Oligohydramnios: low amniotic fluid in pregnancy

The Newcastle and Leeds fetal medicine units explain that the baby is cushioned inside a fluid-filled bag, the amniotic sac. The two membranes that make up the wall keep the baby sealed in the fluid and usually open before or during labour, which is the waters breaking. The fluid protects the baby, including from the effects of trauma to the abdomen (Leeds), helps the lungs, digestive system, muscles and bones develop, and protects against infection (Newcastle).

The RCOG notes in its PPROM leaflet that amniotic fluid continues to be made, and the Newcastle leaflet lists problems with the production or flow of the baby's urine as one cause of low fluid, which shows the baby's urinary tract is part of the picture.

Both trusts say oligohydramnios can only be diagnosed after an ultrasound scan, and that the normal range changes a little with the stage of pregnancy. Your sonographer or doctor estimates the amount by measuring the depth of fluid pockets; the leaflets say that where the deepest pocket is less than 2 cm the fluid is considered lower than normal. If there is no measurable fluid, this is called anhydramnios.

The RCOG leaflet on baby movements says an ultrasound to check growth and the amount of fluid may be arranged if your uterus measures smaller than expected, your pregnancy has stillbirth risk factors, or you still feel movements are reduced when heart-rate monitoring is normal. NICE guideline NG229 on fetal monitoring in labour lists too little or too much amniotic fluid (anhydramnios or polyhydramnios) among the reasons continuous CTG monitoring is offered in labour.

The trust leaflets list causes of oligohydramnios earlier than 24 weeks: a problem with the baby's urinary tract, which limits the production or flow of urine; ruptured membranes (waters breaking); and a problem with placental function. The Newcastle leaflet also lists an infection in the baby passed across the placenta, such as cytomegalovirus (CMV). The Leeds leaflet says that if the fluid is low because of poor placental function the baby is likely to be very small and at risk of complications from poor growth.

Later in pregnancy low fluid is often found without a clear reason, and the leaflets say an unexpected late finding is unlikely to be significant for the baby. Your team will consider your scans, growth, blood pressure and whether the waters have broken before deciding on next steps.

The Leeds and Newcastle leaflets say the implications depend on the cause, the degree of reduction and the gestation at which it is first found. Significantly reduced or absent fluid before 24 weeks can mean the baby's lungs do not develop effectively and the limbs and joints may not move well after birth, and sadly some of these babies will not survive. If the fluid is low because the membranes ruptured before 24 weeks there is also a risk of miscarriage and infection. A significant kidney abnormality would also be a concern for the long-term outlook.

These statements describe serious situations that arise mainly with a very early and marked loss of fluid. They are not a prediction for every person with a low reading, particularly later in pregnancy. If you have been given this diagnosis, ask your fetal medicine doctor what applies to your scan findings and gestation.

The Leeds leaflet says your local hospital will discuss your case and may refer you to the fetal medicine unit for a specialist scan and discussion, and that depending on findings you may be offered further tests and future scans. Newcastle suggests making a note of questions for your next fetal medicine appointment and mentions Antenatal Results and Choices as a source of support.

If you think your waters may have broken, wear a pad and go to hospital: the RCOG says leaking fluid should be checked straight away. Contact your maternity unit if movements change, you have bleeding, a raised temperature or abdominal pain. Arrangements for scans and referral to fetal medicine differ between UK nations and trusts.