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Biochemistry

Six key pregnancy hormones and their roles

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Pregnancy is a rollercoaster of shifting hormone levels which can have numerous effects. This graphic looks at six key hormones during pregnancy, their roles in the development of the baby, and other effects.

Human chorionic gonadotropin (hCG)
hCG is an important hormone in early pregnancy. It’s produced by the placenta after implantation, and supports the function of the corpus luteum. The corpus luteum is a temporary structure in the ovaries which produces other key hormones during early pregnancy.

hCG is also the hormone detected by pregnancy tests. Its concentration increases from conception and peaks 8–11 weeks after. For the first few days after conception its levels are too low to detect with pregnancy tests, but after implantation its levels double every 48 hours. There’s more on hCG and pregnancy tests in this previous graphic.

Progesterone
During the early weeks of pregnancy, the corpus luteum produces progesterone. After 8-12 weeks, the placenta takes over.
Progesterone stimulates growth of the blood vessels that supply the womb lining. It also stimulates the lining to release nutrients, providing nourishment for the early embryo. Additionally, progesterone inhibits contraction of the smooth muscle of the uterus so that it grows as the baby does.

Progesterone levels continue to rise as the pregnancy progresses. Along with oestrogen, it promotes the growth of breast tissue and milk duct development. Progesterone prevents lactations during pregnancy, which only starts when levels drop after birth. This hormone also plays an important role in preparation for birth: it strengthens the pelvic wall muscles required for labour.

Noticing increased hair growth during pregnancy? That’s also due to progesterone!

Oestrogen
As with progesterone, the corpus luteum produces oestrogen in the early stages of pregnancy before the placenta takes over. Oestrogen is actually a collective group of similar compounds: oestrone, oestradiol, and oestriol.

Oestrogen helps the uterus grow and maintains its lining. It supports foetal development, including the development of organs and bodily systems. It also activates and regulates the production of other important pregnancy hormones.

Prolactin
Prolactin is the main hormone needed to trigger the production of breast milk. It enlarges the mammary glands to prepare for this (though as previously noted progesterone levels prevent lactation until the baby is born).

Prolactin has other roles unrelated to milk production. It contributes to the development of the foetal lungs and brain, and also to the mother’s immune system tolerating the foetus.

Relaxin
Relaxin levels are highest during the first trimester of pregnancy, but it is present throughout. It has several roles, including prohibiting contraction of the uterine muscles to prevent premature birth.

Relaxin’s name gives a clue to its more important roles. It relaxes blood vessels, increasing blood flow to the placenta and kidneys. This helps the mother’s body cope with the increased demand for oxygen and nutrients from the developing baby.

Relaxin also helps the mother’s body prepare for birth. It relaxes joints in the pelvis and softens and widens the cervix to make delivery of the baby easier.

Oxytocin
Oxytocin only appears in significant amounts towards the end of pregnancy, though it is present in lower amounts before this. Its levels rise when labour starts, triggering contractions.

If labour doesn’t start naturally, oxytocin (or synthetic equivalents) can be used to induce it.

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References & Further Reading