Oligohydramnios, which has an occurrence rate of about 4%, is a condition where there are low levels of amniotic fluid surrounding the fetus. There are a variety of reasons why oligohydramnios can happen including chromosomal abnormalities, placental problems, leaking membranes, maternal issues like preeclampsia, medications, intrauterine growth retardation, infections, postdates (the pregnancy is about 42 weeks or older), and fetal renal agenesis (the fetus does not have kidneys). Oligohydramnios obviously has negative implications for the fetus.
The diagnosis of oligohydramnios is made on ultrasound when the largest pocket of fluid measures 2 cm or less, the amniotic fluid index (AFI) measures 5 cm or less, or there is a fluid volume of less than 500 cc at 32-36 weeks gestation. After 20 weeks gestation, the uterine fundus grows approximately a centimeter in height every week. Oligohydramnios is suspect if the uterus is not growing at the anticipated rate. This is one of the many reasons why it is important to get prenatal care by a qualified provider.
The fetal lungs develop by having the fetus breathe the amniotic fluid in the second trimester. Breathing the amniotic fluid is not possible in oligohydramnios since the fluid is all but absent. Therefore, the fetus will develop pulmonary hypoplasia (the lungs don’t develop). In addition, there is the catastrophic possibility of umbilical cord compression where the fetus lays on the cord cutting off the blood supply. There can also be developmental abnormalities like a club foot or limb deformities because of the restricted growth space.
Oligohydramnios also carries the risks of preterm labor, meconium stained fluid, and increased rate of cesarean section. There is also a high likelihood of miscarriage if the oligohydramnios occurs early in the pregnancy.
Treatment for this condition depends on the underlying cause. For example, if the oligohydramnios is due to medications, then the medications should be stopped. On the other hand, if the oligohydramnios is due to a fetal abnormality like absent kidneys, then there is little that can be done.
Maternal hydration, either oral or intravenous, appears to help with the oligohydramnios sometimes. Unfortunately, the oligohydramnios appears to return within the week after hydration unless the oligohydramnios was due to maternal dehydration. Amnioinfusion, where a catheter is placed into the uterus via the dilated cervix and fluid is administered to create more volume, is a possibility if the fetus is being delivered. Amnioinfusion is not an option if the cervix is not dilated and the infant is not being delivered.
Your provider is interested in maintaining the pregnancy as long as possible in most cases. With oligohydramnios, however, your provider may elect to deliver the fetus earlier due to the potential complications of oligohydramnios. When the delivery occurs will depend on the underlying condition for the oligohydramnios, the fetal gestational age, and the results of the fetal well being tests.
I have discussed the use of the nonstress test (NST) on previous posts. This is a noninvasive test where a belt is placed around the maternal abdomen and an attached transducer is placed over the location of the fetal heart. The fetal heart pattern will then be traced on a long sheet of paper located on the fetal monitor. With this test, your provider will be able to tell if your baby is doing well in the uterus or not. A nonreactive NST may tell your provider that it is time to deliver the baby as the baby will do better outside the uterus at that point. Your provider may also place you on continuous fetal monitoring where you have the abdominal belt around the abdomen constantly.
Your provider will most likely order serial ultrasounds as well to keep track of the amniotic fluid volume.
Treatment with betamethasone to accelerate the fetal lung is also an option, but bear in mind that pulmonary hypoplasia will not respond to the steroid.
Wishing you a problem free delivery, Pablo.