Health Guide

How Does Human Reproduction Work ?

Humans reproduce when two cells (gametes), an egg (ovum) and a sperm, come together. Each gamete contributes half of the genetic material contained in the foetus. The hormones which control the production of sperm and eggs are called gonadotrophins. There are two types of gonadotrophins: Follicle Stimulating Hormones (FSH) and Luteinizing Hormone (LH). These two hormones are produced in a tiny, pea-size gland at the base of the brain called the pituitary gland. In men, they stimulate the testicles to produce sperm and testosterone. In women they act on the ovaries, where the eggs develop, producing the female sex hormones oestrogen and progesterone.

Sperm
Sperm are produced at the rate of about 300 million per day. They take some 80 days to mature. Each sperm has a head, which contains the genetic material, and a tail, which propels it up through the vagina, uterus, and fallopian tubes, to reach and penetrate the egg.

Eggs
In the woman the production of sex hormones and the release of an egg is known as the menstrual cycle. It is counted from the first day of the period ("Day 1"). In an "average" cycle of 28 days, ovulation the release of an egg from the follicle, happens on about day14. However, cycle length varies between women and it is important to note that ovulation and the fertile time occurs earlier in women with short cycles and later in women with long cycles.

At ovulation the egg is released from the ovary and picked up by the fringed end of the fallopian tube. After ovulation the empty follicle produces the hormone progesterone. This prepares the lining of the uterus (the endometrium) to retain an embryo. If an embryo does not implant, the level of progesterone drops and a period starts again. For more information on ovulation, please click here.

Conception
The mature egg survives only 24-48 hours, while sperm remain viable longer. In natural conception sperm is placed inside the vagina at the time of intercourse. The mucus in the cervix (the neck of the uterus) is slippery around the time of ovulation, which enables the sperm to swim into the uterus. The uterus then contracts in such a way as to help the sperm move up into the fallopian tube to reach the egg.

Once a sperm has penetrated the shell surrounding the egg (zona pellucida) it sets up a barrier stopping other sperm from penetrating. When the head of the sperm has released its contents into the egg, the egg is fertilised. The egg then starts to divide and becomes an embryo. Few days after fertilisation the embryo implants in the endometrium and starts to produce Human Chorionic Gonadotrophins (HCG), the hormone that causes a "Positive" pregnancy test.