human reproduction

Production of offspring by humans. Human reproduction is an example of sexual reproduction, where the male produces sperm and the female eggs. These gametes contain only half the normal number of chromosomes, 23 instead of 46, so that on fertilization the resulting cell has the correct genetic complement. Fertilization is internal, which increases the chances of conception; unusually for mammals, pregnancy can occur at any time of the year. Human beings are also remarkable for the length of childhood and for the highly complex systems of parental care found in society. The use of contraception and the development of laboratory methods of insemination and fertilization are issues that make human reproduction more than a merely biological phenomenon.

The menstrual cycle occurs in female mammals of reproductive age. It prepares the female body for pregnancy. At the beginning of the cycle an egg (ovum) begins to develop in the ovary. At the same time the inner wall of the uterus forms a soft spongy lining. The egg is then released from the ovary, while the uterus lining (endometrium) becomes filled with blood vessels. At this stage fertilization can take place. However, if fertilization does not occur, the lining of the uterus breaks down, and is shed. This is the loss of blood that marks menstruation. The cycle then begins again.

The cycle is controlled by hormones such as oestrogen and progesterone. Human menstruation takes place from puberty to menopause, except during pregnancy, occurring about every 28 days.