“Ulipristal acetate is a synthetic medicine which binds to the receptor for progesterone* (an area on or in the cell to which progesterone attaches), thereby blocking the action of progesterone because its receptor is blocked. It is often called an anti-progesterone (or anti-progestin) or sometimes a progesterone receptor blocker,” says Professor Anna Glasier**, a world expert in emergency contraception.
**Anna Glasier does not endorse any products or brands.
Progesterone is a hormone which play an important role in the menstrual cycle and pregnancy.
Basically, the menstrual cycle has three phases: follicular, ovulatory, and luteal. The follicular stage begins on the first day of your period. During this phase, your body prepares for ovulation (egg release) by releasing follicle stimulating hormone (FSH). This is the hormone which helps with the development of eggs in the ovaries. By around day 6, a single egg in your ovaries becomes dominant and continues growing. Once this has happened, your levels of FSH decrease.
The ovulation phase happens around midway through the cycle when you get a surge of luteinizing hormone (LH). LH is the hormone which triggers the release of an egg from your ovaries to make its way to your uterus. Ovulation usually happens around day 14 of your cycle, but this can vary from person to person and from cycle to cycle.
This is why it can be really hard to pinpoint the exact moment of ovulation, as ovulation can happen from day 11 all the way up to day 21, and just because you ovulated on day 12 one month doesn’t mean it’ll happen on the same day the next month. Find out more about what can affect your menstrual cycle here.
One released, the egg can survive for 12 to 24 hours, and if it isn’t fertilised during this time it dissolves.
During the luteal phase, increases in progesterone and oestrogen allow your body to begin to prepare for implantation in case the egg that has been released is fertilised with sperm. Progesterone is responsible for the lining of your uterus, called the endometrium. The rise in progesterone is telling your uterus to start thickening this lining in preparation for a potential pregnancy.