In Defence of the Amniotic Sac

Updated: September 2023

Artificial rupture of membranes (ARM) aka ‘breaking the waters’ is a common birth intervention. However, an ARM should not be carried out without a good understanding of how the amniotic sac and fluid function in labour. Women need to be fully informed of the risks associated this intervention before agreeing to alter their labour in this way.

This post will discuss how the ‘waters’ work during a physiological labour and the implications of breaking them. Please note that this post is not about induction. An induced labour is not physiological and usually involves an ARM as part of the process.

Anatomy and physiology

By the end of pregnancy the baby is surrounded by around 600mls of fluid. This is mostly made up of urine and respiratory tract secretions produced and excreted by the baby. The amniotic fluid is constantly being produced and renewed – baby swallows the fluid; it is passed through their gut into their circulation; then sent out via the umbilical cord through the placenta. This process continues even if the amniotic membranes have broken. So, even when the amniotic sac has a hole in it, there is still some fluid present, there is no such thing as a ‘dry labour’. You can read more about amniotic fluid volume in this post.

The amniotic sac is made up of two membranes – the amnion (inner) and the chorion (outer). At the end of pregnancy there is around 200mls of amniotic fluid and mucous between these two layers. After the birth of the placenta the two layers stick together because this fluid has gone. However, you can still tease the layers apart.

During pregnancy

The amniotic sac protects and prepares baby by:

  • Cushioning any bumps to the abdomen.
  • Maintaining a constant temperature.
  • Assisting the baby’s movements which are essential for muscle development.
  • Creating space for the baby to grow.
  • Protecting against infection – the membranes provide a barrier + the fluid contains antimicrobial peptides.
  • 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.

After 40 weeks gestation, around 20% of babies will pass meconium into their amniotic fluid as their bowels reach maturity and begin to work. This is perfectly normal and is not a sign of distress.

During labour and birth

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.

General fluid pressure

During a contraction the pressure is equalised throughout the fluid rather than directly squeezing the baby, placenta and umbilical cord. This protects the baby and their oxygen supply from the effects of the powerful uterine contractions.

When fluid is reduced (by escaping through a hole in the membranes), the placenta and baby get compressed more firmly during a contraction. When the placenta is compressed blood circulation is interrupted reducing the oxygen supply to baby. In addition, the umbilical cord may be in a position where it gets squashed between baby and uterus with contractions. When this happens the baby’s heart rate will dip during a contraction in response to the reduced blood flow. A healthy baby can cope with this intermittent reduction in oxygen for hours (it’s a bit like holding your breath for 30 seconds every few minutes). However, this is probably not so great for an extended period of time, or if the baby is already compromised through prematurity or a poorly functioning placenta.

Forewaters and hindwaters

The sac of amniotic fluid is described as having two sections – the forewaters (in front of baby’s head) and the hindwaters (behind baby’s head). A ‘hindwater leak’ refers to an opening in the amniotic membranes behind the baby’s head. Often this is experienced by the woman as an occasional light trickle as the fluid has to run down the outside of the sac and past baby’s head to get out.

During labour, forewaters are formed as the lower segment of the uterus is pulled upwards and the chorion (the external membrane) detaches from it. The well flexed baby’s head fits into the cervix and blocks off the fluid in front of the head (forewaters) from the fluid behind (hind waters). Pressure from contractions cause the forewaters to bulge downwards into the cervix and eventually through into the vagina. This protects the forewaters from the high pressure applied to the hindwaters during a contractions and keeps the membranes intact. The forewaters transmit pressure evenly over the cervix which helps with dilatation. When the baby is in an OP position the head may not flex as well to block off the hindwaters and pressure is able to move into the forewaters and they may rupture. Early rupture of membranes is often a feature of an OP labour.


The forewaters usually break when the cervix is almost fully open and the membranes are bulging so far into the vagina that they burst. This ‘fluid burst’ lubricates the vagina and perineum to facilitate movement of the baby and stretching of the tissues.

Born in the caul

It is fairly common for a baby to be born in the amniotic sac when labour is left to unfold without interference. Particularly during a waterbirth where the pressure on the amniotic sac is altered by the fluid in the pool. The photograph at the beginning of this post is my lovely friend Holly birthing her baby in his caul.

