Oligohydramnios means that, relative to gestational age (meaning how far along the pregnancy is), the amniotic fluid surrounding the fetus (baby) is at low levels. Amniotic fluid is the water that surrounds the fetus in the uterus. At the start, it contains mostly water with electrolytes. However, with the progression of pregnancy, more molecules (including proteins, carbohydrates, lipids and urea) are contained in it.
Amniotic fluid has a large number of functions whose purpose is the protection and development of the fetus.
During the first trimester of pregnancy, the main component of amniotic fluid is fluid supplied by the mother via the placenta (maternal plasma, nutrients and growth factors). In the latter half of pregnancy the baby is the main producer of amniotic fluid, with fetal urine and fluid excreted from its lungs also contributing to the fluid.
As the baby develops, it produces more urine, with the amount of amniotic fluid reaching a peak at about 32-34 weeks of gestation. After 36 weeks of gestation, i.e. near term, the volume declines naturally.
Via ultrasound, physicians are able to have a reliable estimati on of amniotic fluid volume by calculating the amniotic fluid index, or AFI, normally using a four-quadrant technique. This means that the uterus is divided into four imaginary quarters and the measurements taken (vertical length of each fluid pocket) are summed up to a score. This score denotes the amniotic fluid index. Generally speaking, an AFI of less than 5-6 cm it is considered below normal.
There is another easily performed and reliable method for amniotic fluid assessment: this is measurement of the deepest vertical pocket (DVP) that one can find in any of the four quadrants, not counting the fetal parts or umbilical cord, measured in centimetres. The normal range for DVP is 2cm-8 cm (for multiple pregnancies it is around the same), with values below 2 cm indicating probable oligohydramnios.
Amniotic fluid does not need to be measured routinely at any scan as a DVP < 2cm or AFI < 5-6 cm is easily recognisable. Therefore, amniotic fluid has to be measured only if subjectively diminished or for obstetric indication. The best time to carry out amniotic fluid calculations is during the second trimester, between 18 and 22 weeks (fetal anatomy scan), and then during the third trimester (fetal growth scan).
Oligihydramnios can be associated to fetal anomalies (mainly from the urine tract) or fetal growth restriction. So, once olygohydramnios is detected, anomalies should be discarded by a detailed scan and the fetal growth assessed. Sonographic measurement of fetal size and estimations of fetal weight gain that are performed together with Doppler scans (fetal circulation assessment) may suggest the presence of oligohydramnios occurring as a manifestation of fetal growth restriction. Amniotic fluid is included in the biophysical profile which is a tool that takes together parameters as fetal movements to evaluate fetal wellbeing.
The most common cause of oligohydramnios is rupture of membranes, but renal dysfunction or urinary tract blockage can also lead to oligohydramnios at any moment. Oligohydramnios is in addition an early indicator of placental dysfunction, which means that the placenta hasn’t developed properly or is damaged (hypoperfusion). Very rarely (less than 1% of cases), the cause may be idiopathic (of unknown cause).
Because the amniotic fluid is a baby’s life support system, oligohydramnios is a serious development since all the vital functions and protective actions of amniotic fluid are reduced . There are several complications resulting from oligohydramnios that vary according to its cause and severity and the time of its presentation. The earlier in pregnancy that oligohydramnios occurs, the worse is the prognosis derived from the cause which is explaining the oligohydramnios. Oligohydramnios, when isolated in the third trimester, usually has a good prognosis. Some of the risks associated with oligohydramnios are infectious complications (rupture of membranes), preterm birth (rupture of membranes, fetal growth restriction), malpresentation because of the difficult to move).
No effective treatment for oligohydramnios is currently available. If it concerns a mild case of oligohydramnios in an otherwise healthy pregnancy near term, no intervention is needed. Particularly in the case of ruptured membranes, management will involve maternal monitoring for signs of infection, antibiotics eventually, sonographic monitoring of the baby and in a few cases hospital admission or steroids. In the event of lower urinary tract obstruction, Fetal surgery should be considered.
Oligohydramnios may be an indicator of fetal growth restriction. That is, there should be examination of the rate of fetal growth, fetal anatomy (in particular the kidneys and urinary tract) and fetal circulation (Doppler scans).
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Last updated September 2019