Low amniotic fluid (oligohydramnios)

What is amniotic fluid, and what does it do?

During pregnancy, your growing baby is cushioned inside a fluid-filled bag (amniotic sac) in your uterus (womb).

The wall of the amniotic sac is made up of two membranes: the chorion and the amnion. These membranes keep your baby safely sealed in his bag of amniotic fluid.

The membranes usually break open during labour, when your baby is ready to be born, and fluid flows out. This is your waters breaking.

Amniotic fluid works in many ways to help your baby develop throughout your pregnancy. It:

• protects your baby from being hurt if you have a blow to your tummy, or your bump is squashed
• helps your baby's lungs and digestive system mature, and his muscles and bones develop
• protects your baby against infection
• keeps your baby at a constant temperature
  (NHS 2015)

What's a normal level of amniotic fluid?

Your baby regularly swallows amniotic fluid and passes it out of his body as urine. This means the amount of fluid in the amniotic sac normally rises and falls every day.

The amount of amniotic fluid increases as your pregnancy progresses. You start off with just a few millilitres, but will have between 800ml and 1 litre by the time you're about 36 weeks pregnant (Carter 2017, Payne 2016). From 38 weeks onwards, the fluid gradually begins to reduce, until you're ready to give birth (NHS 2015, Payne 2016).

Too little amniotic fluid surrounding your baby is called oligohydramnios, and too much fluid is called polyhydramnios, or hydramnios.

Having low amniotic fluid is a fairly common problem. It affects one in 25 pregnant women, rising to one in eight women who go beyond 41 weeks (Payne 2016).

How can I tell if I have oligohydramnios?

Your doctor or midwife may suspect that your amniotic fluid levels are low if your bump appears smaller than expected. Your caregiver will pick this up when she checks the size of your bump with a tape measure at one of your antenatal appointments.

Oligohydramnios can be associated with having a baby who is small-for-gestational-age (SGA). So your doctor may also want to check your amniotic fluid levels if you have risk factors for having a low-birth-weight baby, such as if you:

• have already had a baby who was small for dates or of a low birth weight
• have certain medical conditions, such as kidney problems, high blood pressure or lupus
• were underweight (with a body mass index (BMI) less than 20) or overweight (with a BMI over 25) before you became pregnant
• are over 35
  (RCOG 2013)

If your midwife or doctor has concerns, she’ll recommend that you have an ultrasound scan.

There are two ways the person doing the scan (sonographer) can check your fluid levels:

• MVP: this stands for maximum vertical pocket, and is the most reliable way to measure the fluid. Your sonographer will measure the depth of the largest pocket of amniotic fluid in your uterus (RCOG 2013). In general, a measurement of less than 2cm is considered to be low, from the late second trimester onwards (Payne 2016, RCOG 2013).
• AFI: this stands for amniotic fluid index, a less reliable measurement that some doctors still use. Your sonographer will look at the picture of your womb, divide it into four quarters, and measure the amount of fluid in each quarter. The average of these measurements is your AFI. In general, an AFI measurement of less than 5cm is considered to be low, from the late second trimester onwards.

How does oligohydramnios affect my baby?

It depends on what's causing the low level of fluid, how little there is, and how far along you are in your pregnancy.

Low levels of amniotic fluid can cause problems with how your baby grows, and with how well his lungs develop (Carter 2017).

If you have low levels of amniotic fluid during your first trimester and the early part of the second trimester, you may be more likely to experience miscarriage.

In the most serious cases, low amniotic fluid can cause a baby to be stillborn after 20 weeks (Carter 2017).

But most cases of low amniotic fluid happen well into the third trimester, usually because your waters have broken (Wojcieszek et al 2014). If this happens to you, your caregiver will carefully monitor your fluid levels and your baby's growth.

To keep you hydrated, you may need extra fluids through a vein (intravenously), if your doctor has concerns (Payne 2016).

Low amniotic fluid can cause labour complications. For example, your baby may be in a bottom-down, (breech) position, and may not have enough room to turn into a head-down position (Shrem et al 2016, Zsirai et al 2016).

Once you're in labour, there's a higher chance of your baby becoming distressed (Carter 2017, Payne 2106). He may do his first poo, a blackish substance called meconium, into the amniotic fluid (Carter 2017, Rabie et al 2017). If he inhales the meconium, it may cause him to have breathing problems when he’s born.

