Last week your baby-to-be still looked like little more than a jumble of cells. This week, the embryo is taking on more recognizable features (at least, if your sonographer guides you through what you're seeing!).
Your baby's head is as large as the rest of his body, but his arm and leg buds are starting to lengthen. His nose is starting to show, and eyelids are forming. Your baby's eyes already have a lens at this point, as well as an iris, cornea, and pigmented retina. Your baby's spine is completely developed and has closed over at both ends. Inside his lungs, secondary branches have started to appear, and they will continue branching out after he's born. Someday these branches will connect to the 300 million tiny air sacs (called alveoli) in his lungs.
Although your blood is supplying nutrients and oxygen to your growing baby, your blood does not flow directly to him. Instead it passes through the umbilical cord to the baby via the placenta, a disk-shape organ with lots of little roots and veins. The placenta produces hormones that support the baby's growth; it also sends nutrients and oxygen to the fetus and helps excrete the baby's waste. Think of it as a glorified filter that prevents your bloodstream from ever directly mixing with your baby's; because of this filter, your baby can have a blood type that's different from yours.
Now the embryo takes on the characteristic C shape that he'll stay in as he continues to develop. You might even notice after birth, your baby prefers to tuck into this familiar shape, knees folded into his chest and head curled toward his tummy. In this shape, the umbilical cord takes its place, growing into your embryo's abdomen and then reaching out toward the gestational sac. Just above the abdomen, the heart is bulging and beating, pumping blood for the first time through the tiny form that will become your baby. Along the back of the C shape, a neural tube creates your baby-to-be's immature spinal cord.
To keep track of your baby-to-be's growth and development, the sonographer will measure the embryo's size from the top of the head to the bottom. Called the CRL, or crown-rump length, this measurement will be used throughout your pregnancy to make sure that the baby is developing on schedule and to predict your baby's due date. At this early stage in pregnancy, your health care provider most likely will not request that you have an ultrasound. If you do receive an ultrasound examination during this week, the sonographer might be able to pick up the baby-to-be's heartbeat. By your 6th week of pregnancy, your baby's heartbeat measures 150-160 beats per minute, which is about twice your heart rate. Your health care provider, however, won't be able to detect the baby's heartbeat during prenatal visits for a few more weeks.
Umbilical cord: A cord that connects the developing baby to the mother in utero. The umbilical cord attaches to the baby at the abdomen and to the mother at the placenta. The cord forms during the fifth week of gestation (seventh week of pregnancy).
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org)