phosphorus are the keys of fetal bone and teeth developing. Iron is important on hematopoietic function. Zinc and iodine are good for mental development and prevention of fetal malformations. Milk, eggs, bones, animal liver, dried small shrimps, aquatic product and nuts contain such substances. Moderate intake of fat can help the absorption and utilization of soluble vitamins.

For those who are in frail physique and poor nutritional status. The pre-pregnancy diet is extremely important. The diet preferably is started a year before pregnancy. In addition to intake adequate nutrition, they should also know that nutrition must be comprehensive, not partial eclipse. Do not be picky eaters, balance your food. But remember all these should be gradual and don’t be too ambitious.

For those who are in good nutritional status and have appropriate amount of body fat, in general, no need to increase in nutrition. However, have high-quality protein, vitamins, minerals, trace elements intake is still essential. Remember, consume less foods that are high in fat and sugar.

Ten coups to restore a new figure

After labour every new Mummy wishes they could quickly revert to their previous graceful figure. But this wish was not always easy to implement. What should we do? The following 10 diet rules will help new mummies quickly fulfill their dreams.

1. Drink two cups of milk. Milk contents only 3% fat and easily creates satiety. It enables the body to get adequate protein, calcium and a large quantity of vitamin B, A and other nutrients without getting fat. Skim milk is a better choice, because skim milk and whole milk have same protein in reserves but less fat.

2. If the baby is not breastfeeding, you can go back to your pre-pregnancy diet. Thus, your body can generate enough energy but not getting extra calorie.

3. Eat 250g dark green vegetables daily. Dark green vegetables are rich in dietary fiber, carotene, vitamin C, calcium, iron and other nutrients, such as kale, broccoli, pea sprouts, cabbage, spinach and so on. It is better to eat these foods first during each meal. This can increase the energy consumption.

4. Eat staple food 150g daily. Do not eat staple food, of course, can consume the body fat, but it will produce excessive metabolic waste, which is bad for your health. Best staple foods are whole grains such as oats, corn, millet, sweet potatoes, beans and so on. These coarse grains are rich in dietary fiber and B vitamins, with which new mommies won’t easily get the post-natal hunger.

5. Eating lots of fruit to satisfy their appetite is wrong. Fruits usually content 8% sugar, some even up to 20%. Bananas are high in carbohydrate. Therefore, the amount of fruit consumption per day must also be paid attention. 300 grams or less (peeled and seeded) is ideal. Don’t have more than two bananas a day.

6. Eat right fruit at right time to control calorie intake. Best not to eat fruit after the meal. The correct way is to eat fruit before a meal. By this way, you won’t feel too hungry during the dinner. Thus, you won’t eat too much, and help control weight.

7. Chewing more before swallowing food. Nutrition experts believe that chewing 300 times, when people began to postpartum satiety, help control food intake.

8. Eat more fresh fruit, drink less fruity drinks. Eating can give more satisfaction feeling than drinking, and fruity drinks usually contents more sugar. To remind you, that when you have fruit salad, better avoid salad dressing and added sugar. If there is fresh fruit, try not to eat dried fruit. Calorie-density rises after drying up.

9. Eat only natural foods other than synthetic food. Because synthetic foods are often added too much artificial coloring and chemical additives, not only pollute the breast milk and has not much nutrition, but also increase the burden on liver and kidney. For example, fresh potatoes are much healthier than French fries.

10. Choose nutritious, controllable food, such as multi-select low-fat and low-protein foods, soy, milk, chicken, fish, etc.; multiple-choice of fresh vegetables, seaweeds. If they are the same type of food, should choose low fat, low calorie varieties, such as the chicken can be used to substitute pork.

Great events for small fetus -2

19 fetus at 12 weeks in the pregnancy began to develop reflective capacity. If her arm or leg grazed the face, she will be pursed lips for sucking like, if the encounter eyelids, she will be made a blink action.

20 most of the mothers in three months of pregnancy, the amniotic fluid is only can be filled an egg cup, but in one month, the water can almost be filled a bottle.

21 pregnant 12 weeks, the baby's weight 14 grams - equivalent to an iPod Shuffle player weight.

