It's difficult to imagine, but this week your baby is still tinier than the "I" at the start of this sentence. Even so, exciting changes are taking place each day. This week, a sonographer can see your baby via ultrasound as a tiny white image tucked within the gestational sac. Your embryo now looks less like a ball and more like a curled tube. One end of it will eventually become your baby's head; the other, your baby's bottom. Between those two ends, the spinal cord is forming, and by the 5th week there will be a series of bumps along your baby's back. This is the start of your baby's central nervous system.
For these first few weeks in your baby-to-be's development, he'll be nourished by the yolk sac. The yolk sac provides all the nutrients the embryo needs and produces blood cells until the placenta fully forms later in the pregnancy. Toward the end of the first trimester, the yolk sac shrinks and can no longer be seen on the sonogram.
As with last week, your baby-to-be is still a cluster of rapidly developing cells. But these cells are becoming even more organized. The cells are formed into a flat disk with three layers.
The first layer will become your baby's brain, nerve tissue, and her skin. From another layer, she'll develop lungs an intestines. And from the remaining layer, her heart, blood vessels, kidneys, genitals, bone, and connective tissue will develop.
This tiny disk contains all the elements the embryo needs to become a fully formed baby. The sonographer will begin charting your baby-to-be's growth by measuring this cluster of cells. The sonographer measures the distance between the embryo's crown -- or the top of the "head"-- and the rump, or "bottom." This measurement, called the crown-rump length, or CRL, will help the sonographer pinpoint your baby-to-be's due date.
During this week, the embryo's heart will begin to flutter. Even though the heart is simply a tube at this stage, it is the first organ to begin working, and by next week it will have right and left chambers. At this early stage in pregnancy, the heart will pulse at about 90-100 beats per minute. By the time the embryo reaches 8 weeks, the heart rate increases to about 120-160 beats per minute. The baby-to-be's heart rate will stay in that range for the rest of the pregnancy. Although your baby-to-be's heart is beating, you won't be able to hear it during your regular prenatal office visits for another few weeks. During an ultrasound examination, a sonographer will often be able to pick up the heart beating deep within the safety of the uterus, but sonograms are usually not performed this early in pregnancy (you might not even know you're pregnant).
Yolk sac: A membrane-lined sac that provides nourishment in the early stages of a baby's development in utero. It also acts as a rudimentary circulatory system before the embryo is able to circulate blood internally.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).