The amniotic sac holds the baby throughout the pregnancy. The amniotic fluid is just just a few milliliters to begin with but by week 36 increases to around 800 to 1000 ml. From week 38 weeks it gradually begins to reduce in preparation for delivery. With the onset of labor the sac breaks open to facilitate delivery, leaking fluid little by little or in a gush. This is called the breaking of the waters.
It is important that an adequate amount of fluid be present in the sac. Too little amniotic fluid surrounding the baby is called oligohydramnios, and too much fluid is called polyhydramnios.
If the abdomen of the mother or the size of the baby appears smaller than expected (small for dates) then oligohydramnios may be suspected. It may also be checked for if the mother has previously given birth to a low birth weight baby or a baby who was small for dates, or if the mother has lupus or high blood pressure.
A regular ultrasound will check fluid levels by looking at the whole amniotic sac. The depths of the largest pockets of amniotic fluid in four sections of the uterus will be checked and the four measurements added together. That will give the amniotic fluid index (AFI). In general, an AFI of 5cm (2in) or less, or a deepest pocket measurement of less than 2cm (0.8in), is considered to be low.
The most common cause of low amniotic fluid is a premature rupture of the membranes, causing the fluid to drain out. Other causes are delayed onset of labour, being more than 42 weeks pregnant, placental insufficiency, certain medications such as ACE inhibitors and non-steroidal anti-inflammatories, such as ibuprofen, presence of pre eclampsia or diabetes in the mother, if the baby has an inherited medical problem with kidney development, or if one of identical twins is not growing as well as he should.
Doctors will carefully monitor fluid levels and the baby's growth. To keep the mother hydrated, extra fluids may be supplied through a vein. Antibiotics may be prescribed if labour does not start. Labour may be induced artificially or the pregnancy be monitored till labour starts naturally.
Low amniotic fluid can cause labour complications. The baby may not have enough room to turn around into proper position. Premature labour may start. Once labour starts the baby may go into distress or the umbilical cord could get compressed and a Caesarian may need to be carried out.
On the other hand too much fluid may be suspected if the girth of the abdomen is bigger than it should be. The symptoms may be abdominal discomfort, increased back pain, shortness of breath in the mother and extreme swelling in the feet and ankles. Ultrasound to measure the amount of fluid will reveal an AFI total of more than 25 cm.
Causes of polyhydramnios are diabetes in the mother, carrying twins especially when one baby gets too much fluid while the other gets too little, if the baby has genetic problems like Down's syndrome, defects such as cleft lip or palate, pyloric stenosis, and others, severe anaemia and neurological problems. A high-resolution ultrasound will check for abnormalities and amniocentesis may be carried out to test for a genetic defect and infections.
The patient will be carefully monitored for signs of premature labour, and during labor as well. The umbilical cord could prolapse or a placental abruption occur when the water breaks. Either event would require an immediate c-section. Postpartum bleeding is more likely as well, because the overly distended uterus may not be able to contract well, so the mother will be watched closely after the delivery too.