Hormones in pregnancy and labour

Don’t know what a hormone is? See our article “What is a Hormone?”.

Useful vocabulary

Conception: the fertilisation of an egg cell by a sperm.

Embryo: the first stage of development. In humans this is from shortly after conception until about eight weeks later, by which time the embryo is usually at least one cm long. 

Foetus: the main stage of development. In humans this is from eight weeks after conception until birth.

Uterus: the organ within which the foetus grows. Also known as the womb.

Placenta: an organ that grows from the embryo and attaches to the wall of the uterus. It is connected to the foetus by the umbilical cord. 

Changes during pregnancy

Pregnancy is a truly incredible process. The mother’s body needs to change in many ways to support the growing foetus, to prepare for birth, and to feed the baby.

The uterus develops a thick lining in preparation for pregnancy. When a pregnancy starts, special tissues must be grown here to support the foetus.

The foetus will need early nutrition, a good blood supply, and other support to thrive. Also, it will need space. As the foetus grows, the uterus and other parts of the mother’s body must stretch and change to give it room to move.

Hormones are small chemical signals which help communicate between the foetus and mother during pregnancy. They are used to send signals to a variety of organs and tissues, which respond by making the changes necessary to support the pregnancy.

Hormones are also important during labour. Their effects provide pain relief, and extra power for the final contractions that deliver the baby.

There are many different hormones involved in pregnancy and labour. This article describes the most important and best-known of these. Just be aware as you read the article that most of these hormones are not only ‘pregnancy hormones’ – they also have other roles not related to pregnancy, including in male bodies.

The pregnancy test hormone - hCG

Pregnancy starts when a fertilised egg develops into an embryo and implants (attaches) to the inside wall of the uterus. This triggers the placenta to form. The next task is for the placenta to signal to the mother that this has been done.

It does this by releasing a hormone called human chorionic gonadotrophin (hCG). This hormone is only released during pregnancy. For this reason, it is sometimes called the pregnancy hormone. It  passes from the placenta into the mother’s blood, from where it can reach any cell in her body.

When the mother’s body detects hCG, it starts changing to support the pregnancy. One early change is to increase blood flow to the kidneys. This causes more urine to be produced, which is one of the reasons why early pregnancy often involves frequent trips to the toilet.

When the ovaries detect hCG, they increase their production of the hormones oestrogen and progesterone. Progesterone in particular is a key pregnancy hormone with many powerful effects (see below). The correct balance of hCG, oestrogen and progesterone will be needed for a successful pregnancy.

Pregnancy tests also rely on hCG. If the test detects its presence, it returns a positive result. The hormone can be found in the mother’s blood from about 11 days after conception, and also in her urine. Bizarrely, pregnancy tests used to involve injecting urine into a frog. If hCG was present, the frog would lay eggs within 18 hours. This works because the human hormone is very similar to a hormone used by the frog. Since the 1960s, hCG can be detected more easily and humanely by peeing onto a pregnancy stick test at home, or by analysis of a blood sample. 

Progesterone

Progesterone is a key hormone during pregnancy. High levels of this hormone help thicken the lining of the uterus, and drive changes in the mother’s body including helping the breasts prepare for breastfeeding. The foetus also needs progesterone for its own development.

At first, most of the progesterone comes from the ovaries. They release it in response to the hCG released from the placenta.

The progesterone signal tells the uterus to support the embryo’s first needs. This includes increasing blood flow, and supporting the development of the placenta. The uterus also releases nutrients to feed the embryo during the time the placenta is getting itself organised.

As the placenta develops, it starts to release a mix of hormones including progesterone. At some point (between week 6-8 of pregnancy), it takes over from the ovaries as the main progesterone production centre, releasing it in ever-increasing quantities.

High progesterone levels prevent the mother from releasing more eggs whilst pregnant. They also signal to the uterus that it is not yet time to start labour.

Oestrogen

Oestrogen is another key pregnancy hormone. It does many things including controlling the release of other hormones. For example, it’s oestrogen that ‘tells’ the placenta to release progesterone.