Another possible complication is that the umbilical cord may get accidentally squashed by your baby while he’s being born (Carter 2017).

Your baby will be carefully monitored, to make sure he's doing well. If he becomes too distressed, you may need to give birth by caesarean section.

What causes low amniotic fluid?

The reason isn’t always obvious, and your doctor may not know exactly why it’s happening to you. But the most common cause of low amniotic fluid is that your waters have broken (Carter 2017).

Other causes of oligohydramnios include:

• being more than 42 weeks pregnant
• a problem with the placenta (placental insufficiency)
• certain types of medication you may be taking
• your baby has an inherited medical problem with his urinary system
• one of your identical twins isn’t growing as well as he should
  (Payne 2016)

Your waters have broken
Your waters may break in a big gush, or fluid may leak out slowly, because there's a tear in the amniotic sac. Sometimes it’s difficult to tell the difference between broken waters and an accidental leak of wee.

If you aren't sure whether your waters have broken, contact your midwife or doctor straight away. If your waters have broken early, before your contractions have started, you're at risk of infection if you don't then go into labour (NCCWCH 2008, 2015).

You may be offered antibiotics if you have signs of infection (NCCWCH 2012, 2014, 2015). If your waters have broken before 36 weeks, antibiotics are recommended to help prolong your pregnancy and guard against infection. The antibiotic used will be safe for you and your baby. You may also be offered steroids, depending on the exact week of your pregnancy (NCCWCH 2015).

Your doctor will aim to delay your baby's birth until he's as well-developed and ready for the outside world as possible. But if there are complications, she may advise you to have your labour induced to start it artificially (Bond et al 2017, Middleton et al 2017, NCCWCH 2008). Otherwise, you'll be monitored until labour starts naturally.

Occasionally, a tear in the amniotic sac can reseal itself so you no longer leak amniotic fluid. Your doctor will keep a close eye on the health of you and your baby, to help her recommend when it’s best for you to give birth (Jain and Sciscione 2011).

A problem with the placenta
This could be because you have a condition that stops the placenta from supplying enough blood and nutrients to your baby. These conditions include lupus, high blood pressure, pre-eclampsia and diabetes (Payne 2016, RCOG 2013). It can also happen if your pregnancy becomes prolonged beyond 42 weeks (Payne 2016).

If the placenta isn’t working well, it will affect your baby's growth and wellbeing. Your health and your baby's development will be carefully monitored, and you'll have regular scans to check fluid levels (RCOG 2013).

Medication you’re taking
Some drugs may cause you to have low amniotic fluid. These include treatments for high blood pressure (ACE inhibitors or angiotensin II receptor antagonists) (NICE 2015, UKTIS 2013, 2015) and non-steroidal anti-inflammatories, such as ibuprofen (UKTIS 2014). But these medications aren’t usually recommended in pregnancy.

Your baby has a health problem
Your baby may not be producing or passing enough urine. This may signal to doctors that he has a problem affecting his kidneys or urinary system, which could be a sign that he has a chromosomal abnormality (Carter 2017, Payne 2016). If this is the case, it's likely to be picked up during your anomaly scan, between 18 weeks and 20 weeks plus six days of pregnancy.

It can be worrying to be told there may be a problem with your baby's health. If necessary, your doctor will refer you to a specialist, who'll make sure you and your baby get the care you need.

A problem with an identical twin
If identical twins share a placenta, sometimes their amniotic fluid levels get out of balance. This happens when one twin gets more blood via the placenta than the other.

This will lead to the twin with extra blood having too much fluid, and the other twin not having enough (twin-to-twin transfusion syndrome) (Carter 2017, Payne 2016).

Unexplained oligohydramnios
If all the above conditions are ruled out, it may not be possible to say why you have low amniotic fluid. We know that low amniotic fluid levels are more common during the summer (Feldman et al 2009), so it's possible you may be dehydrated. Drinking plenty of water may help boost your fluid levels (Payne 2016).

What can I do if I have oligohydramnios?

It can be worrying to be told you have low amniotic fluid. You may feel anxious for the rest of your pregnancy, because of your natural concern for your baby's wellbeing.

The best you can do for your baby is to focus on staying healthy yourself. Try to:

• go to all your antenatal appointments (Payne 2016)
• stay well hydrated (Payne 2016)
• eat healthily (NHS 2015)
• rest as much as you can (NHS 2015)

Learn more about what to drink during pregnancy.