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.
- Protects your baby from being hurt if you have a blow to your tummy, or if your bump is squashed.
- Helps your baby’s lungs and digestive system to mature.
- Protects your baby against infection.
What is a normal level of amniotic fluid?
Your baby regularly swallows amniotic fluid, and passes it out of his body as urine. This means that 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 (1.4 pints) and 1,000ml (1.6 pints) by the time you’re about 36 weeks pregnant. From 38 weeks onwards, the fluid gradually begins to reduce, until you’re ready to give birth.
Too little amniotic fluid surrounding your baby is called oligohydramnios, and too much fluid is called polyhydramnios, or hydramnios.
How can I tell if I have low amniotic fluid?
Your doctor or midwife may suspect that your amniotic fluid levels are low if your tummy or baby appears smaller than expected (small for dates). Your midwife will pick this up when she checks the size of your bump with a tape measure at one of your antenatal appointments.
She may also want to check your amniotic fluid levels if you have:
- already had a baby who was small for dates or a low birth weight
- a particular medical condition, such as lupus
- problems with your blood pressure
If your midwife has concerns, she will recommend that you have an ultrasound scan.
The person doing the scan (sonographer) will check your fluid levels by looking at the whole amniotic sac. She’ll measure the depth of the largest pockets of amniotic fluid in four sections of your uterus. By adding the four measurements together, she’ll work out your score on the amniotic fluid index (AFI). She may do another measurement by checking the deepest pocket of amniotic fluid.
There are established normal levels of fluid for each week of pregnancy, so your levels will be compared with these. In general, an AFI of 5cm (2in) or less, or a deepest pocket measurement of less than 2cm (0.8in), is considered to be low.
How does low amniotic fluid 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.
Sadly, low levels of amniotic fluid during your first trimester and in the early part of your second trimester lead to an increased chance of having a miscarriage, or a baby who is stillborn. It can also cause problems with how your baby grows, and with how well his lungs develop.
However, most cases of low amniotic fluid happen well into the third trimester, when no more than keeping a close eye on things is needed. Doctors will carefully monitor 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.
If your waters have broken early, you may go into premature labour. Doctors will weigh up the risks of this happening against the risk of infection setting in if your baby stays in your uterus. You may be given antibiotics to prevent infection affecting your baby.
Once you’re in labour, there’s a higher chance of your baby becoming distressed. He may do his first poo, a blackish substance called meconium, into the amniotic fluid. If he inhales the meconium, it may cause him to have breathing problems when he is born.
Another problem may be that the umbilical cord gets accidentally squashed by your baby while he is being born. 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 most common cause of low amniotic fluid is that your waters have broken.
Often no cause can be found, but other causes include:
- Being more than 42 weeks pregnant.
- A problem with the placenta (placental insufficiency).
- Certain types of medication that you may be taking.
- If your baby has an inherited medical problem with kidney development.
- If one of your identical twins is not growing as well as he should.
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.
You may be offered antibiotics if you have signs of infection. If your waters have broken before 37 weeks pregnancy, antibiotics are recommended to help prolong your pregnancy and guard against infection. The antibiotic used will be safe for you and your baby.
Depending on your stage of pregnancy, you may be advised to have your labour started artificially. Otherwise, you’ll be monitored until labour starts naturally.
A problem with the placenta
This could be because you have a condition that stops your placenta from supplying enough blood and nutrients to your baby. These conditions include lupus, high blood pressure, pre-eclampsia and diabetes. It can also happen if your pregnancy becomes prolonged beyond 42 weeks.
If the placenta is not 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.
Medication that you are taking
Some drugs may cause you to have low amniotic fluid. These include treatments for high blood pressure (ACE inhibitors) and non-steroidal anti-inflammatories, such as ibuprofen. But these medications are not usually prescribed 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 that affects his kidneys or urinary system, which could be a sign that he has a chromosomal abnormality. If this is the case, it’s likely to be picked up during your anomaly scan, at 20 weeks.
It can be worrying to be told there may be a problem with your baby’s health. So you get the care that you need, you may be referred to a specialist fetal medicine unit, staffed by consultants and midwives skilled in caring for pregnant women. They may offer treatments that are not available at smaller units.
A problem with an identical twin
If identical twins share a placenta, sometimes their amniotic fluid levels become 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
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, so it’s possible you may be dehydrated. Drinking plenty of water may help boost your fluid levels. You should also try to eat well and get plenty of rest.
What can I do if I have low amniotic fluid?
It can be worrying to be told you have low amniotic fluid. However, there’s not much you can do, apart from staying well hydrated, eating healthily and resting. Try not to let worry stop you from sleeping.
You and your baby will be well looked after during your pregnancy. You’ll both be carefully monitored by midwives and doctors to make sure that you receive the best possible care.