Amniotic fluid volume increases with the duration of pregnancy, with about 200 cc at 16 weeks to about a liter between 28 and 36 weeks. Then it falls slightly with approaching term, to about 800 cc at 40 weeks. After 40 weeks, the volume drops further. AFV of more than 2 L is considered polyhydramnios.
Amniotic fluid is removed by the fetal membranes, swallowed by the fetus, and in the presence of ruptured membranes, may leak out through the vagina. It is deposited in the amniotic sac by the fetal membranes and by fetal urination. Any disturbance in the normal equilibrium of fetal swallowing, urinating, or amniotic membrane fluid transport can result in polyhydramnios (sometimes called hydramnios)..
Polyhydramnios is both a symptom and a threat. As a symptom, it can reflect decreased (or absent) fetal swallowing, congenital anomaly, or abnormal membrane fluid transport. When seen early in pregnancy and in the absence of diabetes, it carries a risk of about 50% of a major congenital anomaly. It is commonly seen in pregnancies among diabetic women, particularly if not well controlled. Regardless of it's cause, polyhydramnios presents a threat to the fetus and to the mother. Polyhydramnios can lead to premature rupture of the membranes, premature labor and premature delivery. During labor with polyhydramnios, the risk of prolapsed fetal small parts and prolapsed umbilical cord is increased. For the mother, polyhydramnios may contribute to her discomfort during pregnancy and if severe enough may interfere with breathing. In the most severe of cases, therapeutic amniocentesis can be performed to relieve (temporarily) the maternal respiratory distress associated with polyhydramnios.
Several means of identifying polyhydramnios are used, and they are not in complete agreement. As with oligohydramnios, although the concept of polyhydramnios is universally accepted, the specific definition of polyhydramnios is not. Definitions have included:
When present in a woman not in labor, consideration is given to inducing labor early, depending on the clinical situation. Therapeutic amniocentesis is used to treat maternal respiratory distress, although the AF tends to reaccumulate within a few days.
OB-GYN 101: Introductory
Obstetrics & Gynecology
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