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 pregnancy
Rupture of membranes
Intrauterine growth restriction (IUGR) associated with placental insufficiency
Amnion nodosum; failure of secretion by the cells of the amnion covering the placenta

Placental
Placental abruption
Twin-twin transfusion
Placental thrombosis or infarction

Diagnosis
Clinical Manifestation
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. Other common clinical presentations include fetal malpresentation, undue prominence of fetal parts, premature rupture of membranes, and reduced amount of amniotic fluid.
One study showed an improvement in fetal structure visibility by 26% (51% to 77% before and after the infusion respectively). There is also some low quality data that may indicate a potential benefit of amnioinfusion is to facilitate external cephalic version. Amnioinfusion can be used during labor to prevent umbilical cord compression. There is uncertainty about the procedures safety and efficacy, and it is recommended that it should only be performed in centers specializing in invasive fetal medicine and in the context of a multidisciplinary team.One to two liters of oral hydration can temporarily increase amniotic fluid in dehydrated patients with isolated oligohydramnios.Other investigational therapies may also be useful such as desmopressin, tissue sealants, or sildenafil citrate. These methods are less commonly used and are experimental.In case of congenital lower urinary tract obstruction, fetal surgery seems to improve survival, according to a randomized yet small study. Prenatal care
Patients who are preterm are managed in the outpatient setting with weekly or biweekly testing to monitor for accurate fetal growth and decrease chances of unexpected fetal death. This includes a  weekly non-stress test (NST) and single deepest pocket (SDP) assessment which is also referred to as the modified BPP. Sonographic fetal growth exams may also be indicated. Timing of delivery
Idiopathic, uncomplicated, and persistent oligohydramnios can be delivered at 36 0/7 weeks – 37 6/7 weeks of gestation or at diagnosis if diagnosis is later. Complications
Complications may include cord compression, musculoskeletal abnormalities such as facial distortion and clubfoot, pulmonary hypoplasia and intrauterine growth restriction.