22 pregnant 13 weeks, the fetus have fingerprints, she is now a big plum’s size.

23 As early as 13 weeks of pregnancy, the fetus's head had already appeared pink hair, as well as eyebrows and eyelashes, whole body wrapped in lanugo, which can help keep warm. Lanugo usually will detach off in the third trimester.

24, when in 14 weeks pregnant, the baby can smile and frown.

25 Although the fetal eyelids still closed, she can already feel the light and dark.

26 pregnant 15 weeks, experienced doctors will be able to determine the sex of the fetus with ultrasound equipment.

27 pregnant 20 weeks, the color Doppler image may show a normal human ------ development is complete, has been right proportional.

28 If your baby is a girl, then within 20 weeks in the pregnancy she formed the eggs can reach 3 million.

Fetal development - 2

Embryonic period

The embryonic period in humans begins at fertilization (12-24hrs after ovulation, generally between the 2nd and 3rd week of gestational age) and continues until the end of the 10th week of gestation (8th week by embryonic age).

The zygote spends the next few days traveling down the Fallopian tube. Meanwhile it divides several times to form a ball of cells called a morula. Further cellular division is accompanied by the formation of a small cavity between the cells. This stage is called a blastocyst. Up to this point there is no growth in the overall size of the embryo, so each division produces successively smaller cells.

The blastocyst reaches the uterus at roughly the fifth day after fertilization. It is here that lysis of the zona pellucida, a glycoprotein shell, occurs. This is required so that the trophectoderm cells, which give rise to extra-embryonic structures such as the placenta, of the blastocyst can come into contact with the luminal epithelial cells of the endometrium. (Contrast this with zona hatching, an event that occurs in vitro by a different mechanism, but with a similar result). It then adheres to the uterine lining and becomes embedded in the endometrial cell layer. This process is also called implantation. In most successful pregnancies, the conceptus implants 8 to 10 days after ovulation (Wilcox et al. 1999). The inner cell mass forms the embryo, while the outer cell layers form the membranes and placenta. Together, the embryo and its membranes are referred to as a conceptus, or the "products of conception".

Rapid growth occurs and the embryo's main external features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells). A spontaneous abortion, or miscarriage, in the first trimester of pregnancy is usually due to major genetic mistakes or abnormalities in the developing embryo. During this critical period (most of the first trimester), the developing embryo is also susceptible to toxic exposures, such as:

Alcohol, certain drugs, and other toxins that cause birth defects, such as Fetal alcohol syndrome

Infection (such as rubella or cytomegalovirus)

Radiation from x-rays or radiation therapy

Nutritional deficiencies such as lack of folate which contributes to spina bifida

Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny." For example, the backbone is a common structure among all vertebrates such as fish, reptiles and mammals, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain, develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo.

Gestational age vs. embryonic age

Gestational age is the time that has passed since the onset of the last menstruation, which generally or as standard occurs 2 weeks before the actual fertilization. Embryonic age, in contrast measures the actual age of the embryo or fetus from the time of fertilization. Nevertheless, menstruation has historically been the only means of estimating embryonal/fetal age, and is still the presumed measure if not else specified. However, the actual duration between last menstruation and fertilization may in fact differ from the standard 2 weeks by several days.

Thus, the first week of embryonic age is already week three counting with gestational age.

Furthermore, the number of the week is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the 1st week after fertilization.

The following table summarizes the various expression systems during week number x of gestation.



number Reached age

(whole weeks)

Gestational x x-1

Embryonic x-2 x-3

Week 1-2

Gestational age: 0 to 1 (whole) weeks old. 1–14 days from last menstruation.

Embryonic age: -2 to -1 weeks old. That is, week 1-2 of gestational age are merely theoretical extrapolations of embryonic age, since the fertilization hasn't actually occurred yet.

Week 3

Gestational age: 2 (whole) weeks old. 15–21 days from last menstruation.

Embryonic age: Week nr 1. 0 (whole) weeks old. 1–7 days from fertilization.

