Amniotic fluid cushions the baby, allows it to move and practise breathing, helps the lungs and muscles develop, and protects the umbilical cord from being squeezed. In the second half of pregnancy, most of this fluid is the baby’s own urine, so the amount reflects how well the baby’s kidneys are working and how well the placenta is supplying blood. When a scan report says the fluid is low - oligohydramnios - the next step is to find out why, and to check how the baby is coping.
How amniotic fluid is measured
Fluid is measured on ultrasound in one of two ways:
- Amniotic fluid index (AFI): the deepest pocket of fluid in each of four quarters of the uterus, added together. An AFI below 5 cm is considered low; 5 to 8 cm is often called borderline
- Single deepest pocket (SDP or DVP): the depth of the largest pocket. Below 2 cm is considered low
- Many guidelines now prefer the single deepest pocket, because using AFI labels more women as low without improving outcomes for babies
- Fluid normally reaches its highest level around 32 to 34 weeks and reduces gradually afterwards, so a lower value near term can be normal
Pre-natal sex determination is prohibited under the PCPNDT Act. The sex of the baby is not detected or disclosed at any scan.
Common causes
- Leaking of fluid due to rupture of the membranes, sometimes so slow it goes unnoticed
- Placental insufficiency: a placenta that is not supplying enough blood, often together with fetal growth restriction, high blood pressure or pre-eclampsia
- Pregnancy beyond 40 to 41 weeks, as the placenta ages
- Problems with the baby’s kidneys or urinary tract, usually detected on the anomaly scan
- Some medicines, including certain painkillers (NSAIDs) and blood pressure medicines (ACE inhibitors) - tell your doctor about every medicine you take
- Dehydration in the mother, particularly in hot weather or with vomiting
- In identical twins sharing a placenta, an imbalance between the twins (twin-to-twin transfusion syndrome)
Sometimes no cause is found, especially when fluid is mildly low near term and the baby is growing well. This is called isolated oligohydramnios.
It is worth remembering that a single measurement can vary. The amount seen depends on the baby’s position, the pressure of the probe and the person scanning, and a value just below the cut-off on one day may be normal a few days later. Decisions are therefore based on the whole picture - the trend over several scans, the baby’s growth and Doppler flow - rather than one number on a report.
Why it matters
The significance depends on when it happens. Very low fluid early in the second trimester can affect the development of the baby’s lungs and limbs, and needs specialist assessment. In the third trimester, low fluid is mainly a marker that the baby may not be getting enough from the placenta, and it can make the umbilical cord more prone to compression during labour. When the baby is growing normally and blood flow is normal, the outlook with isolated low fluid near term is usually good.
How the baby is assessed
- A careful history and examination, including checking for leaking fluid and testing the vaginal fluid if needed
- A growth scan to estimate the baby’s weight and compare it with earlier scans
- Doppler studies of the umbilical artery, middle cerebral artery and other vessels, which show how well the placenta is working
- A detailed look at the kidneys and bladder if this has not been done
- CTG (NST) monitoring of the baby’s heart rate
- Blood pressure and urine checks for pre-eclampsia
Treatment and monitoring
There is no medicine that directly increases amniotic fluid. Management focuses on the cause and on choosing the right time for delivery.
- Drinking plenty of water - about 2 to 3 litres a day - may increase the fluid modestly, particularly if you were dehydrated, and is a reasonable first step
- Rest on your left side may improve blood flow to the uterus, though strict bed rest is not needed
- Stopping any medicine that may be reducing fluid, under medical advice
- Repeat scans and CTGs, often once or twice a week, depending on the findings
- Steroid injections to mature the baby’s lungs if delivery before 34 to 35 weeks becomes likely
- Planning delivery: at or near term, induction of labour is often discussed; earlier delivery is considered if the Doppler or CTG shows the baby is under strain
Low fluid does not by itself mean you need a caesarean. Many women with low fluid have a normal delivery with continuous monitoring in labour. A caesarean is advised if the baby shows signs of distress or for other obstetric reasons.
When low fluid is caused by leaking membranes before 37 weeks - preterm prelabour rupture of membranes - the plan is different. Antibiotics are given to lower the risk of infection, steroids are given to help the baby’s lungs, and you are monitored in hospital for signs of infection or labour. Delivery is usually planned around 34 to 37 weeks, or earlier if infection or other problems develop.
Things you can do at home
- Drink water regularly through the day; carry a bottle, especially in summer
- Keep track of your baby’s movements every day
- Report any leaking of fluid, even a small, steady wetness
- Keep all scan and monitoring appointments, even when you feel well
- Do not take painkillers or herbal remedies without checking with your doctor
Come to the hospital the same day if your baby is moving less than usual, if you notice fluid leaking from the vagina, or if you have bleeding, a severe headache, blurred vision or abdominal pain.
When to see a doctor
If your scan shows low or borderline fluid, ask for a review with a doctor who can assess growth and Doppler flow rather than relying on the fluid number alone. At Divine Women’s Hospital, Gota, Dr. Devang Patel performs growth and Doppler scans and plans monitoring and delivery for women with low amniotic fluid and growth concerns, with labour and newborn care available 24 × 7.
Related care: Fetal growth restriction (small baby) care
More patient guides
- The NT scan at 11–14 weeks: what it checks and how to prepare
- High blood pressure in pregnancy: the warning signs you should not wait on
- Expecting twins? Why “do they share a placenta?” is the first question
- NIPT or NT scan: which test do you need, and can one replace the other?
- Pregnancy after 35: fertility, risks and how to plan a healthy pregnancy
This guide is general information for patients and does not replace a consultation with your own doctor.