The hair of the scalp is, however, most sensitive to the effects of male hormones (androgens), which are in the blood of people of both sexes (only in different proportions in men and women). Androgens are the most important factor regulating hair growth, and also the thickness of the hair shaft. Female hormones (oestrogens), which both sexes have too, slow down hair growth during the growing period but also make that period longer. Many women notice a difference in their hair growth during pregnancy. At this time women have vast amounts of oestrogen in their blood, far outweighing their male hormones. It is the balance between the male and female hormones which decides the growth of the hair.