Oligohydramnios (Low Amniotic Fluid)

Amniotic fluid is produced mainly by the baby's kidneys (as fetal urine) in the second half of pregnancy, and is swallowed and reabsorbed in a continuous cycle. Oligohydramnios means this balance has shifted so that too little fluid remains. We measure it on ultrasound in two accepted ways:

• Deepest Vertical Pocket (DVP or SDP): the single deepest pool of fluid, free of cord and limbs. A DVP less than 2 cm defines oligohydramnios. This is the method recommended by ISUOG and SMFM as it reduces unnecessary intervention.
• Amniotic Fluid Index (AFI): the sum of the deepest pockets in four quadrants of the uterus. An AFI less than 5 cm is the traditional cut-off.

When there is virtually no measurable fluid, we use the term anhydramnios, which is a more serious finding.

The likely cause of low fluid depends heavily on when in pregnancy it appears:

• Ruptured membranes (PPROM): Leaking of fluid from the waters breaking early is one of the most common causes and must always be excluded first.
• Placental insufficiency & Fetal Growth Restriction (FGR): When the placenta underperforms, blood is diverted away from the kidneys, urine output falls, and fluid drops. This pairing of a small baby with low fluid is an important warning sign.
• Fetal kidney and urinary tract problems: Absent or non-functioning kidneys, blockages of the bladder outlet (such as LUTO), or other renal anomalies reduce urine production. Low fluid before 20 weeks often points here.
• Post-dates pregnancy: After 40–41 weeks, placental function naturally declines and fluid may reduce.
• Maternal factors: Dehydration, certain blood-pressure medications (ACE inhibitors), and some maternal illnesses can contribute.

Once low fluid is confirmed on a careful ultrasound, our work-up is designed to find the cause and gauge the baby's condition:

1. Sterile speculum examination to exclude ruptured membranes, sometimes with a bedside test for amniotic fluid proteins.
2. Detailed anatomy scan focused on the kidneys, bladder, and urinary tract, and a full review of fetal structures.
3. Growth and Doppler assessment: measuring the baby's size and checking blood flow in the umbilical artery, middle cerebral artery, and ductus venosus to detect placental insufficiency.
4. Fetal wellbeing tests: a biophysical profile and, from viability, cardiotocography (CTG) to monitor the baby's heart rate.
5. Consideration of genetic testing (amniocentesis or microarray) when structural anomalies are found, particularly in early or severe cases.
6. Maternal review: blood pressure, hydration, and a medication history.

Management is individualised according to the cause, the amount of fluid, and how many weeks pregnant you are:

• Increased surveillance: More frequent scans, Doppler studies, and CTG monitoring are the cornerstone. Isolated mild reduction near term with a healthy, well-grown baby may simply be watched.
• Timing of delivery: This is the central decision. Isolated oligohydramnios at or beyond 36–37 weeks is often an indication to plan birth. When it accompanies FGR or abnormal Dopplers, delivery may be recommended earlier to prevent stillbirth, balanced against the risks of prematurity.
• Maternal hydration: Simple oral hydration can transiently improve fluid volume and is sometimes advised.
• Amnioinfusion: Instilling warmed fluid into the uterus is not a routine treatment for oligohydramnios itself. Its main established role is during labour to relieve cord compression, and occasionally as a diagnostic aid to visualise anatomy. It does not cure the underlying problem.
• Corticosteroids for lung maturity if early delivery is anticipated.

The outlook depends almost entirely on the cause and the gestation at which the fluid falls:

• Isolated low fluid near term with a well-grown baby generally has an excellent outcome once birth is safely timed.
• Low fluid with FGR or abnormal Dopplers reflects placental disease; with close monitoring and well-timed delivery, most babies do well, though some need neonatal care.
• Severe, early oligohydramnios (before 20–24 weeks), especially with anhydramnios from kidney or bladder-outlet problems, carries a more guarded outlook because the lungs need fluid to develop (risk of pulmonary hypoplasia). In selected LUTO cases, fetal therapy may be considered.
• Post-dates low fluid is usually managed successfully by planning delivery.

You should contact your maternity team promptly if you notice:

• A gush or continuous trickle of fluid from the vagina, which may signal ruptured membranes.
• Reduced or changed fetal movements.
• A sensation that your bump has stopped growing or feels smaller.

Any confirmed low fluid, a small baby, or abnormal Doppler flows warrants referral to a Maternal-Fetal Medicine unit. Our MFM and fetal-imaging team provides detailed assessment, investigates the cause, and plans personalised surveillance and delivery timing in line with current guidance.