Amniotic fluid stands for an essential substance during pregnancy. This fluid surrounds the fetus during its development and is generated by the pregnant woman throughout the gestation period, which reaches its highest peak when the woman is 34 weeks pregnant.
During pregnancy, women must keep just the right amount of amniotic fluid. While low values cause oligohydramnios, an excess of amniotic fluid generates polyhydramnios (also known as hydramnios), which may lead to further pregnancy problems.
Amniotic fluid volume assessment
In pregnancy checkups, the obstetrician will measure the volume of your amniotic fluid in addition to monitoring fetal development. Amniotic fluid levels can be measured in two ways:
- Deepest vertical pocket (DVP): It measures the maximum vertical pocket of amniotic fluid free of cord and fetal parts. Its normal values range from 2 to 8 cm.
- Amniotic fluid index (AFI): It follows the same patterns as DVP, but in this one the pregnant abdomen is divided into four quadrants. Normal values for AFI range from 5 to 25 cm. It is carried out from the 24th week of pregnancy onwards.
The DVP method is the one used routinely; however, in case some anomaly is detected, then it will be analyzed through AFI. Polyhydramnios is found in approximately 2% of pregnancies, and according to its severity, we can distinguish between:
- Mild polyhydramnios: amniotic fluid index of 25-29.
- Moderate polyhydramnios: amniotic fluid index of 30-34.
- Severe polyhydramnios: amniotic fluid equal to or higher than 35.
Unfortunately, finding out what is causing polyhydramnios is not always possible; in fact, in half of the cases the origin of this alteration remains unknown. In general terms, it may be due to three potential causes which have been classified as follows: fetal, maternal, and placental.
If the origin is related to the fetus, it may be due to altered fetal growth. Nonetheless, gastrointestinal disorders may be the cause behind it as well, although some other causes such as central nervous system alterations, renal dysfunctions, infections, neuromuscular alterations, and some syndromes associated with genetic abnormalities could have an influence too.
When the cause of polyhydramnios is of maternal origin, it is usually connected with gestational diabetes. Any metabolic disorder in the pregnant woman may cause this excess of amniotic fluid.
On the other hand, placental alterations, such as placental chorioangiomas—benign vascular tumors of placental origin—, may be linked to the appearance of polyhydramnios.
Depending on how severe the case of polyhydramnios is and its etiology, the obstetrician will perform the appropriate follow-ups.
In many cases, no specific way of treating fetal glomerular filtration is found, especially if the origin is unknown.
Most severe cases can be addressed by means of amniodrainage to remove the substantial amniotic fluid, although the likelihood of having a preterm birth is high. This type of drainage cannot be performed from week 35 onwards and is only indicated in cases of moderate to severe polyhydramnios, with an AFI value higher than 30.
From week 32, prostaglandin inhibitors can also become effective for the treatment of polyhydramnios. These drugs reduce the fetal glomerular filtration rate by reducing the amount of fetal urine. They also serve as a way of increasing the reabsorption of fetal lung liquid and the passage of amniotic fluid through the membranes.
Since this treatment can cause severe side effects in the fetus, it is only indicated in very special cases.
With polyhydramnios, there is a high probability of having a preterm birth. Thus, depending on the severity of each case and the stage of fetal development, inducing labor may become a solution.