Fertilization of the ovum to form a zygote. (day 1 of fert. )

The zygote undergoes mitotic cellular divisions, but does not increase in size. This mitosis is also known as cleavage. A hollow cavity forms marking the blastocyst stage. (day 1.5-3 of fert. )

The blastocyst contains only a thin rim of trophoblast cells and a clump of cells at one end known as the "embryonic pole" which include embryonic stem cells.

The embryo hatches from its protein shell (zona pellucida) and performs implantation onto the endometrial lining of the mother's uterus. (day 5-6 of fert. )

If separation into identical twins occurs, 1/3 of the time it will happen before day 5.[6]

Week 4

Gestational age: 3 weeks old. 22–28 days from last menstruation.

Embryonic age: Week nr 2. 1 week old. 8–14 days from fertilization.

Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes. The blastocyst is fully implanted day 7-12 of fert.

Formation of the yolk sac.

The embryonic cells flatten into a disk, two cells thick.

If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoined.

Primitive streak develops. (day 13 of fert. )

Primary stem villi appear. (day 13 of fert. )

Week 5

Gestational age: 4 weeks old. 29–35 days from last menstruation.

Embryonic age: Week nr 3. 2 weeks old. 15–21 days from fertilization.

A notochord forms in the center of the embryonic disk. (day 16 of fert. )

Gastrulation commences. (day 16of fert. )

A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end. Neuromeres appear. (day 18 of fert. )

Somites, the divisions of the future vertebra, form. (day 20 of fert. )

Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20 of fert. )

Embryo at 4 weeks after fertilization. The crown-rump length is about 0.2 inches.[7]

A 10mm embryo from an ectopic pregnancy, still in the oviduct. This embryo is about five weeks old (or from the seventh week of menstrual age). Week 6

Gestational age: 5 weeks old. 36-42 days from last menstruation.

Embryonic age: Week nr 4. 3 weeks old. 22-28 days from fertilization.

The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.

The heart bulges, further develops, and begins to beat in a regular rhythm. Septum primum appear.

Branchial arches, grooves which will form structures of the face and neck, form.

The neural tube closes.

The ears begin to form as otic pits.

Arm buds and a tail are visible.

Pulmonary primordium, the first traits of the lung appear.

Hepatic plate, the first traits of the liver appear.

Buccopharyngeal membrane ruptures. This is the future mouth.

Cystic diverticulum, which will become the gallbladder, and dorsal pancreatic bud, which will become the pancreas appear.

Urorectal septum begins to form. Thus, the rectal and urinary passageways become separated.

Anterior and posterior horns differentiate in the spinal cord

Spleen appears.

Ureteric buds appear.

This embryo is also from an ectopic pregnancy, this one in the cornu (the part of the uterus to which the Fallopian tube is attached). The features are consistent with a developmental age of seven weeks (reckoned as the ninth week of pregnancy).

Week 7

Gestational age: 6 weeks old. 43–49 days from last menstruation.

Embryonic age: Week nr 5. 4 weeks old. 29–35 days from fertilization.

The embryo measures 8 mm (1/4 inch) in length.

Lens pits and optic cups form the start of the developing eye.

Nasal pits form.

The brain divides into 5 vesicles, including the early telencephalon.

Leg buds form and hands form as flat paddles on the arms.

Rudimentary blood moves through primitive vessels connecting to the yolk sac and chorionic membranes.

Week 8

Gestational age: 7 weeks old. 50–56 days from last menstruation.

Embryonic age: Week nr 6. 5 weeks old. 36–42 days from fertilization.

The embryo measures 13 mm (1/2 inch) in length.

Lungs begin to form.

The brain continues to develop.

Arms and legs have lengthened with foot and hand areas distinguishable.

The hands and feet have digits, but may still be webbed.

The gonadal ridge begins to be perceptible.

The lymphatic system begins to develop.

Week 9

Gestational age: 8 weeks old. 57–63 days from last menstruation.

Embryonic age: Week nr 7. 6 weeks old. 43–49 days from fertilization.

The embryo measures 18 mm (3/4 inch) in length.

Fetal heart tone (the sound of the heart beat)can be heard using doppler.

Nipples and hair follicles begin to form.

Location of the elbows and toes are visible.

Spontaneous limb movements may be detected by ultrasound.

All essential organs have at least begun formation.