Polyhydramnios

The most common mechanisms for polyhydramnios are decreased fetal swallowing and increased urination

Polyhydramnios may be idiopathic or associated with a variety of fetal disorders, including:

• Fetal structural anomalies, chromosomal abnormalities, neoplasms
• High fetal cardiac output states (eg, any condition resulting in anemia)
• Twin-twin transfusion syndrome (TTTS)
• Maternal diabetes mellitus
• Fetal neuromuscular disorders that impede swallowing
• Fetal infection (eg, parvovirus B19)
• Fetal Bartter syndrome

The diagnosis of polyhydramnios is based upon ultrasound visualization of increased amniotic fluid volume. It is recommended that use of either of the following thresholds for diagnosing polyhydramnios:

• Single deepest pocket (SDP) ≥8 cm, or
• Amniotic fluid index (AFI) ≥24 cm

Olgiohydramnios

ETIOLOGY

The most likely etiologies of oligohydramnios vary according to severity and the trimester in which they are diagnosed. The majority of women with oligohydramnios or borderline/low normal amniotic fluid volume present in the third trimester and have no identifiable cause.

First trimester — The etiology is often unclear. Reduced amniotic fluid prior to 10 weeks of gestation is rare due to gestational sac fluid is primarily derived from the fetal surface of the placenta.

Second trimester — fetal urine begins to enter the amniotic sac, and the fetus begins to swallow amniotic fluid. Therefore, disorders related to the fetal renal/urinary system begin to play a prominent role in the etiology of oligohydramnios

Third trimester — Oligohydramnios first diagnosed in the third trimester is often associated with PPROM or with uteroplacental insufficiency due to conditions such as preeclampsia or other maternal vascular diseases.

Diagnosis — There are both objective and subjective ultrasound criteria for diagnosis of oligohydramnios. Use of an objective criterion is generally preferable (either single deepest pocket <2 cm or amniotic fluid index ≤5 cm);

The most extreme end of the oligohydramnios spectrum is anhydramnios, which can be defined as a lack of a measurable AFI or SDP, although a thin echolucent rim may be imaged on the inner aspect of the uterus.

 

https://obgyn.onlinelibrary.wiley.com/doi/10.1111/j.1471-0528.2010.02751.x

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