Oestrogens are the primary female sex hormones, mainly produced by the ovaries, though smaller amounts are made in the adrenal cortex. They are a group of steroid compounds, named for their importance in the estrous cycle, their name comes from estrus/oistros (period of fertility for female mammals) + gen/gonos = to generate. They are needed for various body processes apart from reproduction, and are found in both men and women, though in different proportions. Deficiencies are involved in various menstrual and reproductive problems, and the lowering of oestrogen levels after the menopause contributes to the aging process, particularly to osteoporosis (weakening and thinning of the bones) which is very common among old people.

Like all steroid hormones, oestrogens readily diffuse across the cell membrane. Once inside the cell, they bind to and activate oestrogen receptors which in turn up-regulate the expression of many genes. Additionally, estrogens have been shown to activate a G protein-coupled receptor, GPR30.

Some plants are known to contain plant oestrogens which are valuable in supplementing the body's own hormones. The most important are hops and catkins {neither of them used in aromatherapy) and Fennel which we do use extensively as an essential oil.

While oestrogens are present in both men and women, they are usually present at significantly higher levels in women of reproductive age. They promote the development of female secondary sex characteristics, such as breasts, and are also involved in the thickening of the endometrium and other aspects of regulating the menstrual cycle. In males, oestrogen regulates certain functions of the reproductive system important to the maturation of sperm and may be necessary for a healthy libido.

Fennel oil, and the corresponding herb tea, can be very helpful in premenstrual tension and menopausal disturbances, and it is recommended that older women take the tea regularly to minimise loss of elasticity of the skin and other connective tissues, as well as the serious bone degeneration already mentioned.