Progesterone is a hormone — a chemical messenger the body makes. It is one of the main hormones of the female monthly cycle, though other bodies make a little too. After an egg leaves the ovary, progesterone rises. It gets the soft inside of the womb ready, so that if a baby starts, there is a place to grow. If there is no pregnancy, the level falls, and a period begins. During pregnancy it stays high and helps keep the womb calm. Doctors also put progesterone, or close copies of it, in birth-control pills (they help stop the monthly egg), and sometimes use it as medicine when periods are irregular or when an early pregnancy needs support. Though actually, people lump ‘female hormones’ together. Oestrogen (also spelled estrogen) and progesterone work as a pair, but they are not the same job: oestrogen builds and changes many tissues; progesterone is the one especially tied to ‘ready for a baby’ and to holding a pregnancy. You do not need the chemistry. You need: body chemical, monthly cycle and pregnancy, also in the pill.