What is oligohydramnios?

When there's too little amniotic fluid in your uterus at any point in your pregnancy, it's called oligohydramnios. (When there's too much, it's called polyhydramnios.) It's most common late in the third trimester, particularly if you're overdue, and not usually a sign of a problem.

What is amniotic fluid?

Amniotic fluid fills the sac surrounding your developing baby and plays several important roles:

It cushions your baby to protect him from trauma (if you take a tumble, for instance).

It prevents the umbilical cord from becoming compressed, which would reduce your baby's oxygen supply.

It helps maintain a constant temperature in the womb.

It protects against infection.

It allows your baby to move around so that his muscles and bones develop properly.

It helps the digestive and respiratory systems develop as your baby swallows and excretes it and "inhales" and "exhales" it from his lungs.

Where does amniotic fluid come from?

During the first 14 weeks of your pregnancy, fluid passes from your circulatory system into the amniotic sac. Early in the second trimester, your baby starts to swallow the fluid, pass it through her kidneys, and excrete it as urine, which she then swallows again, recycling the full volume of amniotic fluid every few hours. (Yes, this means that most of the fluid is eventually your baby's urine!)

So your baby plays an important role in keeping just the right amount of fluid in the amniotic sac. Sometimes, though, this system breaks down, resulting in either too much or too little fluid – both of which can present problems.

How much amniotic fluid should I have?

Under normal circumstances, the amount of amniotic fluid you have increases until the beginning of your third trimester. At the peak of 34 to 36 weeks, you may carry about a quart of amniotic fluid. After that, it gradually decreases until you give birth.

How will I know if I have low levels of amniotic fluid?

Your practitioner may suspect this problem if you're leaking fluid, measuring small for your stage of pregnancy, or not feeling your baby move very much. She may also be on the lookout for it if you've previously had a baby whose growth was restricted or if you have certain pregnancy complications (see maternal complications, below), or if you're past your due date.

To find out what's going on, your practitioner will send you for an ultrasound. The sonographer will measure the largest pockets of amniotic fluid in four different sections of your uterus and add them together to see where you rate on the amniotic fluid index (AFI). A normal measure for the third trimester is between 5 and 25 centimeters (cm). A total of less than 5 cm is considered low.

What could be causing oligohydramnios?

Experts don't always know what causes low levels of amniotic fluid. In fact, most women with oligohydramnios have no identifiable cause. The later in pregnancy the condition develops, the better the outlook for your baby.

Here are the most common causes of oligohydramnios:

A small baby.

Small babies will produce less fluid.

Maternal complications.

Complications such as chronic high blood pressure, preeclampsia, diabetes, and lupus can result in low amniotic fluid levels.

Placental problems

A problem with your placenta, such as a partial abruption, in which the placenta peels away from the inner wall of the uterus, can lead to low levels of amniotic fluid. If the placenta doesn't supply enough blood and nutrients to your baby, he will stop producing urine.

Less common causes of oligohydramnios include:

Leaky or ruptured membranes

A small tear in your amniotic membranes can allow some fluid to leak out. This can happen at any point in your pregnancy, but it's more common as you approach delivery. You may notice the leaking fluid yourself if you find that your underwear is wet, or your practitioner may discover it during an exam. Let your practitioner know if you suspect your amniotic fluid is leaking.

A ruptured membrane can increase the risk of infection for both you and your baby because it provides a way for bacteria to enter the amniotic sac. Occasionally, a tear in the sac will heal on its own, the leaking will stop, and the fluid level will return to normal. (This is usually the case if the leaking happens after an amniocentesis.)

Certain medications

Some medications, such as ACE inhibitors, can contribute to oligohydramnios.

Carrying twins or multiples

You're at risk for low fluid levels if you're carrying twins or multiples. Oligohydramnios is likely in the case of twin-to-twin transfusion syndrome, in which one twin has too little amniotic fluid while the other has too much.

Being well past your due date

Decreased amniotic fluid is one of the risks of post term pregnancy. Being two weeks or more past your due date can put you at risk for oligohydramnios.

Fetal abnormalities

If you're found to have low levels of amniotic fluid in your first or second trimester, it may mean that your baby has a birth defect. If his kidneys aren't there or aren't developing properly or his urinary tract is blocked, for example, your baby won't produce enough urine to maintain the level of amniotic fluid.

Is oligohydramnios dangerous for my baby?

Not usually. Most cases of oligohydramnios in the third trimester won't result in any complications for your baby. If the fluid level gets very low, though, there's some risk to your baby because the umbilical cord can become squeezed, preventing your baby from getting enough food and oxygen. That's why women with oligohydramnios at term have a higher rate of interventions or problems during labor and birth, including:

It's uncommon for a low amniotic fluid level to develop in the first two trimesters, but if it does, it can cause:

What will happen if I'm diagnosed with oligohydramnios?

If you have low levels of amniotic fluid, your caregiver will follow your pregnancy closely to be sure your baby continues to grow normally. How your pregnancy will be managed depends on how far along you are, how your baby's doing, and whether you have other complications.

If you and your baby are healthy and oligohydramnios develops toward the end of your pregnancy, you may be monitored more often but may not need any treatment.

If your oligohydramnios is concerning, procedures might include: