What you can't see on this picture are the changes going on in your bloodstream. Within the first few days of your baby-to-be's development, your body begins to produce and circulate human chorionic gonadotropin (hCG), the so-called "pregnancy hormone." This is the same hormone that triggers a positive reading on your pregnancy test. In fact, until the hCG levels reach 1,000-2,000 mIU/ml, indicating that there's a growing gestational sac, the sonographer (the ultrasound technician) will most likely not be able to pick up an image during an ultrasound exam.
Over the next few days, this sac will grow at a rate of 1 mm daily. As it develops, the levels of hCG will grow proportionately, too. The hCG levels will double every few days, and this increase is thought to be the reason many women feel nauseous. These hormones flood your entire body -- not just your uterus.
Keep in mind that ultrasounds are usually not performed this early in a pregnancy—you're getting an exclusive sneak peek here! If your health care provider does request an ultrasound, she's most likely confirming your pregnancy and looking at the placement of the gestational sac. That small circle you see in the center of the image is exactly where it should be, protected deep within the uterus.
Don't be nervous if you experience a little spotting this week. The embryo burrows into your uterine lining as part of the implantation process, and this sometimes causes a small amount of bleeding. That's normal. Your baby's cells continue to multiply, and the placental tissue grows, inhabiting the lining of the uterus and establishing contact with your circulation, which will supply more blood to your growing baby. The placenta also produces estrogen, progesterone, and hCG (human chorionic gonadotropin), which is often referred to as the "pregnancy hormone."
Although your baby is still a microscopic cluster of cells at this point, three different essential cell layers are already beginning to develop. Their names are probably familiar from biology class: the ectoderm, which will become your baby's nervous system, hair, and skin; the endoderm, which will form your baby's gastrointestinal tract, liver, pancreas, and thyroid; and the mesoderm, which eventually develops into your baby's skeleton, connective tissue, blood system, urogenital system, and muscles.
Sonographer: The person who performs your ultrasound examination. These specialists receive advanced training at reading sonograms.
Sonogram: An image produced by an ultrasound examination. Most often performed by a sonographer; sometimes performed by your health care provider.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).