What is Oligohydramnios?
Oligohydramnios means having less amniotic fluid than usual for the pregnancy stage. Amniotic fluid is the fluid that surrounds and protects a baby in the womb. Its volume increases steadily until around 34 to 36 weeks into pregnancy, where it remains constant (about 400 ml) until the due date. From 40 weeks onwards, the volume of amniotic fluid tends to decrease, leading to less fluid in pregnancies that go past the due date. We can track the volume of this fluid throughout pregnancy using techniques like measuring the height of the uterus (fundal height) and utilizing ultrasound imaging.
When there’s a mismatch between the measured size of the uterus and the stage of pregnancy, it could indicate a disorder with the amniotic fluid. In such cases, an ultrasound can be used to assess the volume of the amniotic fluid.
The ultrasound scan, done across the stomach, measures either the maximum vertical pocket (MVP) or the Amniotic Fluid Index (AFI) to estimate the volume of the amniotic fluid. The MVP is the deepest pocket of amniotic fluid that doesn’t include the baby’s umbilical cord or any body part. Generally, normal MVP ranges from 2 to 8 cm. If it’s less than 2 cm, it suggests oligohydramnios. The AFI, another way to calculate the amount of fluid, is done by dividing the uterus into four sections through the belly button, measuring the MVP in each section, and adding them up. An AFI of less than 5 cm is associated with oligohydramnios.
However, it’s crucial to remember that using the MVP can easily overestimate cases of polyhydramnios (too much amniotic fluid), while the AFI can underestimate cases of oligohydramnios. Because of this, some medical facilities may prefer to use the MVP for pregnancies with low fluid levels and the AFI for pregnancies with high fluid levels. The MVP is especially useful for tracking low fluid levels in pregnancies with multiple babies since we can’t measure all four sections for each baby.
What Causes Oligohydramnios?
Amniotic fluid, which is the liquid within the sac that surrounds the growing baby in the womb, is all about balance. Its levels depend on the balance between how much fluid is being made and how much is leaving the sac. In the first 20 weeks of pregnancy, the primary sources of amniotic fluid are lung secretions and fluid transport from the mother’s blood through the baby’s membranes.
Around the 16th week of pregnancy, the baby’s kidneys start working. They gradually make more and more urine, which contributes to the majority of the amniotic fluid for the remainder of the pregnancy.
If something isn’t working properly with the baby’s urinary system like bladder obstruction, abnormal kidneys, or missing kidneys, it can lead to having too little amniotic fluid – a situation known as oligohydramnios – especially after the 16th to 20th weeks of pregnancy.
Most of the amniotic fluid is absorbed back into the fetus through swallowing and via absorption directly across the sac into the baby’s blood vessels. If there are problems with the baby’s gastrointestinal system, such as a connection between the food pipe and windpipe (a condition called tracheoesophageal fistula seen in about 1 in 3500 live births), it could lead to having too much fluid, a situation referred to as polyhydramnios.
Risk Factors and Frequency for Oligohydramnios
Oligohydramnios, a condition where there is not enough amniotic fluid around the baby during pregnancy, happens in about 4.4% of full-term pregnancies. However, it’s less common in preterm pregnancies, occurring in fewer than 1% of cases.
Signs and Symptoms of Oligohydramnios
If an ultrasound has identified low levels of amniotic fluid (oligohydramnios), the doctor will then need to figure out what’s causing it. They will look into the following areas:
- History: The doctor will ask about the mother’s medical and family history. This can help identify any conditions or medications that might be linked to low amniotic fluid. They’ll also ask about any loss of fluid from your womb which could indicate that your water has broken early.
- Physical Exam: The doctor will conduct a targeted physical examination. They’ll measure the size of your womb, conduct a speculum examination (an examination of the vagina), and perform tests to check whether your membranes have ruptured.
Testing for Oligohydramnios
Checking for oligohydramnios, or low amniotic fluid around the baby in the womb, involves two main steps:
Firstly, an ultrasound is used which can look at specific parts of the baby’s urinary and genital systems for any abnormalities, check for slow fetal growth and markers of aneuploidy, which refers to abnormal numbers of chromosomes in the baby’s cells. The ultrasound also uses an umbilical artery Doppler to check if the placenta is not functioning correctly, as this could be the reason for low amniotic fluid.
Secondly, nonstress tests are done. These tests are carried out at least once a week until the baby is born. They help observe how the baby’s heart rate responds to its movements, indicating if the baby is getting enough oxygen. If the heart rate doesn’t increase with movements, it might suggest that the baby is not doing well.
Treatment Options for Oligohydramnios
To tackle the root cause and monitor the health of the baby, your doctor would conduct regular tests while you’re pregnant. These include measuring your amniotic fluid and conducting nonstress tests, which are used to check the baby’s heartbeat. These tests have been shown to help reduce the chance of sudden unexplained death of a baby in the womb. Your doctor will also frequently check on your baby’s growth.
