Right now, it may seems like baby hardly ever sleeps now that she's big enough to be felt, though in reality a baby kicks only several times per hour, then sleeps for a period of time, awakens, and kicks some more. Because she still has room to move in your uterus, you'll feel the kicks in random places. At this point, it's tough to tell a foot from a fist.
As she continues to plump out, your baby will start to look like a little doll, complete with eyebrows. She's now completely covered with that white, pasty layer of vernix caseosa. Most of that substance will probably disappear before birth, except in certain places like the creases of her neck and behind her knees. Up until this point, her liver and spleen have produced nearly all of her blood cells. The liver will keep doing that until after her birth, and now her bone marrow's getting up to speed, churning out red blood cells as well.
What's more, your baby's brain is growing as fast as the rest of her, and it will keep growing rapidly for several more years. As your baby's senses develop, she's becoming increasingly aware of your eating and sleeping habits and of light and noise levels outside your body.
From a 21 weeks ultrasound, you'll also learn that amniotic fluid has several different functions that are helping baby develop. Yes, the fluid-filled amniotic sac cushions her against any outside pressure on the abdomen, but amniotic fluid does so much more than simply protect your baby from the occasional bump. Some of this fluid is taken into her lungs so that she literally "breathes" it in. This action of filling and emptying baby's immature lungs helps her respiratory system practice breathing before her arrival day. As your baby is being delivered, the action of moving through the birth canal pushes any excess amniotic fluid out of her lungs so that she'll be ready to take in air after birth.
Your baby-to-be doesn't just breathe in the amniotic fluid — she also ingests some of it. Again, this is a normal, helpful action that aids in your baby's development. Taking in amniotic fluid gives her body a chance to use her digestive system. The fluid passes through to her small intestine, which removes and absorbs the water; the remaining substances are passed to the large bowel. Her bowels then form an early stool, called meconium. She will keep building up meconium in her bowels until birth. In some instances, the baby will have a bowel movement in the uterus just before birth or during birth. Both are normal, but require special care to make sure that the baby doesn't ingest any of the meconium.
For your newborn's first few diapers, her bowels will be pushing out meconium. This tarlike substance contains the byproducts of her development, such as shed skin cells, mucus, lanugo, and more.
Meconium: The earliest stool produced by a fetus and expelled after birth. In some instances, the unborn baby will release some meconium before or during birth, which will taint the color of the amniotic fluid. Meconium is thick and tarlike but gives off no odor.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).