What Is the Normal Amount of Amniotic Fluid During Pregnancy?

Amniotic fluid plays a critically important role throughout pregnancy. Its primary functions include: (1) protecting the fetus from external mechanical impacts; (2) participating in fetal metabolism; (3) safeguarding the maternal body from injury; and (4) reducing discomfort caused by fetal movement. Normal amniotic fluid volume is an essential prerequisite for healthy fetal growth and development. Both polyhydramnios (excess amniotic fluid) and oligohydramnios (insufficient amniotic fluid) can severely impair fetal development. Whether amniotic fluid volume is normal can be assessed via ultrasound examination. Throughout pregnancy, amniotic fluid volume naturally fluctuates.

The normal range of amniotic fluid volume during pregnancy is 300–2000 mL. During routine prenatal visits, B-mode ultrasound is used to evaluate amniotic fluid volume. In early and mid-pregnancy, the maximum vertical pocket (MVP), also known as the amniotic fluid vertical depth (AFV), is typically measured. At this stage, the normal AFV range is 2–8 cm. In late pregnancy, the amniotic fluid index (AFI) is generally assessed. Briefly, the AFI is calculated by dividing the uterine cavity into four quadrants—upper, lower, left, and right—with the fetus centered, measuring the maximum vertical depth of amniotic fluid in each quadrant, and summing these four measurements.

This sum constitutes the AFI. The normal AFI range is 8–18 cm. An AFI of 5–8 cm is considered borderline low (mild oligohydramnios), while an AFI < 5 cm is diagnosed as oligohydramnios. An AFI of 18–25 cm is classified as borderline high (mild polyhydramnios), whereas an AFI > 25 cm indicates polyhydramnios. If the AFI falls outside the normal range—either too high or too low—pregnant women are advised to seek prompt medical evaluation at a hospital and may require genetic counseling.