Introduction

The correct balance of hormones is essential for a successful pregnancy. Hormones act as the body’s chemical messengers sending information and feeding back responses between different tissues and organs. Hormones travel around the body, usually via the blood, and attach to proteins on the cells called receptors – much like a key fits a lock or a hand fits a glove. In response to this, the target tissue or organ functions in a way that ensures pregnancy is maintained. Initially, the ovaries, and then later, the placenta, are the main producers of pregnancy-related hormones that are essential in creating and maintaining the correct conditions required for a successful pregnancy.

The early stages of pregnancy

Following conception, a new embryo must signal its presence to the mother, allowing her body to identify the start of pregnancy. When an egg is fertilised, it travels though the female reproductive tract and implants into the womb around 9 days after it was fertilised, releasing a hormone called human chorionic gonadotrophin in the process. This hormone enters the maternal circulation and allows the mother to recognise the embryo and begin to change her body to support a pregnancy. This hormone continues to be produced in large quantities during pregnancy.

Human chorionic gonadotrophin can be detected in the urine as early as 7-9 days after fertilisation and is used as an indicator of pregnancy in most over-the-counter pregnancy tests. It is partly responsible for the frequent urination often experienced by pregnant women during the first trimester. This is because rising levels of human chorionic gonadotrophin causes more blood to flow to the pelvic area and kidneys, which causes the kidneys to eliminate waste quicker than before pregnancy. Human chorionic gonadotrophin passes through the mother’s blood to the ovaries to regulate the levels of the pro-pregnancy hormones, oestrogen and progesterone.

The role of progesterone and oestrogen during pregnancy

High levels of progesterone are required throughout pregnancy with levels steadily rising until the birth of the baby. During the first few weeks of pregnancy, progesterone produced from the corpus luteum (a temporary endocrine gland of the ovaries) is sufficient to maintain pregnancy. At this early stage, progesterone has many different functions which are vital to the establishment of pregnancy, including:

• Increasing blood flow to the womb by stimulating the growth of existing blood vessels.
• Stimulating glands in the lining of the womb (the endometrium) to produce nutrients that sustain the early embryo.
• Stimulating the endometrium to grow and become thickened, producing the decidua (a unique organ that supports the attachment of the placenta) and allowing implantation of the embryo.
• Helping to establish the placenta.

As the placenta forms and grows, it develops the ability to produce hormones. The cells that make up the placenta, known as trophoblasts, are able to convert cholesterol from the mother’s bloodstream into progesterone. Between weeks 6-9 of pregnancy, the placenta takes over from the ovaries as the main producer of progesterone. As well as being vital to the establishment of pregnancy, progesterone also has many functions during mid- to late pregnancy, including:

• Being important for correct foetal development.
• Preventing the muscles of the womb contracting until labour starts.
• Preventing lactation (breastfeeding) until after pregnancy.
• Strengthening the muscles of the pelvic wall in preparation for labour.

Although progesterone dominates throughout pregnancy, oestrogen is also very important. Many of the functions of progesterone require oestrogen and in fact, progesterone production from the placenta is stimulated by oestrogen. Initially, oestrogen is made and released by the corpus luteum of the ovaries. As the pregnancy develops, the foetal adrenal glands produce androgens that are then passed to the placenta and converted to the hormone oestriol (an oestrogen often used to determine foetal wellbeing in pregnancy) and other oestrogens. Levels of oestriol increase steadily until birth and have a wide range of effects, including:

• Maintaining, controlling and stimulating the production of other pregnancy hormones.
• Ensuring the correct development of many foetal organs including the lungs, liver and kidneys.
• Stimulating the growth and correct function of the placenta.
• Promoting growth of maternal breast tissue (along with progesterone) and preparing the mother for lactation (breastfeeding).

Other hormones produced by the placenta

The placenta also produces several other hormones including human placental lactogen and corticotrophin-releasing hormone. The function of human placental lactogen is not completely understood, although it is thought to promote the growth of the mammary glands in preparation for lactation. It is also believed to help regulate the mother’s metabolism by increasing maternal blood levels of nutrients for use by the foetus. Corticotrophin-releasing hormone is thought to regulate the duration of pregnancy and foetal maturation. For example, when pregnant women experience stress, particularly in the first trimester of pregnancy, the placenta increases the production of corticotrophin-releasing hormone. There is a good reason for this. In the first days of pregnancy, corticotrophin-releasing hormone suppresses the mother’s immune system, preventing the mother’s body from attacking the foetus. Later in pregnancy, it improves the blood flow between the placenta and foetus. In the last weeks of pregnancy corticotrophin-releasing hormone levels climb even higher – a rise which coincides with a major spike in cortisol levels. The rise in corticotrophin-releasing hormone and cortisol may help the foetal organs mature just before labour begins, and influence the timing of birth, through production of a ‘late-term cortisol surge’. This prenatal cortisol surge has also been linked to more attentive mothering in both animals and women. It is thought to be an adaptive response that induces an increased liking for their infant’s body odours, strengthening the bond between mother and baby.

