In Defence of the Amniotic Sac

intervention physiology

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 with this intervention before agreeing to alter their labour in this way.

This post will discuss how the amniotic sac functions during physiological labour and the implications of breaking it. 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

The amniotic sac during pregnancy

By the end of pregnancy, the baby is surrounded by approximately 600mls of fluid, primarily composed of urine and respiratory tract secretions. This amniotic fluid is not stagnant; it's constantly being produced and renewed. The baby swallows the fluid, it passes through the gut into their circulation, and then it's sent out via the umbilical cord through the placenta. This process continues even if the amniotic membranes have broken, ensuring that there's always some fluid present. Therefore, the concept of a 'dry labour' is a misconception. You can find more information about amniotic fluid volume in this post.

Diagram available here

The amniotic sac comprises two membranes: the amnion (inner) and the chorion (outer). At the end of pregnancy, around 200ml of amniotic fluid and mucous are between these two layers. After the placenta is born, the two layers stick together because this fluid has gone. However, you can still tease the layers apart. The amniotic sac and fluid protect the baby and prepare them for life outside of the uterus 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, and the fluid contains antimicrobial peptides.
Assisting lung development – the baby breathes fluid in and out of their lungs.
Taste and smell – the baby tastes and smells the amniotic fluid, which contains chemicals from the food their mother eats.
Preparing the baby for breastfeeding – amniotic fluid smells and tastes similar to colostrum (the first breastmilk), which helps the baby 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 a sign of maturity, not a sign of distress.

The amniotic sac 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, 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 the baby. In addition, the umbilical cord may be in a position where it gets squashed between the baby and the 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 amniotic sac is described as having two sections–the forewaters (in front of the baby's head) and the hindwaters (behind the baby's head). A 'hindwater leak' refers to an opening in the amniotic membranes behind the baby's head. Often, the woman experiences this as an occasional light trickle as the fluid has to run down the outside of the sac and past the 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. A 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 causes 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 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, their head may not flex as well to block off the hindwaters, and pressure is able to move into the forewaters, and they can rupture. Early rupture of membranes is often a feature of normal OP labour.

Lubrication

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 the movement of the baby and the stretching of the perineal 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. This photograph is of 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 talismans to protect them from drowning. You can find out more about the social history of the caul in a 1950s journal article by Forbes [1].

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

I don't know the answer to this question. However, increasing research is identifying the importance of intestinal microbiota for health, including immune development and function. 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 of these photographs. There has been a study supporting this practice for preterm babies [2]. 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 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 the introduction of O'Driscoll's Active Management of Labour protocol into maternity hospitals in the 1970s [3].

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 [4] 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. An ARM does not generally speed labour up.

Risks associated with ARM

There are 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).
One study found amniotomy altered fetal vascular blood flow, suggesting there is a fetal stress response following an ARM [5].
The umbilical cord can be swept down as the amniotic 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 (i.e. fingers, instruments etc.).

It seems that ARMs are often performed during labour without consent. The requirement for consent to be valid includes providing adequate information about the procedure.

Summary

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, care providers 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.