Increased fluid around the baby is called polyhydramnios. A normal gestational-diabetes test removes one common explanation, but it does not prove that the finding is harmless or truly unexplained.
The useful question is: How much fluid is present, when did it appear, is it increasing, and what do the baby’s growth, anatomy and placenta show?
First Confirm the Degree and the Trend
Polyhydramnios is usually defined by a deepest vertical pocket of at least 8 cm or an amniotic fluid index of at least 24 cm. Mild, stable polyhydramnios is often idiopathic, while an earlier, rapidly increasing or severe finding is more likely to need a wider search for a cause. A borderline measurement on one scan should therefore not be interpreted without its method and trend. [1,2]
A Normal Diabetes Test Does Not Close the Search
If polyhydramnios appears in the third trimester and more than a month has passed since diabetes screening, repeat glucose assessment may be considered; a previously normal result does not exclude later gestational diabetes. [1]
The ultrasound review should look beyond the fluid number. Fetal growth, stomach filling, swallowing pathways, bowel, brain, heart, kidneys, bladder and placenta may provide clues. Depending on the pregnancy, other considerations include fetal anaemia, red-cell antibodies, congenital infection, complications of monochorionic twins and chromosomal or genetic conditions. Infection or anaemia testing should be targeted to the history and additional ultrasound findings rather than ordered automatically in every mild isolated case. [1,2]
Severity Decides the Next Step
Mild isolated polyhydramnios may not require intensive surveillance solely because of the fluid. Moderate or severe polyhydramnios, progressive increase or associated fetal findings can change monitoring, place of birth and specialist-referral planning. [1,3]
Prompt assessment is needed for rapidly increasing abdominal size, significant breathlessness or pain, contractions, leaking fluid, bleeding or reduced fetal movements.
Dr Tania’s Clinical Insight: A normal diabetes test closes only one branch of the investigation. The meaning of increased fluid comes from its severity, timing and trend—and from what the detailed fetal and placental assessment shows beside it.
Evidence Base
[1] Society for Maternal-Fetal Medicine; Dashe JS, Pressman EK, Hibbard JU. SMFM Consult Series #46: Evaluation and management of polyhydramnios. American Journal of Obstetrics & Gynecology. 2018;219:B2–B8. Reaffirmed 2024.
[2] Khalil A, Sotiriadis A, D’Antonio F, et al. ISUOG Practice Guidelines: performance of third-trimester obstetric ultrasound scan. Ultrasound in Obstetrics & Gynecology. 2024;63:131–147.
[3] American College of Obstetricians and Gynecologists. Indications for Outpatient Antenatal Fetal Surveillance. Committee Opinion No. 828. June 2021. Current online guidance.
Medical Disclaimer
This Clinical Insight provides general education and does not replace patient-specific obstetric or fetal-medicine advice. The appropriate investigations and surveillance depend on gestation, severity, ultrasound findings and the full clinical picture. Seek prompt maternity assessment for rapidly increasing abdominal size, significant breathlessness or pain, contractions, leaking fluid, bleeding, reduced fetal movements or feeling seriously unwell.