Pregnancy-4

Physiology

The term trimester redirects here. For the term trimester used in academic settings, see Academic term

Pregnancy is typically broken into three periods, or trimesters, each of about three months. While there are no hard and fast rules, these distinctions are useful in describing the changes that take place over time.

First trimester

Traditionally, doctors have measured pregnancy from a number of convenient points, including the day of last menstruation, ovulation, fertilization, implantation and chemical detection. In medicine, pregnancy is often defined as beginning when the developing embryo becomes implanted into the endometrial lining of a woman's uterus. In some cases where complications may have arisen, the fertilized egg might implant itself in the fallopian tubes or the cervix, causing an ectopic pregnancy. Most pregnant women do not have any specific signs or symptoms of implantation, although it is not uncommon to experience minimal bleeding at implantation. Some women will also experience cramping during their first trimester. This is usually of no concern unless there is spotting or bleeding as well. After implantation the uterine endometrium is called the decidua. The placenta, which is formed partly from the decidua and partly from outer layers of the embryo, is responsible for transport of nutrients and oxygen to, and removal of waste products from the fetus. The umbilical cord is the connecting cord from the embryo or fetus to the placenta.The developing embryo undergoes tremendous growth and changes during the process of fetal development.

Morning sickness occurs in about seventy percent of all pregnant women and typically improves after the first trimester.

In the first 12 weeks of pregnancy the nipples and areolas darken due to a temporary increase in hormones.

Most miscarriages occur during this period.[citation needed]

Second trimester

Months 4 through 6 of the pregnancy are called the second trimester. Most women feel more energized in this period, and begin to put on weight as the symptoms of morning sickness subside and eventually fade away.

In the 20th week the uterus, the muscular organ that holds the developing fetus, can expand up to 20 times its normal size during pregnancy. Although the fetus begins to move and takes a recognizable human shape during the first trimester, it is not until the second trimester that movement of the fetus, often referred to as "quickening", can be felt. This typically happens in the fourth month, more specifically in the 20th to 21st week, or by the 19th week if the woman has been pregnant before. However, it is not uncommon for some women not to feel the fetus move until much later. The placenta fully functions at this time and the fetus makes insulin and urinates. The reproductive organs distinguish the fetus as male or female.

Third trimester

Comparison of growth of the abdomen between 26 weeks and 40 weeks gestation.Final weight gain takes place, which is the most weight gain throughout the pregnancy. The fetus will be growing the most rapidly during this stage, gaining up to 28g per day. The woman's belly will transform in shape as the belly drops due to the fetus turning in a downward position ready for birth. During the second trimester, the woman's belly would have been very upright, whereas in the third trimester it will drop down quite low, and the woman will be able to lift her belly up and down. The fetus begins to move regularly, and is felt by the woman. Fetal movement can become quite strong and be disruptive to the woman. The woman's navel will sometimes become convex, "popping" out, due to her expanding abdomen. This period of her pregnancy can be uncomfortable, causing symptoms like weak bladder control and back-ache. Movement of the fetus becomes stronger and more frequent and via improved brain, eye, and muscle function the fetus is prepared for ex utero viability. The woman can feel the fetus "rolling" and it may cause pain or discomfort when it is near the woman's ribs and spine.

There is head engagement in the third trimester, that is, the fetal head descends into the pelvic cavity so that only a small part (or nothing) of it can be felt abdominally.

It is during this time that a baby born prematurely may survive. The use of modern medical intensive care technology has greatly increased the probability of premature babies surviving, and has pushed back the boundary of viability to much earlier dates than would be possible without assistance. In spite of these developments, premature birth remains a major threat to the fetus, and may result in ill-health in later life, even if the baby survives.