Low amniotic fluid, called oligohydramnios, happens when the fluid surrounding a baby in the womb drops below normal levels. It’s diagnosed when an ultrasound shows a deepest fluid pocket of less than 2 centimeters, or when the amniotic fluid index (a measurement combining four areas of the uterus) falls below 5 centimeters. The causes range from problems with the baby’s kidneys to issues with the placenta, certain medications, or a leak in the membranes. Understanding the specific cause matters because it determines both the severity and what happens next.
How Amniotic Fluid Works
Amniotic fluid isn’t static. It’s constantly being produced and recycled in a loop between the baby and the surrounding membranes. By the end of pregnancy, a baby’s kidneys produce roughly 500 to 700 milliliters of urine per day, and that urine is the primary source of amniotic fluid. At the same time, the baby swallows about 200 to 450 milliliters of fluid daily, removing roughly half of what the kidneys put out. The placenta and the membranes lining the uterus also move water back and forth.
Anything that disrupts this cycle, whether the baby produces less urine, the membranes absorb too much water, or fluid physically leaks out, can tip the balance toward low levels. That’s why the causes of oligohydramnios fall into a few distinct categories.
Fetal Kidney and Urinary Problems
Because fetal urine is the main ingredient in amniotic fluid during the second and third trimesters, any condition that affects the baby’s kidneys or urinary tract can cause fluid levels to drop significantly. If the kidneys are underdeveloped or missing entirely (a condition called renal agenesis), the baby simply cannot produce urine, and fluid levels fall dramatically. Polycystic kidney disease, where cysts replace functional kidney tissue, has the same effect.
Blockages in the urinary tract are another common fetal cause. If urine is being made but can’t exit the kidneys or bladder, it never reaches the amniotic space. These blockages can occur at different points along the urinary system. In some cases, the obstruction is partial and fluid levels drop gradually. In others, the blockage is complete and the drop is severe. Kidney and urinary tract problems tend to cause the most pronounced cases of low fluid, particularly when they develop early in pregnancy.
Placental Insufficiency
The placenta plays a less obvious but important role in regulating amniotic fluid. When it doesn’t deliver enough blood and oxygen to the baby, a condition called placental insufficiency, fluid levels often decline. The mechanism is more complex than it might seem. Research in models of severe placental insufficiency (where fetal oxygen was cut by 50%) found that amniotic fluid dropped even though the baby’s urine output stayed the same. The problem wasn’t less fluid going in. Instead, the membranes surrounding the baby appeared to absorb excess water back into the mother’s and baby’s bloodstreams.
Water movement across the placenta and the amniotic membrane is driven by pressure and concentration gradients. When placental blood flow is compromised, those gradients shift, pulling water out of the amniotic space faster than it’s replaced. This is why conditions that damage the placenta, including chronic high blood pressure and preeclampsia, are strongly linked to low fluid levels. The placenta may look normal on imaging while still underperforming in ways that quietly drain fluid volume over weeks.
Ruptured Membranes
A tear or rupture in the amniotic membranes lets fluid leak out physically, and this is one of the more straightforward causes of low fluid. When this happens before 37 weeks, it’s called preterm premature rupture of membranes (PPROM). You might notice a sudden gush of fluid followed by a steady trickle that you can’t control. Some people wake up with wet sheets or feel like they’ve urinated without meaning to. The fluid is typically clear to light yellow.
Not every membrane rupture is dramatic. Small, high tears can produce a slow leak that’s easy to confuse with vaginal discharge or urine. If your provider suspects a leak, they’ll examine the vagina for pooled fluid and may run tests to confirm whether it’s amniotic fluid. Membrane rupture needs prompt evaluation because losing that fluid cushion increases the risk of infection and other complications.
Medications That Reduce Fluid
Certain common pain relievers can cause low amniotic fluid by affecting the baby’s kidneys. The FDA issued a warning that nonsteroidal anti-inflammatory drugs (NSAIDs) used at 20 weeks of pregnancy or later can cause rare but serious kidney problems in the baby, leading to reduced fluid. This includes ibuprofen, naproxen, diclofenac, and celecoxib. The risk was considered significant enough that the FDA moved its warning from 30 weeks back to 20 weeks.
If an NSAID is absolutely necessary between 20 and 30 weeks, current recommendations call for the lowest dose for the shortest time, with ultrasound monitoring of fluid levels if use extends beyond 48 hours. Low-dose aspirin (81 mg) is an exception; it’s still used for certain pregnancy-related conditions and isn’t subject to the same restriction. Blood pressure medications that work by blocking certain hormones (ACE inhibitors and similar drugs) are also known to reduce fetal urine output and are generally avoided in pregnancy for this reason.
Other Contributing Factors
Going past your due date is one of the more common and less alarming reasons for fluid to drop. Amniotic fluid volume peaks around 36 weeks, then gradually declines. Fetal urine production dips slightly after 40 weeks. By 41 or 42 weeks, low fluid is common enough that guidelines recommend ultrasound monitoring of the deepest fluid pocket twice weekly starting at 41 weeks. This is one of the reasons providers often discuss induction between 41 and 42 weeks.
Twin or higher-order pregnancies can also lead to uneven fluid distribution, particularly in identical twins sharing a placenta. In twin-to-twin transfusion syndrome, blood flow between the babies becomes imbalanced, leaving one with too little fluid and the other with too much. Dehydration in the mother can temporarily lower fluid levels as well, though this is usually mild and reversible with increased fluid intake.
What Low Fluid Means for the Baby
The risks depend heavily on when low fluid develops and how severe it is. Amniotic fluid does more than cushion the baby. It gives the lungs space to expand and practice breathing movements. When fluid is very low early in pregnancy, the lungs may not develop fully, a condition called pulmonary hypoplasia. This is one of the most serious complications and is more likely with severe, prolonged cases in the second trimester.
Later in pregnancy, the main concern shifts to the umbilical cord. With less fluid to float in, the cord is more likely to get compressed between the baby and the uterine wall, which can temporarily reduce blood flow. This is why providers monitor babies closely when fluid is low, using tests that check heart rate patterns and fluid levels together. In many third-trimester cases, the baby is healthy and the low fluid is managed by closer surveillance or, if the pregnancy is far enough along, delivery.
How Low Fluid Is Detected
Most people with low amniotic fluid don’t feel any symptoms. It’s typically caught during a routine ultrasound. Your provider might suspect it if your belly is measuring smaller than expected for your gestational age, but this isn’t reliable on its own. The two standard measurements are the amniotic fluid index, where the uterus is divided into four quadrants and the deepest pocket in each is added together (normal is above 5 cm), and the maximum vertical pocket, which measures just the single deepest area of fluid (normal is above 2 cm). The maximum vertical pocket method is generally preferred for diagnosing oligohydramnios because it has fewer false positives.
Once low fluid is confirmed, providers look for the cause. This usually involves a detailed ultrasound to check the baby’s kidneys and urinary tract, a review of medications, testing for membrane rupture, and assessment of placental function. In some cases, no clear cause is found, and the low fluid is managed with increased monitoring and follow-up scans to track whether levels stabilize, worsen, or improve.