Updated: May 2022
Artificial rupture of membranes (ARM) aka ‘breaking the waters’ is a common intervention during birth. 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 in labour and the implications of breaking them. Most of the information in this blog is available in any good physiology textbook. I have included references and links for additional content.
Anatomy and physiology
By the end of pregnancy the baby is surrounded by around 500-1000mls 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, ie. 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.
The amniotic sac protects and prepares baby by:
- Cushioning any bumps to the abdomen.
- Maintaining a constant temperature.
- Allowing the movement essential for muscle development.
- Creating space for growth.
- 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. This meconium is diluted and processed with the amniotic fluid as described above.
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.
Fore waters and hind waters
The sac of amniotic fluid is described as having two sections – the fore waters (in front of baby’s head) and the hind waters (behind baby’s head). A ‘hind water 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, fore waters 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 (fore waters) from the fluid behind (hind waters). Pressure from contractions cause the fore waters to bulge downwards into the cervix and eventually through into the vagina. This protects the fore waters from the high pressure applied to the hind waters during a contractions and keeps the membranes intact. The fore waters 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 hind waters 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 to the cervix and open it quicker. However, a cochrane review of the available research states 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 are also 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… and the intervention rollercoaster begins.
- 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 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 eligible 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.