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The Inside Story: Your Baby in Week Eighteen

If you had an ultrasound this week, you might see your baby sucking his thumb, or frowning as if to say, "You again?" His fingerprints are nearly complete, and his heart has formed completely so that you can see the heart's different structures on the ultrasound. Your baby's senses are rapidly maturing too. He has more taste buds than he needs. At long last he's able to use his ears. His inner ear bones and nerve endings from his ears to his brain have developed enough so that your baby might startle if he hears a loud noise. He can also sense sounds like your heartbeat and blood pumping through the umbilical cord. If there were light in your womb, he could detect that too.

Because your baby's visceral organs, such as his heart and liver, are growing fast, he's sitting more erect inside you to make room for them. He now has 200 bones, and they're continuing to harden; at birth he'll have about 300 bones, but some of those will fuse later, which is why the adult skeleton has only 206 bones in all.

Your baby is as big as the placenta, which is still his main source for nutrients and oxygen. The placenta is also a waste disposal system, rushing baby's waste into your bloodstream, where it will be filtered through your kidneys and liver.

Originally published in You & Your Baby: Pregnancy.
