Coitus

Sexual union of a male and a female; also called sexual intercourse.

Coitus begins when the male inserts his erect penis into the female vagina, a canal leading to the uterus (womb) and fallopian tubes along the sides of the womb.

The male then begins thrusting.

This stimulation creates intense pleasure for both partners and usually builds to a peak of excitement called orgasm (climax).

During orgasm, the walls of the vagina pulsate and the male ejaculates semen containing approximately 250 million spermatozoa (sperm) into the female.

The partners may experience orgasm separately or simultaneously, and sometimes a partner does not achieve orgasm at all.

If the female has ovulated -- that is, she has released an egg for fertilization -- the ejaculated sperm will seek to fertilize it.

Only a small number of spermatozoa will actually reach the site of the egg, one of the two fallopian tubes between the ovaries and the womb.

The fallopian tubes deliver the egg to the womb over a three-to-five day period.

If a sperm cell fertilizes the egg as it moves through a fallopian tube, a new cell (zygote) develops to begin the formation of embryonic life in the womb.

A related term, 'coitus interruptus,' refers to withdrawal of the penis from the vagina before ejaculation occurs.

'Coitus' is derived from the Latin word 'coire' (to come together).