Low Amniotic Fluid in Pregnancy

The volume of amniotic fluid in your uterus is not static. It is being made and reabsorbed constantly, and by late pregnancy, the entire pool turns over roughly every three hours. Whatever changes the rate of amniotic fluid production or the rate of reabsorption changes the volume. Most of the time the two stay in balance and the volume sits in a normal range. When they fall out of balance — either too little amniotic fluid is being made or too much is being reabsorbed, the question becomes why.

After about the sixteenth week of pregnancy, the dominant source of amniotic fluid is the baby’s urine. The baby drinks the amniotic fluid. The fluid passes through the gastrointestinal tract, is absorbed into the the baby’s circulation system, filtered through the kidneys, and returns to the uterus as urine. This cycle is like a river. When something interrupts the cycle such as a problem with making urine or a problem with swallowing the amniotic fluid, the volume changes.

Low amniotic fluid (oligohydramnios), occurs in approximately 5 percent of pregnancies. I did not see much low amniotic fluid in my practice. Low amniotic fluid is probably more common in mothers carrying twins or multiple babies because one of the babies might get more nutrition than the others.

Several maternal factors can reduce amniotic fluid volume. Dehydration is the simplest: a mother who is not drinking enough water makes prevents the development of amniotic fluid. Cigarette smoking, recreational drugs, and inadequate weight gain are all associated with reduced amniotic fluid, and all are correctable. Certain medications reduce the fluid, most notably certain blood pressure medications such as ACE inhibitors and NSAIDs used to reduce pain. If the baby cannot make urine, or cannot make enough of it, the amniotic fluid drops.

The symptoms of low amniotic fluid are subtle. The most common is a baby that measures small for its gestational age. Reduced fetal movement may be noticed, but normal movement varies so much from one baby to the next — and from one pregnancy to the next in the same mother — that movement alone is an unreliable signal. The mother herself may notice that her belly seems smaller than expected for her due date.

If there are concerns about low amniotic fluid, I would recommend getting an ultrasound exam of your baby and your uterus to measure the amniotic fluid and check the anatomy of your baby. The best way to treat low amniotic fluid volume would be to look for a cause and to treat it. The amniotic fluid is doing what the cycle of production and reabsorption is designed to do. When that cycle is working normally, the volume sits in a normal range. When the volume has drifted out of range, something has changed in the cycle, and the clinical work is to identify what is causing the change in the amount of amniotic fluid. Sometimes the answer is reassuring, something as simple as drinking more water. Sometimes the answer is serious. Either way, the volume of amniotic fluid itself is not the problem, but rather a clue that points toward the answer to the problem.