Your pregnancy: 4 weeks

How your baby's growing:


This week marks the beginning of the embryonic period. From now until 10 weeks, all of your baby's organs will begin to develop and some will even begin to function. As a result, this is the time when she'll be most vulnerable to anything that might interfere with her development.

Right now your baby is an embryo the size of a poppy seed, consisting of two layers: the epiblast and the hypoblast, from which all of her organs and body parts will develop.

The primitive placenta is also made up of two layers at this point. Its cells are tunneling into the lining of your uterus, creating spaces for your blood to flow so that the developed placenta will be able to provide nutrients and oxygen to your growing baby when it starts to function at the end of this week.

Also present now are the amniotic sac, which will house your baby; the amniotic fluid, which will cushion her as she grows; and the yolk sac, which produces your baby's red blood cells and helps deliver nutrients to her until the placenta has developed and is ready to take over this duty.

See what's going on in your uterus this week. (Or see what fraternal twins look like in the womb this week.)

Note: Every baby develops a little differently — even in the womb. Our information is designed to give you a general idea of your baby's development.

How your life's changing:

Sometime this week, you may be able to find out whether you're pregnant. For the most accurate results, wait until the end of the week to take a home pregnancy test. (You can try one now if you like, but you're more likely to get a correct result a week past your expected period.)

If the test is positive, call your practitioner's office and schedule your first prenatal appointment. Most practitioners won't see you until you're about eight weeks along, unless you have a medical condition, had problems with a previous pregnancy, or are having symptoms that need to be checked out.

If you're taking any medications — prescription or over-the-counter — ask now whether it's safe to keep taking them. And be sure to alert your caregiver to any other issues of concern.

You should already be taking a multivitamin that contains at least 400 micrograms (mcg) of folic acid. Once you're pregnant, you'll need a bit more — 600 mcg a day — so switch to a prenatal vitamin if you haven't already.

The next six weeks are critical to your baby's development. The rudimentary versions of the placenta and umbilical cord, which deliver nourishment and oxygen to your baby, are already functioning. Through the placenta, your baby is exposed to what you take into your body, so make sure it's good for both of you.

If your home pregnancy test is negative, take another at five weeks if you still haven't gotten your period. Many urine tests are not sensitive enough to detect a pregnancy at four weeks.

If you've been trying to conceive with no success for a year or more (or for six months if you're over 35), talk to your healthcare provider about a workup exam for you and your partner to spot possible fertility problems. While the results may be upsetting, finding out about a problem sooner rather than later will get you started on the road to treatment — and to your ultimate goal: having a baby.

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