Spontaneous rupture of membranes

Spontaneous rupture of membranes usually occurs during normal labour – but membranes can rupture before labour also and this can affect progress of labour.

What is spontaneous rupture of membranes?

Whilst the baby, or fetus, is growing in the womb it is surrounded by amniotic fluid –which is commonly known as ‘the waters’. By the end of pregnancy this totals around one litre and it is contained within a sac, or two membranes:

  • The amnion, which is next to baby
  • The chorion, which is next to the wall of the womb.

In normal labour the membranes usually stay intact until the pressure of the contractions towards the end of labour causes the membranes to burst, just like a balloon bursting under pressure. The advantages of this are:

  • the baby remains cushioned in the sac of water during contractions
  • the baby is protected from infection during labour
  • the pressure on the cervix during contractions is effective in helping the cervix to open, or dilate.

When ‘the waters go’ in labour the hole in the membranes is usually at the bottom of the uterus, over the cervix, causing a gush of fluid. This may be quite small, such as 50mls, or it can be significantly large, 200-300mls, depending upon how much fluid is around the fetus and how well the head is fitting in the pelvis. If the head is well down and tight fitting it will act like a plug and retain most of the fluid behind baby’s head.

Spontaneous rupture of membranes is often abbreviated to S.R.M. or S.R.O.M.

What is the amniotic fluid?

Amniotic fluid surrounds the baby in the womb. It is composed of water and solutes and in later pregnancy contains fetal urine as the baby passes urine regularly.

It is a pale yellow or ‘straw’ colour and has a faint smell. Some women confuse the amniotic fluid with urine and are unsure whether they have had a bladder leak or their waters have broken. A simple test is to empty your bladder, put a pad in place and then check your pad after half an hour. If your waters have gone the pad will be damp.

If in doubt contact your LMC for advice.

Amniotic fluid can be meconium stained if the baby has moved their bowels whilst in the womb. This would cause the fluid to be green/brown and can be a sign that baby is distressed, so it should be reported to your maternity carer immediately.

Amniotic fluid can also be blood stained. A small amount of blood streaking is normal as part of the ‘show’ (the mucus plug which comes away from the cervix), but usually the waters will not be blood stained, therefore it is important to report to your LMC or other health professional if your waters are blood stained.

What is pre labour SRM?

In pregnancies that reach term (37 weeks and over) 10% of women have spontaneous rupture of membranes before labour begins – that is, before contractions commence.

This is often due to an ill fitting presenting part caused by the baby’s head (or bottom) not fitting evenly over the cervix, causing undue pressure and hence the waters break early.

It can also be linked with infection – both as a cause and as a result of pre labour spontaneous rupture of membranes.

What are the risks of pre labour spontaneous rupture of membranes?

When the baby is enclosed in a sealed sac there is little chance of the fetus catching an infection from bugs that either live in the mother’s vagina or have ascended from the outside, up the vagina through the cervix. The membranes form an effective barrier from ascending infection. Once there is a hole in the sac the risk of infection is increased. Some women carry infections such as Group B streptococcus (group B strep) which can be harmful to the baby.

There is also a very small risk of cord prolapse. If the baby’s head (or bottom) is ill-fitting, the cord may fall down beside or below the head and become squashed, reducing the blood supply to the baby.

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What usually happens after pre labour SRM?

When the membranes rupture prior to labour, after 37 weeks of pregnancy, most women will go into labour spontaneously:

  • 86% of women will labour within 12 – 23 hours
  • 91% will labour within 24 – 47 hours
  • 94% will labour within 48 – 95 hours
  • 6% of women will not be in spontaneous labour within 96 hours of PROM.

Most maternity units therefore practise what is called ‘expectant management’. The mother is asked to wait either at home or in the unit for 24-48 hours to ‘see what happens’, knowing that the majority of women will start to labour without any further intervention. During this time they will be asked to report back if the colour of the fluid changes or they feel unwell.

After this time most units would encourage induction of labour as the longer the time interval of ruptured membranes, the higher the risk of infection.

In addition to this most maternity units also encourage antibiotics for prolonged rupture of membranes, after varying lengths of time, depending upon research evidence and local policies. These are usually given orally before labour and intravenously during labour, to reduce the risk of infection.

Some units also give the mother the choice of immediate induction of labour after SRM. Whilst this is done to reduce the risks of infection, it is important to acknowledge the risks of intervention and failed induction which could result in Caesarean section.

What is preterm pre labour rupture of membranes?

This occurs to rupture of the membranes before 37 weeks of pregnancy, prior to labour commencing. As the baby is not yet fully grown and ready to be born this may be managed differently. There are many different scenarios depending upon:

  • The length of your pregnancy (gestation)
  • The growth of the baby so far
  • The condition of the baby at the moment
  • Any other medical or pregnancy related factors.

It is possible that preterm pre labour SRM would be treated with any or a combination of the following:

  • Expectant management
  • Antibiotics
  • Steroid therapy, in the form of injections for the mother, to mature the bay’s lungs
  • Transfer to a large maternity unit
  • Immediate delivery

Your maternity health professionals will explain your options and advise you of the safest course of action.

Contact your LMC or other maternity health professional if you suspect that your membranes have ruptured at any time in pregnancy.

Useful articles

To read more about Induction of Labour, visit our informative Kiwi Families article

To understand more about the process of Normal Birth, click here

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Paula Skelton

Paula Skelton is a qualified NZ nurse and midwife, a midwifery & childbirth educator and the mum of three lovely girls.

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