Important causes

1. Twin-to-twin transfusion syndrome (TTTS)
   • Occurs only when twins share a placenta, usually monochorionic-diamniotic twins.
   • One twin, the donor, has reduced urine output and low fluid. The other, the recipient, often has too much fluid.
   • TTTS commonly appears from about 16 weeks. In one sac, a deepest vertical pocket of 2 cm or less is concerning, particularly if the other twin has excess fluid.
   • SMFM recommends ultrasound surveillance beginning at 16 weeks and at least every 2 weeks for monochorionic-diamniotic twins, including fluid in each sac, fetal bladders, and ideally Doppler studies. SMFM TTTS guidance
2. Leakage or rupture of membranes
   • A slow fluid leak can cause low fluid, sometimes with a persistent watery vaginal discharge or gush of fluid. It can occur even without pain or fever.
3. Placental insufficiency or fetal growth restriction
   • Reduced placental blood flow can reduce fetal kidney blood flow and urine production, leading to lower fluid. Blood pressure disorders and growth differences between twins may be associated.
4. Fetal kidney, urinary tract, or bladder problem
   • Examples include severe urinary obstruction or absent/poorly developed kidneys. Ultrasound usually checks kidneys, bladder filling, and anatomy.
5. Medication-related reduction in fetal urine
   • Some medicines, especially ACE inhibitor or ARB blood-pressure drugs, and prolonged NSAID use such as ibuprofen, can lower amniotic fluid. Do not stop prescribed medication without speaking to your clinician, but contact them urgently for advice.
6. Measurement or sac-identification issue
   • In twins, fluid must be measured separately in each sac. An experienced twin-pregnancy ultrasound is important.

What to do now

• Whether the twins share a placenta and/or sac
• Deepest fluid pocket in each sac
• Presence and filling of each baby’s bladder
• Growth difference, anatomy, and Doppler blood-flow studies
• Whether there is evidence of membrane rupture

• Ascending vaginal/cervical infection, especially bacterial vaginosis, Group B Streptococcus, E. coli, chlamydia, gonorrhea, trichomonas, and genital mycoplasmas.
• Intra-amniotic infection (chorioamnionitis), meaning infection/inflammation of the membranes, placenta, or amniotic fluid.
• Less commonly, congenital infections such as cytomegalovirus, toxoplasmosis, parvovirus B19, syphilis, or rubella can be associated with low fluid, usually with other fetal ultrasound abnormalities or growth problems.

1. Membranes have ruptured or are leaking
   • Often the most likely cause of sudden or severe loss of fluid.
   • May be only a small ongoing watery vaginal leak, not necessarily a large gush.
   • Infection can cause membrane weakening and rupture, and rupture raises infection risk.
2. Twin-to-twin transfusion syndrome (TTTS)
   • Relevant if the twins share one placenta.
   • The donor twin produces very little urine and can develop almost no fluid, sometimes appearing “stuck” against the uterine wall.
   • Usually the other twin has excess fluid. If the other twin has normal fluid, clinicians must also look for other causes.
3. Severe kidney or urinary-tract abnormality in the affected twin
   • Bilateral absent/nonfunctioning kidneys, severe kidney dysplasia, or severe bladder outlet obstruction can prevent urine production.
   • After about 16 weeks, fetal urine is the major source of amniotic fluid.
   • On ultrasound, an absent bladder plus no fluid makes a serious renal/urinary cause a concern. In contrast, a bladder that fills despite no fluid may favor membrane rupture.
4. Severe placental insufficiency or selective fetal growth restriction
   • Reduced blood flow to one twin can reduce kidney perfusion, urine output, growth, and fluid.
5. Fetal demise or a problem with one sac/placenta
   • This also needs urgent ultrasound clarification.