As well as its effects on the mother’s body, oestrogen helps encourage growth of the placenta. It is also needed by the foetus for correct development of its organs, including its lungs, liver and kidneys.

The pregnancy starts with the ovaries being the main production centre for oestrogen, before the placenta starts producing it in greater quantities. This is just the same as what happens with progesterone.  

Getting ready for birth

Progesterone is involved in strengthening the muscles that will be used during labour. It is also used as a signal to prevent the uterus muscles contracting too early.

Both oestrogen and progesterone help promote the growth of breast tissue and preparation for the production and release of milk (lactation).

High levels of oestrogen and another hormone, oxytocin, lead to the release of a group of hormones called prostaglandins, which help soften the cervix (the part of the uterus that opens into the vagina). This will allow it to stretch as the baby is squeezed out.

The placenta also releases the stress hormone Corticotrophin-releasing hormone. This hormone has a wide variety of important roles for both the mother and the foetus. Its levels surge very shortly before birth.

Labour (giving birth)

The events leading up to labour are not very well understood. What makes labour start? What happens first? We don’t exactly know. But if doctors need to trigger labour, this can be done with an injection of the hormone oxytocin.

Oxytocin is a powerful hormone with multiple roles in both women and men. Sometimes called “the love hormone”, it has been linked to feelings of calm, bonding and motherhood. It is also important during labour as it helps organise muscle contractions in the uterus and surrounding tissues.

As the contractions become stronger, natural pain relief hormones are released. As well as reducing pain, these can create feelings of euphoria (excited happiness). Then for the final pushes, large amounts of the ‘fight or flight’ hormones adrenaline and noradrenaline are released. These hormones cause a surge of energy for the final, powerful contractions that deliver the baby. 

After birth

Skin-to-skin and eye-to-eye contact between mother and child cause the release of extra oxytocin and prolactin in the mother. Many mothers describe being in a state of intense happiness just after labour, which can be linked to the effect of oxytocin and other hormones. Oxytocin and prolactin are also involved with mother and child bonding.

Shortly after birth, the placenta is also delivered. The loss of its supply of progesterone and oestrogen means that the levels of these hormones in the mother fall rapidly.

Without high levels of progesterone and oestrogen signalling a pregnancy, the uterus slowly returns to its normal state. And the drop in these hormones also affects the breasts.

During pregnancy, high levels of progesterone and oestrogen prevent lactation (the production and release of milk). Now, without progesterone and oestrogen blocking the process, lactation can start, driven by the hormone prolactin. If the baby is breast-fed, this action leads to the release of more prolactin and oxytocin, which encourages the production and release of more milk. This is a rare example of a positive feedback loop in biology. Find out more in our article What Are Feedback Loops.

Hormone side-effects and problems

It’s very common for pregnant women to feel unwell as a side-effect of hormonal changes. This is especially true at the start of a pregnancy. The rapid change in hormone levels has a wider effect on the body which takes a while to adjust to.

The jump in progesterone and oestrogen in early pregnancy often causes mood swings. It is also thought to be at least partly responsible for morning sickness (which can unfortunately happen at any time of day). Most women suffer this nausea - sometimes vomiting - somewhere between weeks 5 and 16. A smaller number of women suffer throughout pregnancy and/or have very severe morning sickness.

There may also be discomfort as the woman’s body changes to make space for the foetus. Early in pregnancy, a hormone called relaxin helps relax the mother’s muscles, joint and ligaments to make room for the growing foetus. This process can lead to pain and loss of stability. Some women might notice it is harder to balance. Relaxin also affects the gut, and it’s common for pregnant women to have changes in gut function. Relaxin’s levels surge before birth, when it helps relax the cervix for delivery of the baby.

The rise and fall of levels of different hormones during pregnancy is complicated. The changing levels help control the timing of different processes. This means that if a birth happens too early, or needs to be triggered by medical staff, hormones may not be at the best levels to support the process. Sometimes additional hormones are given by injection to help correct for this.