Oligohydramnios, defined as amniotic fluid volume less than the 5th percentile for gestational age, has several potential causes. Here's a breakdown of the most common:
Common Causes of Oligohydramnios
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Premature Rupture of Membranes (PROM): This is arguably the most frequent cause. Even a small leak, often missed by the mother, can significantly reduce amniotic fluid volume.
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Placental Insufficiency: When the placenta doesn't function optimally, it may not provide adequate blood and nutrients to the fetus, impacting fetal urine production (a major contributor to amniotic fluid). This results in decreased amniotic fluid.
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Fetal Abnormalities: Certain fetal structural anomalies, particularly those affecting the kidneys and urinary tract, can dramatically reduce or prevent fetal urine production. A classic example is bilateral renal agenesis (Potter sequence), where the fetus develops without kidneys.
Less Common, But Important, Considerations
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Medications: Certain medications taken by the mother can contribute to oligohydramnios. Examples include ACE inhibitors and NSAIDs, especially later in pregnancy.
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Post-term Pregnancy: As pregnancy progresses beyond the due date, placental function can decline, increasing the risk of oligohydramnios.
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Twin-Twin Transfusion Syndrome (TTTS): In monochorionic twin pregnancies, TTTS can lead to oligohydramnios in the donor twin and polyhydramnios in the recipient twin.
Diagnostic Considerations
When oligohydramnios is diagnosed, it's crucial to determine the underlying cause. Diagnostic steps may include:
- Ultrasound: To assess amniotic fluid volume and fetal well-being, and to look for fetal abnormalities.
- Amniocentesis: To rule out chromosomal abnormalities and assess fetal lung maturity.
- Maternal History: To identify potential risk factors, medications, or past obstetric complications.
In summary:
Oligohydramnios is most commonly caused by PROM or placental insufficiency, but it’s essential to rule out fetal abnormalities, maternal medications, and other potential contributors to ensure proper management and improve pregnancy outcomes.
