4 Weeks Pregnant

How Big is Baby?

Your fertilized egg, known as a zygote, is a ball of 32 cells that's about the size of a poppy seed. It's at this stage that the cells separate into three distinct layers from which your baby's internal organs and skin will develop.

Baby's Length: 0.01in.
Baby's Weight: 0.04oz.

Mom's Changing Body

You just missed your period and the test came back positive—that's right, you're pregnant! Can't bear to fasten your bra in the morning? For most women, breast tenderness is the first physical sign of pregnancy—even before telltale morning sickness strikes. (If your breasts aren't sore now, odds are they will be within a few days or weeks.) You may also experience more extreme senses of taste and smell. In other words, that vase of peonies has never smelled better, but your coworker's tuna sandwich—which he's eating three cubicles away—might make you beeline for the bathroom.

Baby's Development

So what's going on in your womb this week? Your embryo may be minuscule, but trust us: Super-important developments are already under way. This week the embryo splits into two parts. One half will become the placenta, a special tissue that delivers must-have nutrients and oxygen to your baby throughout your entire pregnancy. In the other half, the embryo itself continues to grow, and a sheet of cells has just begun to create the neural tube, where your baby's brain, spinal cord and backbone will ultimately form.

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Weekly Tip

Don't be surprised if your doctor doesn't schedule an appointment to see you until your 8- to 12-week mark. Many healthcare practices have potential mommies-to-be come in for a blood test with a nurse first to confirm pregnancy and then wait until you're far enough along to better estimate your due date through an ultrasound test.

Week 4 Ultrasound

The small circle at the center of the sonogram may not look like much, but that little sac is a kind of baby cocoon called a gestational sac. The cells that make up this sac will begin to specialize. Some cells will become part of the placenta. Some will form the amniotic sac that will fill with fluid to cushion your developing baby. Other cells are destined to form everything from delicate eyelashes to muscles and skin. But that's still a long way away.

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