Amniotic fluid is the liquid that surrounds the baby within the amniotic sac, which sits inside the uterus. The amount of fluid can be measured using ultrasound.

Amniotic fluid is important for a healthy pregnancy. It helps the developing lungs of the fetus expand and grow normally. This fluid also reflects the general health of the pregnancy.

Here's a fact that may surprise you. The amniotic fluid in the second and third trimesters is basically a baby's urine. Healthy, growing fetuses urinate inside the womb to create the amniotic fluid. They swallow the fluid so it can be circulated back through the placenta to the mother. Without this process, the fluid might collect and become excessive. This can occur if, for example, swallowing is obstructed or impaired. So, a normal volume of amniotic fluid shows a healthy fetal urinary system and healthy placenta.

Problems with the kidneys or bladder of the fetus can cause decreased urine production or a blocked bladder that decreases urination. This can lead to low levels of amniotic fluid. Such problems are often detected early in pregnancy, perhaps during the second trimester ultrasound. Decreased levels of fluid found early can mean a serious problem, for example, a fetus missing both kidneys.

Later in pregnancy, decreased amniotic fluid may mean that the placenta is not working properly. This can happen when a pregnancy continues further and further past the due date. The theory is that as placental function decreases, less fluid from the mother can be passed to the baby. This leads to decreased circulation to the fetal kidneys and decreased fetal urination.

Low levels of amniotic fluid may be particularly concerning late in pregnancy if they are associated with a baby that is small. Ruptured membranes ("breaking the water") can also be a cause of decreased fluid levels anytime during pregnancy, but particularly as the due date approaches.

Deciding what to about low levels of fluid depends on a number of factors:

• The volume of fluid (none, present but decreased, lower than average but still normal)
• The suspected reason for the decrease
• A mother's underlying condition
• The gestational age at which the decrease is detected

Sometimes it is best to watch, wait and re-evaluate. But if the volume of fluid is very decreased, or the result of ruptured membranes or detected near the due date, doctors may recommend delivery. In most cases, delivery can be accomplished by inducing labor. Low fluid volumes alone only rarely require a cesarean delivery.