If the egg is fertilised, progesterone is vital in maintaining the pregnancy. During menopause, the ovaries' production of both oestrogen and progesterone decreases. The drop in hormones is thought to be responsible for a range of menopausal 'symptoms'. HRT usually involves a combination of synthetic ostrogen and progesterone. Subsequently, synthetic progesterone was combined with oestrogen to protect the endometrium.
Synthetic progesterone differs in composition from the progesterone naturally found in the body.
Synthetic progesterone, derived from 'natural' progesterone, is chemically altered in a laboratory, resulting in the addition of other molecules. The altered molecular structure of progestins, however, appears to cause numerous side effects. Women taking progestins may experience weight gain, fluid retention, headaches, irritability and depression. Some progestins also have adverse effects on cholesterol levels. When they are taken with oestrogen in hormone replacement therapy, the progestins may reduce the cardiovascular benefits that oestrogen is believed to provide.