Here’s a quick guide to your body during pregnancy and birth – how it changes and grows and the role of different hormones.
Your body during pregnancy: hormones
As soon as the placenta starts to form and the cells, which are fast becoming your baby, have implanted into the side of the uterus, your body will produce human chorionic gonadtrophin (hCG) – these levels will double every couple of days, reaching its peak by about 10 weeks when it levels out and the placenta starts to function.
The hormone oestrogen is needed to:
- develop your baby’s lungs, liver and kidneys
- work with progesterone to grow your breast tissue for milk production
- stimulate the placenta, to ensure it grows and works
These rapidly growing levels of oestrogen in early pregnancy may cause sickness and nausea.
Oestrogen combined with the hormone relaxin creates the loosening of the ligaments and joints throughout the body. This is needed to enable your body to expand for your growing baby and to provide the space for your baby to be born.
Progesterone is a powerful hormone and its levels are high during pregnancy. In early pregnancy it is needed to:
- to stimulate blood vessels to increase the blood flow to your womb
- for the glands in the lining of the womb to produce the nutrients that are needed for your embryo baby
- to enable the lining of your womb to thicken and to aid the attachment of the placenta and the implantation of your embryo baby
- to form the placenta
Throughout pregnancy, progesterone is needed:
- to prevent your womb contracting before labour needs to start
- for the development of your baby
- to prevent lactation until after your baby is born
- to help strengthen the muscles of your pelvic wall for labour
Your posterior pituitary gland releases oxytocin during pregnancy. This will increase towards the end of your pregnancy to stimulate the contractions needed for labour.
Human placental lactogen (hPL)
This hormone, also known as human chorionic somatomammotrophin, is made by the placenta. It provides nutrition for your baby and it stimulates milk glands in your breasts in preparation for breastfeeding.
What’s going on inside your body during pregnancy
During the second trimester of your pregnancy, your heart is working 40% harder than normal – it is working efficiently to pump more blood with each beat.
- your heart rate may increase by about 15 beats per minute
- blood volume increases throughout pregnancy
- plasma can increase by 40-50% and red blood cells increase by 20-30% which can mean you need more iron and folic acid
Throughout pregnancy, the air going in and out of your lungs increases by about 50% with higher blood oxygen levels so you could consume 10-20% more oxygen.
The shape of your chest can change during pregnancy as your diaphragm can rise by 4cm, which can reduce your lung capacity by about 5%
Your cervix and uterus
Your cervix acts as a barrier against infection during pregnancy and it protects your baby by staying firmly closed until labour starts. The hormone progesterone causes the cells of the cervix to create mucus, which forms the mucus plug.
Your uterus will contract throughout pregnancy – these Braxton Hicks contractions will help to boost the blood circulation to the placenta.
Nausea and vomiting
This can affect about 70% of pregnant women and it is usually caused by relaxed stomach muscles and increased levels of oestrogen.
About 20% of pregnant women will experience nausea and vomiting throughout their pregnancy.
About 50% of pregnant women will experience an increased appetite.
Heartburn is experienced by a lot of pregnant women – it is thought that between 30-70% of women will have heartburn at some point during their pregnancy.
It is caused by progesterone, which relaxes the oesophageal sphincter.
Weight gain during pregnancy
The expected weight gain in pregnancy is approximately 4kg/9lb during the first 20 weeks and 8.5kg/1 stone 5lb in the last 20 weeks.
This is a rough guide to weight-gain and it is important to remember that babies vary in size and some women will gain more weight than others. Talk to your midwife or doctor if you have any concerns about weight gain or weight loss during your pregnancy.
- Boobs: 0.4kg (1lb)
- Fat: 3.5kg (12lb)
- Placenta: 0.6kg (1lb5oz)
- Baby: 3.4kg (7lb 7oz)
- Amniotic fluid: 0.6kg (1lb 5oz)
- Increased uterus size: 1kg (2lb 3oz)
- Increased blood volume: 1.5kg (3lb 5oz)
- Extra fluid: 1.5kg (3lb 5oz)
Half of all pregnant women will experience deeper colouring on their face. It is more common in dark haired women.
This increases by about 0.5C – due to increased progesterone and
The anterior pituitary gland can double in size, which can be the cause of
headaches during pregnancy.
Your breasts may swell due to increased blood supply and hormones:
- oestrogen = growth of lactiferous tubes and ducts as well as fat
- progesterone= alveoli buds and their ability to secrete milk
- prolaction = production of colostrum
Hormones in labour
For your baby to be born, the muscles of your womb need to contract and your cervix needs to soften and open. A balance of hormones working together makes this happen.
- Oxytocin stimulates the uterus to create contractions. With a high level of oestrogen, your body creates prostaglandins which helps to soften and open your cervix.
- Relaxin levels increase in labour to help soften and improve the flexibility of your cervix, uterus and pelvis to aid the birth of your baby.
- Beta-endorphins – your natural opiate – is released to help you manage the contractions. It aIso enables the production of prolactin and
- prepares your baby’s lungs for birth.
- Adrenaline – high levels are produced when birth is close. This is to give you a surge of energy, to make you more alert and to create powerful contractions to birth your baby.
- Prolactin – this is produced throughout pregnancy but levels peak when your baby is close to being born. Levels then stay high to produce milk while you breastfeed.
Hormones after labour
- Oxytocin continues to be produced after your baby is born, to create contractions for the delivery of your placenta. Along with prolactin, it helps to boost your feelings of happiness and love so you will protect and look after your baby.
- Beta-endorphins are also crucial for breastfeeding – levels peak in your body about 20 minutes after giving birth.
- Prolactin is produced to enable your breasts to make milk for your baby. With oxytocin and beta-endorphins, it also helps to create euphoria in new mums.
And this is just the basics – there’s a lot going on in your body during pregnancy, labour and shortly after birth. If you are ever worried or concerned about anything, seek some medical reassurance from your midwife or your GP.
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