American Institute of Ultrasound in Medicine - AIUM.org
Amniotic fluid has several different functions that help your unborn baby develop. Perhaps the most obvious: the fluid-filled amniotic sac cushions your baby against any outside pressure on the abdomen.
But the amniotic fluid does so much more than simply protect your baby from the occasional bump. Some of this fluid is taken into the lungs so that the baby literally "breathes" it in. This action of filling and emptying the unborn baby's immature lungs helps her respiratory system practice breathing before her arrival day. As the baby is being delivered, the action of moving through the birth canal pushes any excess amniotic fluid out of your baby's 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.
Terms to Know
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.
Important Information About Your Pregnancy
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Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).