Skip to main content Skip to Search Box

Definition: menstrual cycle from The Penguin Dictionary of Science

The approximately four-weekly ➤oestrous cycle in human females associated with the release of an egg from the ➤ovary (ovulation) and the preparation of the wall of the ➤uterus for a fertilized egg. The cycle is under the control of ➤hormones secreted by the ➤pituitary gland (➤follicle stimulating hormone and ➤luteinizing hormone) and the ovaries themselves (primarily oestrogen and progesterone). If no fertilized egg is present, the lining of the uterus breaks down and passes out through the ➤vagina over a period of five or six days (a ‘period’) during menstruation.

Summary Article: menstrual cycle
From Philip's Encyclopedia

In humans, and some higher primates, of reproductive age, the stage during which the body prepares for pregnancy. In human women, the average cycle is 28 days. At the beginning of the cycle, hormones from the pituitary gland stimulate the growth of an ovum contained in a follicle in one of the two ovaries. At approximately mid-cycle, the follicle bursts, the ovum releases (ovulation) and travels down the Fallopian tube to the uterus. The follicle (now called the corpus luteum) secretes the hormones, progesterone and oestrogen, during this phase, and the endometrium thickens, ready to receive the fertilized ovum. If fertilization (conception) does not occur, the corpus luteum degenerates, hormone secretion ceases, the endometrium breaks down and menstruation occurs. In the event of conception, the corpus luteum remains and maintains the endometrium with hormones until the placenta is formed. In humans, the onset of the menstrual cycle occurs at puberty; it ceases with the menopause (around 50 years of age). See also ovary

menstrual cycle

The changes occurring during the menstrual cycle are controlled by the balance of

the follicle stimulating hormone (FSH) and luteinizing hormone (LH) secreted by the pituitary. The diagram shows the changing levels of these, and of oestrogen and progesterone induction from the ovarian follicle, together with changes in the structure of the uterine wall (A) and development of the follicle (B), in a circular form through a normal 28-day cycle. The sharp increase in LH at about mid-cycle causes ovulation (C) and, if fertilization does not occur, the corpus luteum (D) formed

degenerates around day 26 as pituitary hormone levels fall. The consequent withdrawal of oestrogen and progesterone causes the uterine wall to shed itself in the menstrual flow.

The wall proliferates again under the influence of oestrogen from a new follicle. If fertilization and egg implantation do occur the placenta produces chorionic gonadotrophin, possibly as early as day 21, that allows the corpus luteum to continue to produce oestrogen and progesterone until the placenta takes over.

Copyright © 2007 Philip's

Related Articles

Full text Article oestrous cycle or estrous cycle
Collins Dictionary of Biology

a reproductive cycle caused by the cyclic production of gonadotrophic hormones by the PITUITARY GLAND . It occurs in adult female mammals and...

Full text Article Menstruation
Encyclopedia of Women's Health

The menstrual cycle is a combination of a series of events occurring between a part of the brain called the hypothalamus, the pituitary gland in...

Full text Article Menstrual Cycle
Encyclopedia of Abortion in the United States

The menstrual cycle is a process that prepares the uterus for pregnancy. This process begins at puberty and occurs about every 28 days (it may vary

See more from Credo