The Amniotic Sac

The amniotic sac is a bag inside your uterus that contains your baby and the amniotic fluid that surrounds it. Early in pregnancy, amniotic fluid comes from the amniotic membrane that covers the placenta and cord. Later in pregnancy, the fluid is mainly composed of fetal urine and fluid excreted by fetal lungs.

As your pregnancy progresses, the amount of amniotic fluid produced increases. This continues until close to the time of delivery, when it begins to decrease.
Amniotic fluid keeps the sac from collapsing and enables the baby to move around so muscles and joints can develop. Fluid regulates temperature and cushions the fetus from injury. It also helps a baby's lungs to mature as fluid passes into and out of fetal lungs when the baby's chest moves in and out.

By 21 weeks, the fetal digestive system has developed enough to enable the fetus to swallow amniotic fluid. The fetus absorbs much of the water contained in the swallowed fluid.
Swallowing amniotic fluid may encourage development of the fetal digestive system. It may condition the digestive system to function after birth. By term, a baby may swallow large amounts of amniotic fluid-as much as 17 ounces (500 ml) of amniotic fluid in a 24-hour period.

Amniotic fluid is an important gauge of fetal well-being. If you have amniocentesis, amniotic fluid is removed from your uterus for study. The amount of fluid can also be an indication of fetal health; ultrasound is used to evaluate the amount of fluid in the sac. Too much fluid may indicate a malformation in the spinal cord or digestive system. Too little may signal fetal bladder or kidney problems.

The Presence of Meconium. The term meconium refers to undigested debris from swallowed amniotic fluid in the fetal digestive system. Meconium is a greenish black to light brown substance that your baby may pass from its bowels into the amniotic fluid. This can happen before or at the time of delivery.

The presence of meconium in amniotic fluid may be caused by fetal distress, although not always. The baby can swallow meconium in the amniotic fluid just before birth or at the time of birth. If inhaled into the lungs, meconium may cause pneumonia or pneumonitis.

Meconium can be detected when your water breaks. Before then, the only way to know about it is by amniocentesis. If meconium is present at the time of delivery, an attempt is made to remove it from the baby's mouth and throat with a small suction tube so the baby won't swallow it.