Polyhydramnious or hydramnious is as an abnormally large volume of amniotic fluid. There is a range of ‘normal’ fluid volumes and an abnormally large volume may raise suspicion of a problem with the pregnancy. Greater deviations from the norm are more strongly associated with abnormality. The definition of “too much” is generally considered to be more than 2 liters; the average amount is about 1 liter, see “Assessment of amniotic fluid volume”.). Most cases of polyhydramnios are mild and involve less than 3 liters of amniotic fluid. So, in many cases, a diagnosis of polyhydramnios means that you’re on the high side of normal for amount of amniotic fluid and presents only minor secondary concerns.
Polyhydramnios occurs in about 1 pregnancy out of 100; 95% of those are considered mild to moderate. The symptoms of hydramnios can include rapid growth of the uterus, discomfort in the abdomen, and possibly uterine contractions, but more often than not, there are no symptoms at all.
Physiologically, the volume of fluid increases with gestation to a maximum of 800-1,000 ml at 36-37 weeks. It has a number of purposes, including protecting the fetus from trauma and infection, allowing lung development and facilitating the development and movement of the limb and other skeletal parts. Fetal swallowing causes a reduction in the volume of fluid and absence of swallowing or a blockage of the fetal gastrointestinal tract may lead to polyhydramnios. Polyhydramnios is therefore linked to fetal abnormality.
Most women diagnosed with the condition deliver healthy babies. Most of the time, a little extra amniotic fluid is nothing to be concerned about. Such extra fluid is likely to be reabsorbed without any treatment. But when fluid accumulation is severe, it may signal a problem with the baby such as a central nervous system or gastrointestinal defect, kidney or bladder malfunction, or a problem with the baby’s ability to swallow.
What causes polyhydramnios?
The causes of polyhydramnios are not completely understood. In many cases it’s difficult to say what causes polyhydramnios but there are a few circumstances that make the condition more likely:
- Multiple / twin pregnancies – you’re more likely to have abnormal amniotic fluid levels if you’re carrying twins or other multiples. The cause of this is often twin-to-twin transfusion syndrome, where one twin has too little amniotic fluid and the other has too much.
- Gestational diabetes – greatly increases the likelihood of polyhydramnios. Around one in ten pregnant women with diabetes will develop some degree of excess amniotic fluid. If diabetes is uncontrolled or poorly controlled in pregnancy, there is a much higher incidence of polyhydramnios and the excessive amount of amniotic fluid is a direct result of the unstable diabetes.
- Infection – certain infections such as rubella, toxoplasmosis and syphilis may lead to polyhydramnios. These can be checked for with blood tests.
- Fetal abnormalities – in about a fifth of cases, excess amniotic fluid may build up when the baby has difficulties swallowing or digesting the amniotic fluid, preventing the fluid from being recycled. This could be caused by an obstruction in the baby’s throat (such as cleft lip or palate) or gastrointestinal tract, or by a neurological problem. Polyhydramnios is also associated with problems with the baby’s heart, kidneys and with chromosomal abnormalities.
In addition, too much amniotic fluid can put your pregnancy at risk for premature rupture of your membranes, premature labour, placental abruption, breech baby presentation, postpartum haemorrhage or umbilical cord prolapse.
Polyhydramnios increases the risk of postpartum haemorrhage simply because the uterus has been distended more than is usual for a singleton pregnancy.
Polyhydramnios increases the risk of placental abruption because of the mechanical forces at work in separating the placenta from the uterus. Polyhydramnios increases the risk of cord prolapse for several reasons. First, because the baby’s presentation is unpredictable, the baby may be in an unfavorable position when the membranes rupture, and the presenting part may not fit into the pelvis well enough to keep the cord from falling out below. Second, because there is so much fluid, there is more pressure on the movable umbilical cord to move it out past the presenting part. If your waters do break before the start of labour you will be advised to lie down and stay reasonably still before going to hospital to reduce the likelihood of a prolapsed umbilical cord.
Growth restriction (IUGR) resulting in skeletal malformations
Stillbirth occurs in about 4 in 1000 pregnancies that suffer from polyhydramnios vs. about 2 in1000 pregnancies with normal fluid levels.
Signs and symptoms
Women might complain of abdominal girth, shortness of breath, oedema of ankles, tense abdomen. The woman might be restless, abdominal skin might look shiny, difficult to palpate, malpresentation and abnormal lie of the foetus.
The first step is to identify any underlying cause. Mild polyhydramnios can be simply monitored and treated conservatively. Pre-term labour is common due to overdistension of the uterus, and measures should be taken to minimise this complication. This includes regular antenatal checks and inspection of the uterus, and bed rest towards the latter stages. Polyhydramnios during pregnancy does not have a harmful effect on the development of the baby or on the woman after delivery, and there is no evidence to suggest that it will recur in a subsequent pregnancy. Bedrest is needed.
Mbilu, J. N. K. (2002). Essentials of Obstetrics and Gynaecology for Clinical Officers and Midwives. Volume 1. Writers Club Press. Lincoln. NE.
Beloosesky, R., Ross, M. G. (2010). Polyhydramnios. UpToDate.
Yeast J. (2006). Polyhydramnios: etiology, diagnosis and management. Neoreviews. 7: 6 e300