Oligohydramnios (low amniotic fluid in pregnancy): causes, scans and outlook

Throughout pregnancy the baby is cushioned inside a fluid-filled bag called the amniotic sac. The Newcastle Hospitals leaflet explains that the wall of the sac is made of two membranes that keep the baby sealed in amniotic fluid, and that these membranes usually break open before or during labour, which is what people mean by the waters breaking. Amniotic fluid, also called liquor, helps the baby's lungs, digestive system, muscles and bones to develop and protects the baby against infection. Leeds adds that it protects the baby from being hurt if you have some trauma to your abdomen.

Oligohydramnios is diagnosed when the amount of fluid around the baby is lower than the expected normal range. The normal range changes a little depending on the stage of pregnancy. If there is no measurable fluid at all the term is anhydramnios. Both leaflets say neither can be diagnosed except by an ultrasound scan, so it is not something you would be able to tell from symptoms alone.

During a scan the sonographer or doctor estimates the amount of fluid by measuring the depth of fluid pockets. Both leaflets say that in general terms, if the deepest pocket of fluid in the womb is less than 2 cm, this is considered lower than normal. Leeds describes it as an unexpected finding on ultrasound examination.

The Newcastle leaflet is written for women with a finding of oligohydramnios and says it should be given in consultation with a senior obstetrician or fetal medicine specialist. This means the numbers on a scan report should be discussed with the team looking after you rather than interpreted alone.

The leaflets describe several potential causes when low fluid is found early, before 24 weeks. These are problems with the baby's urinary tract, meaning the production or flow of urine is inhibited; ruptured membranes, the waters breaking; and a difficulty with the function of the placenta. Newcastle also lists an infection in the baby that has been transmitted through the placenta, such as cytomegalovirus (CMV).

The causes of low fluid later in pregnancy are not set out in these leaflets, and the likely reason is something your own team will look for when they review your scans, your blood pressure and your baby's growth.

• A problem with the baby's urinary tract that limits urine production or flow

• Ruptured membranes (the waters breaking)

• Reduced function of the placenta

• Infection in the baby passed on through the placenta, such as cytomegalovirus

Both NHS leaflets are careful to say that the implications depend on several factors: the possible underlying cause, how low the fluid is and when in pregnancy it is first found. A finding of oligohydramnios late in pregnancy is unlikely to be significant for the baby.

More serious consequences relate to significantly reduced or absent fluid before 24 weeks. Newcastle says that this may mean the baby's lungs will not develop effectively and the baby's limbs and joints may not move well after delivery, and that sadly some of these babies will not survive. Leeds uses similar language. If the fluid is low because the membranes have ruptured before 24 weeks, there is also a risk of miscarriage and of developing an infection. A significant kidney abnormality may be a concern for the long-term outlook, and Leeds says that if the lack of fluid is due to poor placental function, the baby is also likely to be very small and at risk of complications from poor growth.

These are difficult things to read. It may help to remember that the leaflets are written for the more serious end of the spectrum, which is why they stress that the outlook depends so much on cause and timing, and that a late finding is unlikely to be significant for the baby.

Leeds says your local hospital will discuss your case and may refer you to the Fetal Medicine Unit for a specialist scan and discussion. Depending on those findings you may be offered further tests and future appointments for scans. Newcastle suggests making a note of questions to ask at your next Fetal Medicine Unit appointment.

Both leaflets point to Antenatal Results and Choices, a charity that supports parents who have received difficult results from antenatal tests, and say that your local hospital will have a specialist midwife you can contact for further discussion.

The leaflets do not give a full list of warning signs, so follow the specific advice from your own unit. As a general principle, contact your maternity team or maternity triage promptly if you notice a change in your baby's movements, think your waters have broken or have any bleeding, rather than waiting for the next scan. Do not use this page to decide on timing of care: your team knows your scan results.

Scans, referral to a fetal medicine unit and decisions about delivery for oligohydramnios are normally part of NHS maternity care. Some families also pay for private scans, which can show an unexpected finding, but any concern identified outside the NHS pathway needs to be reviewed by your NHS maternity team who hold your full record. Bring any private scan report with you to your appointment.