Ensuring that you, the mother, are properly hydrated is especially important. This is key especially in cases where there is a low level of amniotic fluid (the protective fluid surrounding your baby in your womb) in the last three months of pregnancy. It’s a condition known as isolated oligohydramnios.
While there is some disagreement about when to deliver the baby in cases of isolated oligohydramnios, current guidelines suggest childbirth should take place at 37 weeks of pregnancy, assuming your water hasn’t broken yet.
What else can Oligohydramnios be?
Oligohydramnios is a pregnancy-related condition where there is less amniotic fluid than normal. This can be related to many different factors and complications during pregnancy. The potential reasons for oligohydramnios can be categorized into maternal, fetal, placental, and unexplained causes.
Here are some potential causes:
- Maternal causes: These are factors relating to the mother’s health or behavior. They can include a range of conditions that disrupt the proper functioning of the uterus, such as chronic high blood pressure, vascular disease, blood clotting disorders, and preeclampsia. These issues can potentially affect around 8% of all pregnancies. Certain medications and substances, such as angiotensin-converting enzyme inhibitors, nonsteroidal anti-inflammatory drugs (NSAIDs), and cocaine, as well as maternal diabetes, can also lead to oligohydramnios.
- Fetal causes: These are factors that are tied to the baby’s development or the state of the pregnancy. Ruptured membranes before delivery, commonly seen in preterm premature rupture of membranes (PPROM), is the most common cause. It accounts for over 37% of oligohydramnios cases identified in the middle and late stages of pregnancy. Other fetal factors include abnormalities in the genitourinary tract, late-term pregnancies, slow fetal growth, chromosomal abnormalities, and, in rare cases, fetal death.
- Placental causes: These are issues with the placenta that could lead to oligohydramnios. Conditions such as placental abruption and twin-to-twin transfusion syndrome can be responsible.
- Idiopathic/Unexplained causes: In over half of the cases found in the third trimester, there is no clear cause and these cases are typically associated with better outcomes.
This break down helps both doctors and patients understand the possible origins of oligohydramnios and guides the diagnostic and treatment process.
What to expect with Oligohydramnios
The treatment and outcome of a condition called oligohydramnios, which is characterized by a lower-than-normal amount of amniotic fluid, can greatly differ based on its root cause, the stage of pregnancy when it is diagnosed, and its severity. If oligohydramnios is diagnosed during the second trimester of pregnancy, it is more likely to be linked with abnormalities in the baby or the mother. However, oligohydramnios diagnosed in the third trimester often has no identifiable cause.
In one research, the cause of oligohydramnios was unknown in just 4% of mothers diagnosed in the second trimester, while in over half of the mothers diagnosed in the third trimester, the cause was not identified. The study also showed that the survival rate of babies was lower (10.2%) when oligohydramnios was diagnosed in the second trimester as compared to an 85.3% survival rate when diagnosed in the third trimester.
For the cases of oligohydramnios detected during the second trimester, there’s an increased risk of a condition called pulmonary hypoplasia (underdeveloped lungs) in the baby, which is a key fact that could influence the baby’s chances of survival. The death rate for babies diagnosed with oligohydramnios during the second trimester can go up to 90%, and in nearly 87% of these cases, the prime reason was pulmonary hypoplasia. The condition tends to be more severe if the diagnosis of oligohydramnios occurs between the 16th to 24th weeks of the pregnancy, a crucial period for lung development. Low amniotic fluid volume during the second and beginning of the third trimester also increases the risk of limb stiffness and birth defects due to pressure on the developing fetus.
Possible Complications When Diagnosed with Oligohydramnios
Pregnancies complicated by oligohydramnios, a condition where there is a lower amount of amniotic fluid, can have certain complications during childbirth. These complications can include an increased possibility of the umbilical cord being squeezed, the baby inhaling its first stool (meconium) potentially leading to breathing problems, needing a cesarean delivery, abnormalities in the baby’s heart rate, and unresponsive fetal heart monitoring results.
On a positive note, giving 1 to 2 liters of oral or intravenous fluids to the mother during labor has been found to temporarily increase the amount of amniotic fluid and reduce the chances of umbilical cord compression.
Complications you should know about include:
- Increased chance of umbilical cord compression
- Possible meconium aspiration by the baby
- Higher likelihood of cesarean delivery
- Abnormalities in fetal heart rate
- Non-reactive fetal heart rate monitoring results
Preventing Oligohydramnios
When discussing oligohydramnios, it’s crucial that we stress the importance of regular prenatal care throughout your pregnancy. Oligohydramnios is a condition where there is less amniotic fluid around the baby in the womb than normal. Regular prenatal visits allow the healthcare team to spot and diagnose conditions like oligohydramnios during the course of your pregnancy. Once identified, the medical team will formulate a suitable follow-up plan. This is done to keep the risk of complications that could affect both the mother and the baby due to oligohydramnios to a minimum.
