The menstrual cycle

In human females, reproduction follows a monthly rhythm: the menstrual cycle. The first one, at puberty, is menarche. A cycle lasts on average 28 or 29 days, counted from the first day of one period to the first day of the next. Cycles stop during pregnancy and end for good at menopause, around 50 years of age.

The cycle has phases, controlled by hormones from the pituitary (LH and FSH) and from the ovary (estrogens and progesterone):

• Menstrual phase (days 1-5, roughly). If the last ovum was not fertilised, the thick lining of the uterus, the endometrium, breaks down. It leaves the body with blood: menstruation. So menstruation happens only when there has been no fertilisation.
• Follicular phase. Under FSH and LH, follicles in the ovary grow into a mature Graafian follicle, and they release more and more estrogens. The estrogens make the endometrium regrow.
• Ovulation (around day 14). LH and FSH peak in the middle of the cycle. The sharp LH surge makes the Graafian follicle burst and release the ovum.
• Luteal phase. What is left of the follicle becomes the corpus luteum (a yellow body), which secretes large amounts of progesterone. Progesterone keeps the endometrium thick and ready for an embryo to implant.

If no fertilisation happens, the corpus luteum degenerates, progesterone falls, the endometrium breaks down, and a new cycle begins with menstruation. If an embryo does implant, the corpus luteum keeps working, and there is no menstruation during pregnancy.

Missing a period can therefore be a sign of pregnancy, though it can also be caused by stress, poor health or other reasons.