Amniotic fluid typically peaks between 32 and 34 weeks of pregnancy, then gradually declines as you approach your due date. This is a normal part of late pregnancy, driven mainly by changes in how your baby’s kidneys concentrate urine. By 39 weeks, average fluid levels are about 22% lower than they were at 34 weeks.
The Normal Decline Week by Week
Ultrasound measurements show a steady downward trend in fluid volume from 34 weeks onward. Providers measure amniotic fluid using an amniotic fluid index (AFI), reported in centimeters. Here’s what the averages look like in healthy, low-risk pregnancies:
- 34 weeks: AFI around 14.6 cm
- 35 weeks: AFI around 14.3 cm
- 36 weeks: AFI around 13.2 cm
- 37 weeks: AFI around 12.5 cm
- 38 weeks: AFI around 12.2 cm
- 39 weeks: AFI around 11.4 cm
Week-to-week changes are small enough that they may not show up as significant on a single ultrasound. But over a two-week window, the drop becomes noticeable and measurable. This is why providers sometimes space fluid checks about two weeks apart in the late third trimester, unless there’s a specific concern.
Why Fluid Decreases Near the End
Your baby’s kidneys mature throughout the third trimester. As they get better at concentrating urine, they produce less of it in volume, which means less fluid flowing into the amniotic sac. At the same time, your baby is swallowing more amniotic fluid and growing larger, taking up more space inside the uterus. The combination of reduced production and increased reabsorption accounts for most of the natural decline.
Placental changes may also play a role. As pregnancy extends past the due date, the placenta gradually becomes less efficient. In post-term pregnancies (beyond 42 weeks), fluid can drop rapidly, sometimes falling dramatically within just 24 to 48 hours. This is one reason providers monitor fluid levels closely if you go past your due date, often with twice-weekly checks.
When Low Fluid Becomes a Concern
A normal AFI falls between 5 and 24 cm. When fluid drops to an AFI of 5 cm or below, or when the single deepest pocket of fluid measures less than 2 cm, your provider will diagnose oligohydramnios, the clinical term for low amniotic fluid. This can happen at any stage of pregnancy, but it’s most common in the third trimester.
Several conditions can push fluid lower than the expected natural decline:
- Fetal growth restriction: When the placenta isn’t delivering enough oxygen, blood flow gets redirected to the baby’s brain and heart, away from the kidneys. Less blood reaching the kidneys means less urine and less fluid.
- Ruptured membranes: A tear in the amniotic sac causes fluid to leak out. Early membrane rupture in preterm pregnancies is linked to serious breathing problems and higher mortality for newborns.
- Going past your due date: Post-term pregnancies carry an elevated risk of rapid fluid loss.
- Preeclampsia and high blood pressure: These conditions can impair placental function, indirectly reducing fluid production.
- Congenital kidney or urinary tract problems: If the baby’s urinary system isn’t developing normally, urine output may be reduced from earlier in pregnancy.
In some cases, fluid is low without any identifiable cause. Research suggests this “isolated oligohydramnios” may involve changes in how the amniotic membranes themselves absorb fluid, rather than a problem with the baby’s kidneys. Studies have found no significant difference in hourly urine production between babies with isolated low fluid and those with normal levels.
Signs You Might Notice
Low amniotic fluid often produces no obvious symptoms, which is why it’s usually caught on routine ultrasound. But there are a few things that might prompt your provider to check:
- Your belly measures smaller than expected for your stage of pregnancy
- You feel less fetal movement than usual
- You notice fluid leaking from your vagina, which could signal ruptured membranes
If you’re unsure whether you’re leaking amniotic fluid or experiencing normal vaginal discharge, it’s worth getting checked. Amniotic fluid tends to be clear and odorless, and it doesn’t stop the way a bladder leak would.
What Happens if Fluid Is Low
Management depends on how far along you are and what’s causing the decrease. If you’re near term and fluid is borderline low, your provider may recommend more frequent monitoring or discuss the timing of delivery.
One straightforward intervention is hydration. A meta-analysis found that drinking extra water (around 1,500 mL or more per day) can meaningfully improve fluid levels in cases of isolated oligohydramnios. Oral hydration works better than IV fluids for this purpose, and plain water outperforms electrolyte-heavy sports drinks. The most effective approach in studies combined a short initial course of IV fluids with at least two weeks of increased oral water intake. This won’t fix an underlying structural problem, but for cases where no clear cause is found, staying well-hydrated is a simple and effective first step.
For pregnancies that go past the due date, twice-weekly fluid monitoring is standard practice. Because amniotic fluid can drop sharply in a short window at this stage, providers combine fluid measurement with fetal heart rate monitoring to get a fuller picture of how the baby is doing. If fluid drops too low or other signs of concern appear, induction or early delivery may be recommended to reduce the risk of complications like umbilical cord compression, which becomes more likely when there’s less fluid cushioning the cord.