Amniotic fluid surrounds and protects your baby throughout pregnancy. The amount of fluid normally changes as pregnancy progresses and can be estimated during an ultrasound.
Sometimes an ultrasound shows less fluid than expected (oligohydramnios) or more fluid than expected (polyhydramnios). These findings can have many causes, and their significance depends on how abnormal the fluid level is, gestational age, fetal growth, and other pregnancy findings.
Amniotic fluid is estimated using ultrasound. Two common methods are:
Amniotic Fluid Index (AFI): The uterus is divided into four sections, and the deepest pocket of fluid in each section is measured. These four measurements are added together to calculate the AFI.
Deepest Vertical Pocket (DVP), sometimes called the Maximum Vertical Pocket (MVP): The single deepest pocket of amniotic fluid is measured.
Your care team may use either method depending on the pregnancy and clinical situation.
Oligohydramnios means there is less amniotic fluid than expected. It is generally diagnosed when the AFI is 5 cm or less or the deepest vertical pocket is less than 2 cm.
Low fluid may develop for several reasons. Sometimes it reflects a problem with the placenta, particularly later in pregnancy. It can also occur if the membranes have ruptured and fluid is leaking, with certain fetal kidney or urinary tract conditions, or with some maternal medical conditions.
Oligohydramnios also becomes more common when a pregnancy continues beyond the due date. As the placenta becomes less efficient later in pregnancy, blood flow to the baby's kidneys may decrease. Because most amniotic fluid later in pregnancy comes from fetal urine, this can lead to decreased urine production and lower amniotic fluid levels.
Sometimes no specific cause is identified.
What happens next depends on how low the fluid is, gestational age, fetal growth, and other pregnancy findings. Additional ultrasounds and fetal monitoring may be recommended.
When oligohydramnios develops near term, delivery may be recommended rather than continuing the pregnancy, particularly when the fluid is significantly decreased or other concerns are present. Low fluid can also increase the chance of umbilical cord compression during labor, which may cause changes in the fetal heart rate and occasionally increase the likelihood of cesarean delivery.
Polyhydramnios means there is more amniotic fluid than expected. It is generally diagnosed when the AFI is 24 cm or greater or the deepest vertical pocket is 8 cm or greater.
Polyhydramnios may be associated with maternal diabetes, fetal conditions that affect swallowing or fluid regulation, certain fetal or genetic conditions, infections, or multiple pregnancy. In many cases, particularly when the increase is mild, no specific cause is identified.
Mild polyhydramnios often requires little more than observation or additional ultrasound monitoring. More significant polyhydramnios may lead to additional evaluation and closer monitoring depending on the underlying cause and gestational age.
When there is a large amount of amniotic fluid, the uterus becomes more stretched than usual. This can sometimes trigger contractions or increase the chance of preterm birth. The extra space can also make it easier for the baby to move into a breech or other non-head-down position.
If the water breaks while there is a large amount of fluid, the sudden release of fluid can rarely allow the umbilical cord to slip down in front of the baby (cord prolapse), which requires urgent delivery. After birth, an overstretched uterus may also have more difficulty contracting firmly, increasing the risk of heavier postpartum bleeding.
The amount of extra fluid, the underlying cause, and other pregnancy findings may affect recommendations for monitoring and delivery.
Usually, amniotic fluid abnormalities are not simply caused by drinking too much or too little water.
Staying well hydrated during pregnancy is important, and maternal hydration can sometimes temporarily increase amniotic fluid measurements, particularly when fluid levels are mildly decreased. However, drinking extra water does not correct most underlying causes of oligohydramnios, such as placental problems or ruptured membranes.
Similarly, drinking less water is not a treatment for polyhydramnios and can lead to dehydration. If your amniotic fluid level is abnormal, your care team will focus on identifying the possible cause and determining whether additional monitoring or changes in pregnancy management are needed.