The topic of the uterus is massive, and having recently read Dynamic Positions in Birth by Margaret Jowett, it seems to me to be at the core of what we are working with as midwives with women during pregnancy and birth. I will continue discussing the uterus in the next issue as well because it is such an important organ that one column cannot do it justice.
The uterus is the most amazing and underestimated part of the human body. It has evolved to have two functions: firstly, to be a safe haven, a lifeline and an incubator for the foetus during the nine months of pregnancy; and secondly, to propel the baby to the outside world.
During labour, its job is to combine both simultaneously. It can repeat this process many times in a woman’s lifetime. In Jennifer Worth’s book Call the Midwife, she describes a mother with 22 children.
One hundred and fifty million years ago, the mammalian uterus evolved from the oviducts, which expelled eggs into the outside world. The uterus was formed at the junction of the tubes from the two ovaries. It evolved to delay the egg on its journey to the outside world, and to incubate the fertilised egg within the body.
At the same time in evolutionary history, a hormone called progesterone prepared the uterus to receive an egg, and to communicate the possibility of a pregnancy to the brain to enable it to switch to ‘pregnancy mode’.
The primate uterus has been evolving for 70 million years, and the human uterus for a million years. It is a phenomenal organ which is responsible for our survival and succession. It had to evolve its size and shape when we came out of the sea to live on land and became primates, and again when the first humans came down from the trees and began to walk on two legs on the ground. Being upright meant the whole of the spine, abdomen and pelvis had to adapt. As hunter-gatherers, we spent much time squatting on our haunches and running, so the muscles developed in such a way as to aid the birth process by opening up the pelvis. (When we kneel, lean forward or squat, 28% more space is created than to when we sit.)
The uterus is made of smooth muscle, the same type of muscle which forms the bladder, blood vessels, intestines and the bowel – systems of the body which run automatically, all the time, with no conscious input. The uterus differs in that its muscular contractions can become strong enough to be felt, for example during menstruation, orgasm and birth.
Progesterone and stress hormones, like adrenaline, tend to stop the uterus contracting, while oestrogen, oxytocin and prostaglandins promote contractions. The absence of stress hormones during labour will reduce labour stress and pain and improve uterine function.
Before pregnancy, a uterus weighs 60 grams, by the time of birth, it weighs one kilo, and, due to its elastic memory, it reverts to its pre-pregnancy state within six weeks after birth. It is the strongest muscle in the body and it stretches to accommodate the growing baby.
In labour, the uterus has a relationship with the spine, pelvis, abdomen and internal organs. It will manage these better when a mother finds a good position to labour. This means being upright. The muscle layers are thickest at the top of the uterus and thinnest at the bottom – the cervix. Only 10–15% of the cervix is muscle. The rest is collagen, which breaks down, enabling the muscular part of the cervix to open up and the baby to be propelled down and be born.
How does it do it?
In the latter weeks of pregnancy, preparation begins:
- Oestrogen levels rise.
- Oxytocin, a chemical messenger, communicates with the uterus.
- Collagen starts dissolving and vaginal discharge increases. As the cervix becomes less rigid, a plug of mucous may be discharged (a ‘show’).
- Contractions become stronger, aligning the baby with the cervix and the exit.
- Calcium is the fuel for all cells. It sustains the exchanges taking place within the body.
- The uterus coordinates with the baby to enable a smooth birth for mum and baby.
Today, fewer than 20% of all births happen at home or in birth centres. The body has not changed so drastically that it cannot function naturally. The female body is amazing. It must be celebrated.
Margaret Jowitt, Dynamic Positions in Birth: A Fresh Look at How Women’s Bodies Work in Labour, second edition (Pinter & Martin, 2020).
Eleanor Copp supports families across the UK, currently online. relaxedparenting.co.uk 07929 857 608
Illustration by Nadezhda Moryak
First published in Issue 70 of JUNO. Accurate at the time this issue went to print.