What do your hormones do at birthing time?

Obstetrician, Dr Nick Walker explains the role our hormones play in bringing a life into this world. 

Firstly, what exactly are ‘hormones’? Medically speaking, they are substances produced by certain organs which impart physical or behavioural changes either in neighbouring or distant (via the bloodstream) body tissues. A good and well known example is adrenaline, which is produced only by the adrenal gland but spreading throughout the body causes a fast heartbeat and increase in nervous and muscular excitation. Everybody has experienced this during a fright or a shock!

Several hormones are important during each pregnancy trimester and post birth. We’ll start with progresterone, a really key player from early pregnancy through until the end, when it’s absence becomes just as important as it’s recent presence. Progesterone quietens the uterus, softening the muscular walls which would otherwise tend to cramp and try to expel the expanding pregnancy within. It is so important, that women carrying IVF pregnancies are often advised to use progesterone supplements up to 12 weeks gestation to absolutely ensure there is adequate amounts to do this job. The developing placenta produces massive quantities of progesterone in a self-preserving and self-serving purpose, which is why most usual pregnancies do not need to supplement with laboratory-sourced extra. As long as progesterone is produced, the uterus can stretch and accomodate the baby as he or she grows from millimetres in length to kilograms in weight. At full term, by mechanisms not fully understood, the uterus becomes somewhat immune to the effects of progesterone, and less is produced, which allows the muscles to regather their potency for the big challenge of labour and birth.

Then comes our next major hormone: oxytocin. Oxytocin is produced by a tiny part (the rear half to be precise) of a tiny gland called the pituitary gland at the base of the brain, a long way from the target tissues in the uterus and breasts. It’s pushed straight from the gland into the bloodstream, and tends to be sent forth in waves or pulses, and these tend to be more exaggerated at night time. This is why pregnant mums often notice more cramps and aches overnight right at the end of the third trimester. Like adrenaline, it only lasts in your body for several minutes before it is used up, so steady production of it is required for it to exert its effects. These effects are simply to bring your baby into the world, and to feed your baby once born. Oxytocin is really vital stuff!

Oxytocin is the necessary initiator and catalyst of uterine contractions and there is no substitute for it. Without oxytocin there could be no labour and birth – we all owe our lives to our mother’s oxytocin (amongst other things!). The importance of its structure and nature was recognised by scientists in the 1950s, who then named it 'oxy', meaning intense or sharp, and 'tocin', meaning labour. The synthetic version is identical to the natural, and is named ‘syntocin’ in New Zealand and ‘pitocin’ in the USA (because it is originally from the 'pituitary' gland). 

Because oxytocin is absolutely vital for contractions to be at their best, and because it arises from your central nervous system, it is thought that women in labour can enhance their oxytocin production by staying as calm and ‘in the moment’ as possible, whilst also avoiding stressful or distracting influences which may slow or limit that production. These strategies are probably most useful in the early and middle part of the labour, because at the final stages of labour the body is in such full internal swing that most externalities seem to make little difference. This is the time for support people to move from passive calm mode into hyped-up cheerleading mode!

Oxytocin’s job does not end with the birth of the baby though. After a brief respite when your baby is born, more starts flowing into your bloodstream to get the last few contractions necessary to birth the placenta (‘whenua‘ or ‘afterbirth’). Further release occurs during the weeks after delivery in order to encourage the uterine muscles to keep shortening and tightening on themselves with the goal of achieving their non-pregnant size and state about four weeks later. It doesn't finish there though! During breastfeeding, stimulation of the nipple area (suckling), as well as visual cues (looking at your beautiful new baby) cause more oxytocin release, and this time the main target tissue is in the breasts. Specifically, the little sacs and tubes in the breasts that are full of milk are lined with a layer of muscle which is responsive to oxytocin, and when these muscles squeeze, milk is able to flow from you into your baby. 

Now we need to return to progesterone again. After the birth, once the placenta is born, the progesterone that had been produced from it falls pretty quickly. It’s a good thing that this happens, because besides the other functions listed above, progesterone also inhibits breast milk production. This is the mechanism by which your body ‘knows’ when to (and when not to, of course) begin lactation. There would be no point in lactation before the baby is born – there is no baby to suckle yet and the milk would literally go sour in the breast, which would be potentially harmful for the mother. The placental progesterone handily keeps milk production on hold because nobody knows when the baby will arrive, let alone your breasts, it is logical that the birth itself holds the key to when to start lactation. Whether a baby is born two months premature or two weeks after due date doesn't matter in terms of lactation. When the placenta is gone, the inhibiting progesterone falls, and milk supply begins. Therefore, and interestingly, the method of birth does not seem to matter – women who have their babies by planned Caesarean section (without labour hormones) experience the same progesterone level drop and the same lactation response, as long as they also suckle their babies.

I hope this overview provides some insight into the inner workings of the more important and well known hormones during your pregnancy, birth and postpartum – the knowledge of these facts has come to us from many decades of patient and enquiring research. However, having this knowledge ought not in any way distract your attention from the lived experience of labour, birth and breastfeeding. The wonder of making new humans ought to be enjoyed as a complete end in and of itself.

Dr Nick Walker is a specialist obstetrician working in public practice at National Women’s Hospital and private practice in Auckland. He divides his time between these roles and helping his wife care for their four children. 

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