Steroid sex hormones, derived from cholesterol, which are produced mainly in the ovaries and placenta. This group of hormones, which includes oestradiol (the most active female hormone), oestriol, and oestrone, not only promote maturation of the female reproductive system but are important for controlling the menstrual cycle, production of ova, and maintenance of pregnancy.

— Saffron A. Whitehead

See sex hormones; steroids.

The Oxford Companion to the Body. Copyright © 2001, 2003 by Oxford University Press. All rights reserved.