What is an ARM (Artificial Rupture of Membranes)?
By the end of pregnancy your baby is surrounded by around 500-1000mls of fluid. Artificial rupture of membranes (ARM), also known as an amniotomy or 'breaking the waters', is a common intervention performed by a midwife or an obstetrician using a specialised tool called an amnihook.
It is usually the second step in the induction process, and is also done in an attempt to speed up spontaneous labour.
The amnihook is a sterile plastic hook inserted into the vagina and used to puncture the membranes containing the amniotic fluid.
What is the role of the Amniotic Sac during pregnancy?
Cushioning any bumps to the abdomen.
Maintaining a constant temperature.
Allowing the movement essential for muscle development.
Creating space for growth.
Protecting against infection.
Assisting lung development – baby breathes fluid in and out of the lungs.
Taste and smell – the baby tastes and smells the fluid, which is similar to colostrum = helps to find their mother’s nipple after birth.
What are the benefits of an intact Amniotic Sac during labour?
Protecting the baby and their oxygen supply from the effects of the powerful uterine contractions by equalising the pressure from surges rather than directly squeezing the baby, placenta and umbilical cord.
Allowing more space for baby to rotate and move into an optimal position.
Lubrication: waters usually break when the cervix is almost fully open. This ‘fluid burst’ lubricates the vagina and perineum to facilitate movement of the baby and stretching of the tissues.
"Around 80-90% of women start labour with their membranes intact. This is probably because the amniotic sac plays an important role in the physiology of childbirth."
Dr Rachel Reed
What is the logic behind using it as part of the induction process?
In an induced labour, intact membranes can prevent the artificially created surges from getting into an effective pattern. There is also the potential risk of an induced contraction (that is too strong) forcing amniotic fluid through the membranes/placenta and into the mother’s circulation causing an amniotic embolism. So an ARM is recommended before a synthetic oxytocin infusion is started (although this may not be a worldwide practice).
Why it might be offered in the context of a spontaneous labour?
To speed up labour aka augmentation of labour. The rationale is that once your waters have gone, the baby’s head will apply direct pressure to the cervix and open it quicker. The pressure also sends signals for the body to release the hormones responsible for your surges so they might become stronger and more regular.
Breaking the waters allows your provider to assess the colour and odour of your amniotic fluid; giving them the ability to know if your baby has passed meconium (baby's 1st poo) in utero.
If baby needs to be monitored more closely and external monitors won’t work, amniotomy will let your care provider place an internal monitor
(The above also applies in the context of induced labour.)
What are the risks associated with an ARM?
It may increase contraction intensity and pain which can result in the woman feeling unable to cope and choosing an epidural which may lead to further interventions and a more difficult birth experience.
The baby may become distressed due to compression of the placenta, baby and/or cord.
Fok et al (2005) found amniotomy altered foetal vascular blood flow, suggesting there is a foetal stress response following an ARM.
Cord prolapse: the umbilical cord may be swept down as the fluid drains out and could end up trapped next to the baby’s head or down into the vagina. This is an emergency situation as the compression of the cord might stop the supply of oxygen to the baby. Baby must be born asap by c-section.
If there is a blood vessel running through the membranes and the amnihook ruptures the vessel, the baby will lose blood volume fast – another emergency situation.
Slight increase in the risk of infection but mostly for the mother (not baby). This risk is minimal if nothing is put into the vagina during labour (ie. hands, instruments etc.).
Other things to consider:
ARM should not be offered routinely, only if there is concern over the progress of your labour. You should always ask for the benefits, risks and alternatives relevant to your specific situation.
It is not actually proven to be effective in shortening labour. Although some studies show that ARM is an effective method of induction. The evidence on whether AROM truly speeds up labour is pretty conflicting and seems a bit anecdotal (meaning it’s based on what people observe, not well done studies).