Amniotic Fluid Levels: Separating Fact from Fear
Amniotic fluid levels are a common concern at the end of pregnancy, but how much do they really matter? In this post, we’ll explore what’s “normal,” what happens when levels are too high or too low, and why trusting your intuition may be more important than relying on an ultrasound diagnosis.
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Amniotic fluid is an essential part of a healthy pregnancy. It has many functions, including cushioning the baby, helping regulate his or her temperature, allowing for movement in the womb, and more.
But what happens if there’s not enough…or too much?
Most of the time, nothing needs to happen. Yet women are treated differently based on their fluid levels quite often. Too often.
So let’s talk about amniotic fluid levels, how common it is for the level to really be a problem, and other things you’ll need to know if a diagnosis based on fluid level happens to you.
Two Extremes + What’s Normal
First of all, I’m going to cover two opposite ends of the spectrum in this blog post: too much amniotic fluid and not enough amniotic fluid. There are similarities and, of course, there are differences.
Too much amniotic fluid is called “polyhydramnios” (polly-high-DRAM-nee-oh-s).
Low amniotic fluid is called “oligohydramnios” (AWH-lig-oh-hi-DRAM-nee-oh-s).
“Normal” fluid level depends on the woman (conditions she has and health habits) and how far into pregnancy she is. Most healthy pregnancies follow the same general pattern of amniotic fluid levels: the amount increases until 34–36 weeks of pregnancy then starts to decrease over the last few weeks.
At 40 weeks, most pregnant women will have about 600 milliliters of amniotic fluid (about 2.5 cups).
Interestingly, two things combine to make it absolutely normal for amniotic fluid levels to decrease as pregnancy nears its end:
- Babies pee less in the two weeks leading up to birth.
- Babies swallow more as pregnancy reaches full term.
What is Polyhydramnios?
Polyhydramnios is when there is an abnormally high level of amniotic fluid surrounding a baby in the womb. It is diagnosed through ultrasound.
I like this summary from Cleveland Clinic of why polyhydramnios can be a problem:
“Too much amniotic fluid in your uterus puts pressure on your nearby organs and [can cause] pregnancy complications. The condition is usually more serious if it occurs early in pregnancy because there’s more time for amniotic fluid to continue to build up. The excess fluid itself is not harmful to the fetus.”
Usually you won’t know you have polyhydramnios until you get an ultrasound (usually in the third trimester) because it doesn’t often cause any symptoms.
How is polyhydramnios diagnosed?
When doing an ultrasound, a sonographer (the person doing the ultrasound) will look at how much of a “pocket” of amniotic fluid there is between the wall of your uterus and your baby. Usually, these dark spots (liquid is dark in an ultrasound) measure between 2 and 8 centimeters.
If any one of them measures more than 8 centimeters (3.15 inches), that indicates polyhydramnios. Polyhydramnios can be mild, moderate, or severe, as follows:
- Mild: 8–11 centimeters
- Moderate: 12–15 centimeters
- Severe: 16+ centimeters
An Alternate Method: Amniotic Fluid Index
The above method of measuring fluid levels is called the single deepest vertical pocket.
Alternatively, the sonographer may use a method called the amniotic fluid index (AFI).
With the AFI, the sonographer will look at the deepest “pocket” of amniotic fluid in each of four imaginary quadrants of the uterus (like it was divided in fourths). Then they’ll add up those four measurements.
The fluid level is said to be normal if the sum is between 5 and 24 centimeters. (However, those limits were chosen arbitrarily.)
Again, polyhydramnios may be mild, moderate, or severe according to this measurement, as follows:
- Mild: 25–30 centimeters
- Moderate: 30.1–35 centimeters
- Severe: 35.1+ centimeters
Are these methods accurate?
Interestingly, studies have been done comparing the two methods of measurement and many have concluded that AFI (the 4-measurement one) results in more misdiagnoses of oligohydramnios which in turn leads to more inductions and c-sections without improving outcomes.
That said, neither is highly accurate at identifying true cases of polyhydramnios (or oligohydramnios) nor at excluding false cases.
Finally, it’s important to recognize that while ultrasounds are the tool used for diagnosis of polyhydramnios or oligohydramnios, researchers have repeatedly found that ultrasound estimations of amniotic fluid levels are only accurate about 65% of the time.
This inaccuracy can be due to an inexperienced sonographer, the baby’s position, or how much pressure is applied with the ultrasound probe.
How common is polyhydramnios?
Polyhydramnios only happens in 1–2% of all pregnancies (1 or 2 out of 100).
But here’s the thing…about 80% of those cases of polyhydramnios are going to be mild. And mild polyhydramnios usually doesn’t cause issues. Mom and baby can have a normal, healthy birth experience.
A group of researchers who conducted a large study about polyhydramnios state that women with mild polyhydramnios do not need extra monitoring leading up to birth, nor should they be induced.
So your chance of having moderate or severe polyhydramnios—and therefore needing extra monitoring and, possibly, intervention in late pregnancy and birth—is only 0.2% or 1 in 500.
