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. 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. Etiology 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. Maternal Conditions such as preeclampsia, chronic hypertension, collagen vascular disease, nephropathy, and thrombophilia cause uteroplacental insufficiency. These conditions decrease the blood flow to vital organs such as the placenta which supplies blood, oxygen, and nutrients to the developing fetus. Decreased blood flow to the fetus causes impaired urine production which leads to reduced amniotic fluid and oligohydramnios. Medications such as angiotensin converting enzyme inhibitors (lisinopril), prostaglandin synthetase inhibitors (NSAIDs, anti-inflammatory steroids), and trastuzumab decrease blood flow to the kidneys of the fetus. When the fetal kidneys are not able to produce adequate amounts of urine, this leads to reduced amniotic fluid or oligohydramnios. Maternal dehydration, especially in the hotter summer months Infections such as the TORCH infections (toxoplasma gondii, rubella, cytomegalovirus, herpes simplex virus) and parvovirus B19 Fetal Chromosomal abnormalities such as Down syndrome which are associated with gastrointestinal abnormalities Congenital abnormalities such as renal agenesis and cystic renal disease are associated with impaired urine production, and posterior urethral valves or urethral atresia which are associated with obstruction of the lower urinary tract Intrauterine demise Post-term