Side-effects of pregnancy hormones

High levels of progesterone and oestrogen are important for a healthy pregnancy but are often the cause of some common unwanted side-effects in the mother, especially as they act on the brain. Until the mother’s body has adapted to the higher levels of these hormones, mood swings can be very common. The majority of women will experience morning sickness – a feeling of nausea, any time of day, which may lead to vomiting. The exact cause of morning sickness is unknown but it is likely to be because of the rapid increase in oestrogen, progesterone and human chorionic gonadotrophin, or a decrease in thyroid stimulating hormone (which has a similar structure to human chorionic gonadotrophin). Although it is probably caused by a combination of all these hormonal changes. Morning sickness usually starts around week 5-6 of pregnancy and often subsides by week 12-16, although some women suffer throughout pregnancy and/or have very severe forms of morning sickness.

Many women experience pain and discomfort in the pelvis and lower back during the first trimester. This is mostly due to a hormone called relaxin. Relaxin becomes detectable by week 7-10 and is produced throughout pregnancy. This hormone relaxes the mother’s muscles, joints and ligaments to make room for the growing baby. The effects of relaxin are mostly concentrated around the pelvic region, and softening the joints of the pelvis can often lead to pain in the area. Softer pelvic joints can also decrease stability and some women may notice it is harder to balance.

Although uncomfortable and frustrating at times, all these side-effects will usually lessen or even subside by the end of the first trimester.

Hormones and labour

The exact events leading up to the onset of labour are still not fully understood. For the baby to arrive, two things must happen: the muscles in the womb and abdominal wall have to contract and the cervix needs to soften, or ripen, allowing passage of the baby from the womb to the outside world.

The hormone oxytocin plays a key role in labour. Often called the ‘love hormone’, oxytocin is associated with feelings of bonding and motherhood. This is also true of another hormone released during labour called prolactin. If labour needs to be induced (brought on artificially), oxytocin or a synthetic oxytocin equivalent is often administered to ‘kick-start’ the process. Oxytocin levels rise at the onset of labour, causing regular contractions of the womb and abdominal muscles.

The cervix must dilate (open) to around 10cm for the baby to pass through. Oxytocin, along with other hormones, stimulates softening of the cervix leading to successive dilation during labour. Oxytocin, with the help of the high levels of oestrogen, causes the release of a group of hormones, known as prostaglandins, which may play a role in softening of the cervix. Levels of relaxin also increase rapidly during labour. This aids the lengthening and softening of the cervix and the softening and expansion of the mother’s lower pelvic region, thereby further aiding the baby’s arrival.

As labour contractions become more intense, natural pain relief hormones are released. Known as beta-endorphins, they are similar to drugs like morphine and act on the same receptors in the brain. As well as pain relief, they can also induce feelings of elation and happiness in the mother. As birth becomes imminent, the mother’s body releases large amounts of adrenaline and noradrenaline - so-called ‘fight or flight’ hormones. A sudden rush of these hormones just before birth causes a surge of energy in the mother and several very strong contractions, which help to deliver the baby.

Hormones after labour

When the baby is born, oxytocin continues to contract the womb in order to limit blood flow to the womb and reduce the risk of bleeding, and to help detach the placenta (which is delivered shortly afterwards). Blood levels of oxytocin and prolactin are very high, which supports bonding between the mother and baby. Skin-to-skin and eye contact between the mother and baby also stimulate the release of oxytocin and prolactin, further encouraging bonding. Many mothers describe being in a euphoric state just after labour; this is due to the effects of oxytocin, prolactin and beta-endorphins.

Women are actually able to breastfeed at around four months of pregnancy but high levels of progesterone and oestrogen during this time prevent milk production. After the placenta is delivered during birth, the blood levels of progesterone and oestrogen fall, allowing the mother to produce the first meal of colostrum, a high density milk that contains more protein, minerals and fat-soluble vitamins (A and K) than mature milk, which is produced later. When the baby suckles, oxytocin and prolactin are released from the pituitary gland, and pass through the mother’s blood to the breast, where prolactin stimulates milk production and oxytocin stimulates milk delivery to the nipple. As well as stimulating bonding, these hormones also aid milk release and further milk production. Mature milk that nourishes the baby and induces sleep starts to be produced about four days after birth.