Complications of Polyhydramnios
Having too much amniotic fluid (moderate or severe cases) is connected to these complications:
- Premature rupture of membranes (water breaking before labor begins)
- Premature birth (before 37 weeks)
- Placental abruption (placenta coming away from uterus before birth)
- Postpartum hemorrhage
- Umbilical cord prolapse
- Breech presentation
- Stillbirth
If you have polyhydramnios, the likelihood of a c-section also slightly increases.
Causes of Polyhydramnios
Most of the time, there is no apparent cause of mild polyhydramnios.
Severe polyhydramnios is rare but interestingly, 91% of these severe cases are linked to a known underlying disease or congenital anomaly (birth defect). In this case, you would likely know of the disease or defect before polyhydramnios became a problem.
Though uncommon, a few other things are associated with increased chance of polyhydramnios.
Diabetes and Polyhydramnios
Gestational diabetes is a common cause of—or at least is commonly associated with—polyhydramnios.
Studies have come to differing conclusions about how often polyhydramnios happens in a woman with gestational diabetes. But it seems to be somewhere between 8 and 16%.
Because polyhydramnios is linked to diabetes, it’s also often associated with, though not necessarily caused by, macrosomia (an abnormally large baby). Because a bigger baby will pee more (urine makes up most of the amniotic fluid), macrosomia and polyhydramnios can create a bit of a vicious cycle.
Twin-to-Twin Transfusion Syndrome and Polyhydramnios
Having twins who share a placenta but have different amniotic sacs can increase your risk of polyhydramnios, too, because of the risk of twin-to-twin transfusion syndrome (TTTS). TTTS happens when one twin gets significantly more blood supply than the other and therefore grows faster and produces more amniotic fluid.
Rh Factor and Polyhydramnios
Finally, if you have Rh-negative blood and your baby is Rh-postive, you have a higher chance of polyhydramnios.
What is Oligohydramnios?
Now, what about low levels of amniotic fluid? Some of the information is similar to polyhydramnios, so if you skipped to this section, I recommend you go back and read about polyhydramnios first.
Oligohydramnios is when there is an abnormally low level of amniotic fluid surrounding a baby in the womb.
You may not know you have oligohydramnios until you get an ultrasound (usually in the third trimester) because it doesn’t often cause any symptoms. However, the following things can indicate low levels of amniotic fluid:
- Fluid is leaking out of your vagina
- Your belly measures smaller than expected
- Your baby isn’t moving very much
How is oligohydramnios diagnosed?
Like polyhydramnios, oligohydramnios is found through ultrasound, specifically through measuring the “pockets” of amniotic fluid. Remember, “normal” is 2–8 centimeters.
If any one pocket measures less than 2 centimeters, that indicates oligohydramnios.
If using the amniotic fluid index (AFI), oligohydramnios is an AFI of less than 5 centimeters. It would be classified as mild, moderate, or severe as follows:
- Mild: 4–5 centimeters
- Moderate: 2–4 centimeters
- Severe: 0–2 centimeters
How common is oligohydramnios?
Oligohydramnios happens in about 4% of all pregnancies (4 out of 100). That number increases to about 12% when a woman goes past her due date because fluid levels naturally decrease as pregnancy goes on.
Still, like polyhydramnios, only a small portion of instances of low amniotic fluid are severe. According to one study, about 20% of the time oligohydramnios will be severe, requiring intervention.
So, all together, your risk of complications because of oligohydramnios is about 0.8% or 1 out of 125.
Note: Oligohydramnios is typically less severe the later it occurs. Most cases of oligohydramnios diagnosed in the third trimester have good outcomes.
Complications of Oligohydramnios
Having low levels of amniotic fluid is connected to these complications:
- Increased risk of umbilical cord compression
- Meconium aspiration
- Low heart rate (baby)
- Premature birth (before 37 weeks)
- Stillbirth
- Infection (if amniotic sac has broken)
Rarely, having too little amniotic fluid for too long can cause your baby’s muscles to lose their function and cause lasting disability.
Like is the case with too much amniotic fluid, not enough amniotic fluid can also increase your risk of c-section.
Finally, like polyhydramnios is associated with macrosomia, oligohydramnios is associated with fetal growth restriction (FGR). Whether one causes the other or if both are caused by the same thing and thus often found together simply isn’t clear.
Causes of Oligohydramnios
There isn’t one clear cause of oligohydramnios. And more than half the time, there isn’t any known cause.
One of the main known causes of oligohydramnios (37%) is preterm premature rupture of membranes or PPROM. This is when a woman’s water breaks not only before labor begins but before she reaches full term (before 37 weeks).
In this situation, amniotic fluid may slowly leak out without mom noticing, which would obviously decrease amniotic fluid levels.
Sometimes oligohydramnios is connected to placental problems (what doctors call “placental insufficiency”). These placental problems can be caused by various issues, including preeclampsia, chronic medical conditions, and improper placental development.
Placental abruption, when the placenta detaches from the wall of the uterus before birth, is found in 8.6% cases of oligohydramnios.
