Up until this point in your unborn baby's development, his head has overshadowed the rest of his body. Now his arms and legs are catching up with his head and he's more in proportion. He's also gaining muscle tissue in his appendages. In fact, you might be feeling a few kicks and jabbed elbows, showing off just how strong your baby has become. And if you haven't felt your baby move much, don't worry—in the coming weeks he'll gain even more strength and might very well become a kickboxer in utero!
If you look at sonogram images of your unborn baby, you might be surprised to see his bones so clearly. That's normal for this stage of gestation. There are several reasons why his skeleton is visible and the bones bright on the ultrasound. First, his skin is still translucent. As he continues to develop, his skin will add layers and become thicker. Other tissues such as muscle and fat are still forming. He'll be adding more as he grows. The lack of significant tissue between his bones and skin makes it easy for the ultrasound-imaging device to capture his frame.
While your baby-to-be's skin is developing, it's covered by a cheeselike substance known as vernix caseosa. This thick layer protects the baby's skin from the acidic liquid environment of the amniotic fluid. Your baby will keep this covering well into the third trimester. Then, in the weeks before delivery, he'll begin to lose some of the vernix, although you might still see some of it on your baby after delivery, especially in folds of skin. (The vernix also helps your baby slide through the birth canal during delivery.)
As your unborn baby is maturing, his nervous system is working to form connections between the different parts of his body and his brain. His senses are beginning to function, but just barely. He'll be trying out his newfound hearing before long.
Note: If your baby is a girl, her uterus and vagina are most likely developing this week. Her ovaries, still tiny within her growing body, now house around 6 million eggs.
Brown fat: A specialized form of adipose tissue that forms around the unborn baby's neck, behind the sternum, and around the kidneys. This fat helps produce heat and keep the baby warm during the newborn period.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).