Now more than 5 pounds, your baby might be more than 18 inches long and seriously competing for space with every other organ scrunched into your torso. Your abdomen may be stretched so large that you're starting to worry your baby is too big, but nature usually does a good job of matching up babies and moms. In any case it's hard to accurately assess a baby's weight at this point because as your baby grows, your amniotic fluid level rises and the placenta grows too.
You may notice that your baby's behavior is becoming increasingly structured, with definite sleep-and-wake cycles. His periods of increased movement may last as long as 20 minutes.
The air sacs of your baby's lungs are becoming lined with surfactant, a chemical substance that keeps lungs expanded after each breath. This will help your baby breathe on his own outside the uterus.
Even without an ultrasound, your provider should be able to tell by now how your baby is "presenting"--whether he is positioned upside down, which means his head is ready to come out first, or if he is breech, which means he'll come out bottom-first. That's partly because your baby's skull, once so soft and pliable, is hardening as calcium is deposited there. His brain still isn't completely closed over with bone, though. His head has fontanels, which are "soft spots" where the skull bones don't completely join together. The largest one is diamond-shape and slightly toward the front of his head. This is the anterior fontanel, and it's the "soft spot" everyone talks about. There is one other noticeable but smaller fontanel toward the back of your baby's head.
Originally published in You & Your Baby: Pregnancy.
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