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Causes of low amniotic fluid in twin pregnancy

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twin pregnancy AND oligohydramnios

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SMFM twin pregnancy oligohydramnios causes twin-to-twin transfusion selective fetal growth restriction

Low amniotic fluid in a twin pregnancy can affect one twin or both. The cause depends heavily on whether the twins share a placenta.

Important causes

  1. Twin-to-twin transfusion syndrome (TTTS)
    • Occurs only in monochorionic twins who share one placenta, most often monochorionic-diamniotic twins.
    • Unequal placental blood-vessel connections cause one twin, the donor, to make less urine and develop low fluid, while the recipient twin has excess fluid.
    • Typical ultrasound pattern: donor twin deepest fluid pocket ≤2 cm and recipient twin with polyhydramnios. [Creasy & Resnik’s Maternal-Fetal Medicine] and TTTS diagnostic criteria describe this oligohydramnios-polyhydramnios sequence.
  2. Rupture or leakage of membranes
    • Preterm prelabor rupture of membranes (PPROM) can reduce fluid around one twin or both.
    • A history of persistent watery vaginal leakage needs prompt assessment, even if it is intermittent.
  3. Placental insufficiency and fetal growth restriction
    • If one twin has a smaller or poorly functioning share of the placenta, it may receive less blood flow, produce less urine, grow poorly, and have reduced fluid.
    • This can occur as selective fetal growth restriction, especially in monochorionic pregnancies, but can also occur in dichorionic twins.
  4. Fetal kidney or urinary-tract problems
    • Reduced urine production from kidney malformations, absent/nonfunctioning kidneys, or severe urinary obstruction can cause low fluid around the affected twin.
    • A detailed anatomy scan and assessment of fetal bladder and kidneys are important.
  5. Maternal medications that reduce fetal urine output
    • ACE inhibitors and ARBs, used for blood pressure or kidney disease, can cause fetal kidney injury and oligohydramnios, particularly later in pregnancy.
    • NSAIDs, especially after 20 weeks and with prolonged use, may also reduce fetal renal blood flow and amniotic fluid. Do not stop prescribed medicines without contacting the obstetric team, but seek advice promptly.
  6. Maternal or placental complications
    • Severe hypertension/preeclampsia, placental abruption, significant maternal illness, and poor uteroplacental blood flow can be associated with low fluid.
    • Rarely, fetal demise or serious fetal compromise may be involved.

How it is assessed

In twin pregnancies, fluid should be measured separately for each sac, usually by the deepest vertical pocket, not by a single overall AFI. Ultrasound usually also evaluates:
  • chorionicity, meaning whether the placenta is shared
  • fetal bladder filling and kidneys
  • growth difference between twins
  • umbilical artery and other Doppler blood-flow studies
  • signs of membrane rupture and TTTS.
Contact your obstetrician or maternal-fetal medicine unit urgently for reduced fetal movement, fluid leaking vaginally, bleeding, contractions, severe abdominal pain, or sudden rapid abdominal enlargement/shortness of breath. In monochorionic twins, low fluid in one twin merits timely specialist assessment because TTTS may need close surveillance or fetal therapy.
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