Premature rupture of membranes

Why do waters break in pregnancy?

During pregnancy the baby develops inside a bag of membranes filled with amniotic fluid. Normally this would burst sometime around the start of labour (or is artificially broken open by midwives to start the labour or get it going). There's usually a sudden gush of fluid followed by an uncontrollable dribble, and most women are in little doubt as to what is going on.

In about ten per cent of pregnancies, waters break at least one hour before contractions of the uterus begin. This is known as prelabour rupture of the membranes (PROM) or (if it occurs before 34 weeks) premature prelabour rupture of the membranes.

The risks of premature rupture of membranes

Possible complications include:

• Infection of the uterus and unborn child.
• Prolapse of the umbilical cord (the baby's cord drops down and this can interfere with the blood supply to the baby).
• Placental abruption (the placenta comes away early with bleeding and loss of blood supply to the baby).

If rupture of the membranes happens before about 26 weeks, the baby is left developing in a confined space with no fluid to allow expansion as it grows, so there are often postural and limb deformities as well as underdevelopment of the lungs.

Rupture of the membranes at the end of pregnancy

When the membranes rupture close to the end of pregnancy (near term), up to 85 per cent of women will go into labour spontaneously within 24 hours. The risk of infection in these cases is very small.

By 48 hours after the rupture only about five per cent won't have delivered their baby. In most hospitals the policy is to keep an eye on the woman and offer induction of labour, especially if signs of infection develop.

Treatment following premature rupture of membranes

Treatment is aimed to keep the baby safe in the womb for as long as possible. Vaginal examinations aren't done because of the risk that infection might be introduced. Cardiotachographs (CTG), which measure the baby's heart rate, and ultrasound scans are usually used to keep an eye on the baby. If there's any sign of infection, fetal distress or advanced labour, then it's essential to deliver the baby as soon as possible.

Another risk is to the unborn baby’s lungs. There's a risk that they may not be developed enough and the baby may develop respiratory distress syndrome (RDS) when it's born.

Most hospitals will treat a woman at risk of delivering between 24 and 34 weeks of pregnancy with steroids to mature the baby's lungs and prevent RDS. They may also give the mother antibiotics, although the value of these isn't so clear.

Very occasionally, the defect in the membranes seals up and the amniotic fluid fills up again.

Premature birth

About half of all premature births follow a premature rupture of membranes. How well the baby does depends on its gestational age (how far into the pregnancy it is). Survival rates for babies at 24 weeks are about 36 per cent, rising to about 90 per cent survival for babies born at 30 weeks.

Sometimes PROM happens after an amniocentesis test at 16 to 18 weeks. At such an early stage, the unborn child is almost always lost.