Oligohydramnios

Oligohydramnios is a medical condition in pregnancy characterized by a deficiency of amniotic fluid, the fluid that surrounds the fetus in the abdomen, in the amniotic sac. The limiting case is anhydramnios, where there is a complete absence of amniotic fluid. It is typically diagnosed by ultrasound when the amniotic fluid index (AFI) measures less than 5 cm or when the single deepest pocket (SDP) of amniotic fluid measures less than 2 cm. Amniotic fluid is necessary to allow for normal fetal movement, lung development, and cushioning from uterine compression. Low amniotic fluid can be attributed to a maternal, fetal, placental or idiopathic cause and can result in poor fetal outcomes including death. The prognosis of the fetus is dependent on the etiology, gestational age at diagnosis, and the severity of the oligohydramnios.

The opposite of oligohydramnios is polyhydramnios, or an excess of amniotic fluid.

Amniotic fluid is a clear, watery substance that surrounds the fetus. It helps to maintain a constant temperature around the fetus, cushion it from injury, and allows for proper fetal movement and organ development. The cause of anhydramnios is not always clear, but several factors can contribute to its development such as fetal renal abnormalities or placental insufficiency. Untreated anhydramnios can lead to serious complications for the baby, including pulmonary hypoplasia or skeletal deformities.^{[citation needed]}

The amount of amniotic fluid available is based on how much fluid is produced and how much is removed from the amniotic sac. In the first trimester, the main sources of amniotic fluid are fetal lung secretions, transportation of maternal plasma across the fetal membranes, and the surface of the placenta. By the second trimester, the fetal kidneys start to produce urine which becomes the main source of the amniotic fluid for the remainder of the pregnancy.

The development of oligohydramnios may be idiopathic or have a maternal, fetal, or placental cause.

The volume of amniotic fluid typically increases until 36 weeks and starts decreasing after 40 weeks in post-term gestations. For this reason, discrepancies between fundal height measurements and gestational age can be a clinical indication of amniotic fluid abnormality and should be evaluated by ultrasound. The symptoms of anhydramnios may not always be apparent, but some potential signs include:

Diagnosis of oligohydramnios or anhydramnios is made by conducting a transabdominal ultrasound of the abdomen.

There are two methods that can be used to make the diagnosis: the amniotic fluid index (AFI) and the single deepest pocket (SDP) measurement. An AFI of less than 5 cm or an SDP of less than 2 cm indicates oligohydramnios, and an AFI of 0 cm or an absent SDP indicates anhydramnios. In measuring the AFI, the sonographer measures the amniotic fluid in each of the four quadrants of the abdomen (right upper quadrant, left upper quadrant, right lower quadrant, left lower quadrant) and adds the values together. For reference, a normal AFI is 5–25 cm. An AFI <5 cm is considered oligohydramnios and an AFI >25 cm is considered polyhydramnios. Randomized control trials have shown that use of AFI can cause an increased number of false positive diagnosis of oligohydramnios and recommend using the measurement of a single deepest pocket (SDP) of amniotic fluid to diagnose oligohydramnios instead.