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Amniotic fluid and amniocentesis
Last reviewed: 06.07.2025
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Amniotic fluid is produced by amnion cells and the filtration of maternal blood plasma, the volume of this fluid depends on the vital activity of the fetus. At 10 weeks of pregnancy, the volume of amniotic fluid is 30 ml, at 20 - 300 ml, at 30 - 600 ml. Its maximum volume is reached between 34 and 38 weeks (800-1000 ml), and then it begins to decrease by 150 ml per week.
[ Indications for amniocentesis
Significant (from the obstetric point of view) age of the mother (if the mother is over 35-37 years old, the fetus has an increased risk of developing Down syndrome); a previous child was diagnosed with a defect in the formation of the nervous system (subsequent fetuses are affected with a frequency of 1:20), increased alpha-fetoprotein levels in the mother; cases when one of the parents is a carrier of a proportional chromosomal translocation (1 in 4-10 chances of the corresponding fetal pathology); the risk of inheritance of metabolic diseases by a recessive type (currently 70 metabolic disorders can be recognized); the mother is a carrier of a disease associated with the X chromosome (for early determination of the sex of the fetus). The miscarriage rate after amniocentesis is 1-2%.
Amniocentesis
Amniocentesis is a puncture of the amniotic sac to obtain a sample of amniotic fluid. The procedure is performed for the purpose of prenatal diagnosis of fetal malformations, to determine the tactics of managing a Rh-conflict pregnancy, and to assess the degree of fetal maturity (for example, its lungs). For the purpose of diagnosing fetal malformations, amniocentesis is performed at the 16th week of pregnancy, when there is already enough amniotic fluid to technically perform the procedure, but it is not too late to terminate the pregnancy in case of unfavorable results. Ultrasound control makes it possible to take the required amount of amniotic fluid for analysis, while avoiding placental penetration. Observing the rules of asepsis and antisepsis and using a G21 puncture needle, 15 ml of the desired fluid is obtained. After this procedure, Rh-negative pregnant women are given 250 units of anti-D immunoglobulin intramuscularly.
Low water content
The volume of amniotic fluid is less than 200 ml. Rare. May be associated with post-term pregnancy, prolonged rupture of the membranes of the fetal membrane, placental insufficiency, urethral aplasia or renal agenesis in the fetus. Potter syndrome (which has a fatal prognosis) is manifested by low-lying ears in the fetus, renal agenesis, pulmonary hypoplasia and nodular amnion (agglutination of fetal skin scales).
Polyhydramnios
Occurs with a frequency of 1:200 pregnancies. The volume of amniotic fluid exceeds 2-3 liters. In 50% of cases, polyhydramnios is combined with fetal malformations, in 20% - with diabetes mellitus in the mother. In 30% of cases, no visible causes for polyhydramnios are found. Causes of polyhydramnios associated with the fetus: anencephaly (no swallowing reflex), spina bifida (open or skin-covered cleft in the spine; absence of several vertebral arches, mainly in the lumbar region); umbilical hernia; ectopia of the gallbladder, urinary bladder; atresia of the esophagus or duodenum; fetal hydrops; excessive extension posture of the fetus. Causes of polyhydramnios associated with the mother's condition: diabetes mellitus, multiple pregnancy. At the very beginning of the third trimester of pregnancy, polyhydramnios may manifest itself in complaints from the mother about shortness of breath and edema. A patient's abdominal circumference of more than 100 cm should suggest the possibility of polyhydramnios. Ultrasound examination is used to rule out multiple pregnancy and fetal malformations. Polyhydramnios predisposes to premature birth, abnormal fetal presentation, cord prolapse, and postpartum hemorrhage (the overstretched uterus does not contract properly). During labor with polyhydramnios, measures to control possible cord prolapse should be planned in advance. After delivery, a nasogastric catheter should be inserted into the newborn to check the patency of the esophagus (to rule out its atresia).