Historically, being born in the caul was considered good luck for the baby. It was also believed that a baby who was born in the caul would be protected from drowning. Midwives used to dry out amniotic membranes and sell them to sailors as a talisman to protect them from drowning. You can find out more about the social history of the caul in an old journal article by Forbes (1953).

How does birth in the caul influence the baby’s microbiota?

I don’t know the answer to this question. However, increasingly research is identifying the importance of intestinal microbiota for health, including immune development and function. I have written about this topic in more detail in another post. During a vaginal birth the baby is colonised by microorganisms as they pass through the vagina. So, this raises questions about what happens if the baby does not come into contact with vaginal microorganisms because the amniotic sac is intact. In theory, during a waterbirth the pool water is likely to contain microorganisms from the mother, therefore the baby could become colonised. But on land, I don’t know.

C-section and the amniotic sac

There are photos circulating on the internet of babies in their caul during a c-section (google caul+caesarean or cesarean). I would like to know the background stories to these photographs. There has been a study supporting this practice for preterm babies (Wang, et al. 2013). However, there is no research supporting this method for full term babies.

Artificial rupture of membranes (ARM) aka amniotomy

Breaking the membranes with an amni-hook is a common intervention during labour. It is usually the second step in the induction process, and is also done in an attempt to speed up spontaneous labour. In an induced labour, intact membranes can prevent the artificially created contractions from getting into an effective pattern. There is also the theoretical 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 and maternal death. So an ARM is recommended before a syntocinon/pitocin infusion is started (although this may not be a worldwide practice).

In a spontaneous labour the rationale for an ARM is that once the forewaters have gone the hard baby’s head will apply direct pressure onto the cervix and open it quicker. This is a relatively new idea. It appeared during introduction of O’Driscoll’s Active Management of Labour protocol into maternity hospitals in the 1970s.

Prior to the 1970s textbooks included explanations of the function of the amniotic sac as I describe above. They also included warnings about ARM. For example, A Short Practice of Midwifery for Nurses by Henry Jellett (1926) states that rupturing the membranes “…may result in various complications, the commonest perhaps of which is the prolongation of labour owning to the loss of the normal dilating power of the bag of membranes” and that “if the entire quantity of liquor amnii escapes, the foetus will be subject to an undue pressure during labour that may prove dangerous to it.”

As ARMs were introduced in the belief that they could speed up labour, Emanuel Friedman (famous for the 1950s research used to create the partogram) criticised the practice: “There is a generally accepted clinical impression that amniotomy stimulates labor in progress. The experimental evidence to support this contention just does not exist. Nevertheless, the conviction with which it is held is almost unshakable” (1978).

Fast forward to 2013 and a Cochrane review of the available research confirms Friedman’s findings, concluding that “the evidence showed no shortening of the length of first stage of labour and a possible increase in caesarean section. Routine amniotomy is not recommended for normally progressing labours or in labours which have become prolonged.” There has been no research to date refuting these findings.

There are also known risks associated with an ARM:

  • It increases pain which can result in the woman feeling unable to cope and choosing an epidural.
  • The baby may become distressed due to compression of the placenta, baby and/or cord (as described above).
  • Fok et al (2005) found amniotomy altered fetal vascular blood flow, suggesting there is a fetal stress response following an ARM.
  • The umbilical cord may be swept down as the fluid drains out. If it ends up wedged next to the baby’s head or washout out into the vagina, this is called a ‘cord prolapse’ and is an emergency situation. The compression of the cord interrupts or stops the supply of oxygen to the baby, and the baby must be born asap by c-section.
  • If there is a blood vessel running through the membranes and the amni-hook ruptures the vessel, the baby will lose blood volume fast (another emergency situation).
  • There is a 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.).

It seems that an ARM is often performed during labour without consent. The requirement for consent to be valid includes providing adequate information about the procedure. Have any readers been given the information above prior to agreeing to an ARM?


The amniotic sac and fluid play an important role in facilitating physiological birth and protecting the baby. There is no evidence that rupturing this sac will reduce the length of labour. While every intervention has its place, including ARM, midwives need to carefully consider the risks before offering it to women. Also, women must be fully informed of the risks before choosing an ARM during their labour.

If you found this post interesting, you can find more of my work in the following resources:

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