In defence of the amniotic sac
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.
Anatomy and physiology
By the end of pregnancy the baby is surrounded by around 500-1000mls of Fluid. This is mostly made up of amniotic fluid secreted by the amniotic sac (the membranes). The baby also contributes urine and respiratory tract secretions into the fluid. The amniotic fluid is constantly being produced and renewed – Baby swallows the fluid; it is passed through the gut into the baby’s circulation; then sent out through the placenta. This process continues even if the amniotic membranes have broken. So, even when the waters have ‘gone’ there is still some fluid present ie. there is no such thing as a ‘dry labour’.
The amniotic membrane is adhered to the chorion – another membrane between the amniotic membrane and the uterus. These membranes look like one, but you can tease them apart after birth.
During pregnancy
The amniotic sac protects and prepares baby by:
After 40 weeks gestation around 20% of baby’s will pass meconium into their amniotic fluid as the 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.
- Cushioning any bumps to the abdomen.
- Maintaining a constant temperature.
- Allowing movement to aid 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 smell of amniotic fluid has been found to have a calming effect on newborns (Varendia et al. 1998).
During labour
Around 80-90% of women start their labour with their membranes intact. This is probably because the amniotic sac plays an important role in the physiology of a natural birth.
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 his/her oxygen supply from the effects of the powerful uterine contractions. When the membranes have ruptured the placenta and baby get compressed during a contraction. Most babies can cope well with this, but the experience of birth for the baby is probably not as pleasant. 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 supply for hours (it’s a bit like holding your breath for 30 seconds every few minutes). However, this is probably not so great to do for an extended period of time....
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… intervention rollercoaster begins.
- The baby may become distressed due to compression of the placenta, baby and/or cord (as described above).
- The umbilical cord may be swept down by the waters and either past the baby’s head or wedged next to the baby’s head. 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. The only cord prolapse I have been involved with happened after an ARM (not done by me - honest!). The outcome for the woman was a live baby born by emergency c-section. Her previous 2 babies had been vaginal births.
- If there is a blood vessel running through the membranes (see picture below) 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.)....
HERE to Read the entire article with (great links for pictures ) on Midwifes Thinking Blog
