Combined pill mechanism of action
The combined pill, a daily tablet containing an oestrogen and a progestogen (a hormone that acts like progesterone), works mainly by stopping ovulation, the release of an egg.
Remember
The combined pill works mainly by negative feedback on the hypothalamus and pituitary gland, which suppresses the mid-cycle surge of luteinising hormone (LH, a hormone that triggers ovulation), so no egg is released.
A chain of hormone signals called the hypothalamic-pituitary-ovarian axis runs from the brain to the ovary. The hypothalamus releases gonadotrophin releasing hormone (GnRH, the brain's signal to the pituitary), and the pituitary gland responds by releasing follicle stimulating hormone (FSH) and LH. FSH makes the follicles, the small egg-containing structures in the ovary, grow and mature. Ovulation is normally triggered by a sharp rise in LH in the middle of the cycle, the mid-cycle LH surge.
Because the pill supplies its oestrogen and progestogen at steady levels, the usual rise and fall that drives the cycle is lost. The pill acts by negative feedback: a raised hormone level tells the brain to make less of the signals that drive the ovaries. The hypothalamus therefore releases less GnRH, so the anterior pituitary gland releases less FSH and less LH.
Without the surge, ovulation does not occur, so the ovary does not release an egg for sperm to fertilise. Suppression of ovulation is the primary mechanism of the combined pill. The pill is over 99% effective only if it is taken correctly all the time, and missed pills make it less effective.
The pill also thickens the cervical mucus, the fluid at the neck of the womb, and thins the endometrium, the lining of the womb. These are back-up effects, not the main action.
Key facts
- Main action
- oestrogen and progestogen suppress FSH and LH by negative feedback, so there is no LH surge and ovulation does not occur.
- Site of action
- the pill suppresses the hypothalamic-pituitary-ovarian axis, which runs from the brain to the ovary.
- Timing
- the pill acts before ovulation, so no egg is released.
- Secondary effects
- thicker cervical mucus and a thinner endometrium add to the effect but are not the primary mechanism.
- Progestogen-only pill
- thicker cervical mucus is the main action of the traditional progestogen-only pill instead.
- Reversible
- fertility usually returns to its previous level about a month after stopping the pill.
Exam traps
- Thicker cervical mucus is a real extra effect of the combined pill, but it does not prevent ovulation. In the traditional progestogen-only pill it is the main action instead.
- Acting after fertilisation is not how the combined pill works: it acts mainly before fertilisation by stopping ovulation, and a review found no scientific evidence that preventing implantation actually results from it.
- Prolactin, a pituitary hormone that stimulates milk production, can suppress ovulation when another cause raises it, such as a prolactinoma (a prolactin-secreting pituitary tumour). The pill does not work by raising prolactin: it works by negative feedback from its oestrogen and progestogen.
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