Hormones in human reproduction
Sex hormones drive puberty and the menstrual cycle, and form the basis of most modern contraceptives and fertility treatments.
Puberty
During puberty, the body produces reproductive hormones that cause secondary sexual characteristics to develop.
- Oestrogen — the main female reproductive hormone, produced in the ovaries. At puberty, eggs begin to mature and one is released approximately every 28 days (ovulation).
- Testosterone — the main male reproductive hormone, produced in the testes. Stimulates sperm production.
The menstrual cycle (≈ 28 days)
Four hormones interact to control the cycle:
- FSH (Follicle-Stimulating Hormone) — secreted by the pituitary. Causes an egg to mature in an ovarian follicle and stimulates the ovaries to release oestrogen.
- Oestrogen — secreted by the ovaries. Causes the lining of the uterus (the endometrium) to thicken. Inhibits FSH and stimulates LH (HT).
- LH (Luteinising Hormone) — secreted by the pituitary. The surge in LH around day 14 triggers ovulation.
- Progesterone — secreted by the empty follicle (the corpus luteum). Maintains the uterine lining. Inhibits FSH and LH (HT). When progesterone falls, the lining breaks down (menstruation, days 1–5 of the cycle).
A useful examining diagram shows hormone levels across the cycle:
- FSH: peak early.
- Oestrogen: rises through follicular phase, peaks just before ovulation.
- LH: sharp peak around day 14.
- Progesterone: rises after ovulation, falls if no implantation.
Hormonal contraception
Contraceptive methods use sex hormones (or barriers) to prevent fertilisation or implantation.
- Combined pill — contains oestrogen and progesterone. The continuous dose suppresses FSH and LH, so eggs do not mature or release. >99 % effective when used correctly. Risks: small increased risk of blood clots and breast cancer; doesn't protect against STIs.
- Progesterone-only pill / implant / injection / IUS — thicken cervical mucus to block sperm and thin the uterine lining.
- IUD (copper coil) — non-hormonal; prevents implantation.
- Barrier methods (condom, diaphragm) — prevent sperm reaching the egg. The condom also protects against STIs.
- Sterilisation (vasectomy / tubal ligation) — surgical, permanent.
- Natural / abstinence methods — track ovulation; only ~75 % effective.
You should be able to compare effectiveness, side effects, and protection against infection.
Treating infertility
Some couples cannot conceive naturally. The two main approaches in the spec:
FSH and LH injections. Used when a woman doesn't ovulate. FSH stimulates eggs to mature; LH triggers their release. Pros: relatively cheap; can lead to natural pregnancy. Cons: doesn't always work; risk of multiple pregnancies (twins, triplets).
IVF (In Vitro Fertilisation).
- The mother is given FSH and LH to mature multiple eggs.
- Eggs are collected from the ovaries.
- Eggs are fertilised by sperm in a laboratory dish.
- The fertilised eggs develop into embryos.
- One or two embryos are inserted into the mother's uterus.
Pros: allows previously infertile couples to have biological children. Cons: physically/emotionally demanding; relatively low success rate (≈ 30 % per cycle); expensive; ethical concerns over unused embryos and multiple births.
⚠Common mistakes
- FSH and LH come from the pituitary, not the ovaries. Oestrogen and progesterone come from the ovaries.
- Ovulation is the release of the egg, not the start of menstruation.
- Progesterone maintains the lining; it does not build it (that's oestrogen).
- IVF uses both FSH and LH — many students forget LH.
Links
Builds on B5.6 (the endocrine system), B6.1 (sexual reproduction and meiosis) and B6.4 (inheritance — sex determination).
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