Chromosomal abnormalities seem to contribute to about 10% of low amniotic fluid cases.
And, like I mentioned earlier, the farther past your due date you get, the more likely you are to have oligohydramnios.
Twin-to-twin transfusion syndrome is also a cause of oligohydramnios. (One twin will experience polyhydramnios while the other experiences oligohydramnios.)
Congenital anomalies (birth defects) can also cause low fluid levels. For example, while a baby’s inability to swallow properly may lead to increased fluid levels, a baby’s inability to pee fluid out may lead to decreased fluid levels.
Finally, while drinking more water doesn’t always mean you’ll have more amniotic fluid, hydration does play a role. If you have oligohydramnios, intentional hydration can significantly improve fluid levels.
What to Do When Your Fluid Levels Appear Abnormal
With that information all laid out, let’s get to the real question: what to do if you’re told you have abnormal fluid levels.
While every woman’s circumstances are going to be a little different, the first thing I recommend you do if you are told you have too much amniotic fluid or not enough is this:
Ask more questions!
Ask these kinds of questions:
- How is my baby doing? How’s his or her breathing, movement, heart rate, etc.?
- Can we see how the fluid measures by using just a single deepest pocket measurement (if AFI was used)?
- Have the fluid levels been increasing or decreasing abnormally?
Amniotic fluid level is just one piece of the puzzle. If everything else seems to be a-okay, you probably don’t need to stress.
If you or your doctor are still worried about your fluid levels, you’ll probably be given a few options:
- Monitor baby every week with ultrasounds and/or non-stress tests
- Undergo tests to check for gestational diabetes
- Receive treatment for known gestational diabetes
- Undergo amniocentesis, a procedure to remove some of the amniotic fluid for testing
- Take medication to reduce how much your baby pees
- Get induced if mom or baby is at risk
Be sure to ask about the risks and benefits of—and evidence behind—each thing before deciding what to do. (Having a doula or other support person with you will make this a lot easier.)
Induction for too much or too little amniotic fluid?
While induction is a common recommendation when fluid levels are abnormal, this isn’t backed by evidence (except in severe cases). When polyhydramnios is mild, there is no evidence that induction (active intervention) is better than waiting (expectant management).
In the case of oligohydramnios, it is routine for doctors to induce at 39 or 40 weeks even if no other factors indicate risk. This too goes against the evidence (when no other complications are present).
Even in severe cases, the risks of waiting have to be weighed against the risks of induction.
Induction is known to create a more stressful environment for the baby. So if a baby is already at risk, induction may only cause more distress and thus lead to an emergency c-section.
Top Tip: Don’t Get a Third Trimester Ultrasound
If you want to avoid a diagnosis of polyhydramnios or oligohydramnios, there’s one simple thing you can do that drastically reduces your chances:
Don’t get an ultrasound in the third trimester.
Ultrasound measurements aren’t always accurate and the defined limits of “normal” are arbitrary, so the chance of a false diagnosis is high.
Plus, humans, especially doctors, tend to see pathology (disease or injury) when there is none—I suppose it would be hard not to after going through so many years of medical school.
Too many women have agreed to an ultrasound in the third trimester and later wished they hadn’t. Because if you accept an ultrasound, you also have to accept the responsibility for opening yourself up to what the doctor might say about your baby, whether it’s accurate or not.
If you do end up getting a third trimester ultrasound—because other factors indicate your baby may be struggling or simply because you want to—be ready to ask questions! Ask the questions I listed earlier:
- How is my baby doing? How’s his or her breathing, movement, heart rate, etc.?
- Can we see how the fluid measures by using just a single deepest pocket measurement (if AFI was used)?
- Have the fluid levels been increasing or decreasing abnormally?
The majority of the time, even slightly higher or lower levels of amniotic fluid don’t indicate a problem. It is rare that your baby will be in danger, and when that is the case, you’ll probably have other warning signs long before you get an ultrasound.
Conclusion: Mother Knows Best
I’ll finish the same way I’ve finished other posts: with the truth that a mother truly knows best.
As Rachel Reed puts it, “Mothers are…experts regarding their own body/baby and notice differences themselves – particularly if they have been pregnant before and can compare pregnancies.”
If something is truly wrong, a mother’s intuition (which I believe is one and the same with divine inspiration) will usually let her know before the doctor says anything. And if everything is fine, a mother will likely know that even if her doctor says otherwise.
So learn to trust your intuition!
In addition to your innate wisdom, receiving care from a midwife is going to make a big difference in regards to how amniotic fluid levels are both assessed and treated.
A midwife will usually not ask you to get many (if any) ultrasounds, which removes a huge part of the issue. And most midwives can recognize true polyhydramnios or oligohydramnios simply by feeling, measuring, or even seeing a mother’s belly.
Because midwifery care is involved and hands-on during pregnancy, midwives are much better equipped to accurately assess abnormal fluid levels…because they know their clients well!
So hire a midwife, trust your intuition, and don’t stress. That’s the best advice I can give you.
Until next time,
Allison
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