The male and female parts of the human reproductive system can bring together an egg and a sperm so that they join and begin the development of a new human being. The female system also protects and nourishes the developing foetus during the nine months of pregnancy. About 150,000 eggs are present in a girl’s ovaries at birth; however, sperm production in boys begins only at the start of puberty.
Sperm are made in the testes and mature in a coiled tube, the epididymis. At the end of sexual intercourse, they travel along the vas deferens and mix with fluids made by the prostate gland and seminal vesicle. The sperm then pass along the urethra to the tip of the penis and into the female vagina. From there they begin their journey up the female reproductive tract.
From puberty, an egg is released monthly from one of the ovaries and passes along the adjoining fallopian tube towards the uterus. The ovaries also produce female sex hormones. The uterus is the home for the foetus during pregnancy. The outlet of the uterus is called the cervix. It stretches wide open during childbirth to allow the baby to pass through.
Millions of sperm leave the penis during sexual intercourse and are deposited in the vagina. From there they travel through the cervical canal into the uterine cavity and then into the fallopian tubes. About 500 sperm reach the tubes. If one fertilizes an egg, the tiny fertilized egg begins to grow by dividing – into two cells, then four, and so on. As it divides, it travels along the fallopian tube to the uterus. There, it embeds in the uterine lining as an embryo.
Fertilization occurs in the outer part of the fallopian tube. Once they reach the egg, the sperm release substances that allow them to break through the egg’s outer layers. Only one sperm penetrates the egg. The egg, and the sperm that successfully penetrates it, each provide half the genetic information needed to form a new individual.
During birth, the opening or canal in the mother’s cervix widens, and the uterus contracts. As the baby moves down, first its head and then the rest of the body passes through the mother’s cervical canal and vagina to the outside world.
A fertilized egg divides to form a ball of cells called a morula. This reaches the uterus a few days after fertilization, moved along by wafting cilia, hairlike structures in the lining of the fallopian tube. The early embryo then embeds itself in the uterine lining.
By this time, the embryo’s cells have developed and grouped together to form tissues and organs. Most organs are formed by eight weeks, and the embryo is now called a foetus. At 10 weeks, the foetus is 5 cm (2 in) long and has facial features and limbs.
At about this time the mother may start to feel the baby move. The umbilical cord connects the foetus to the placenta. This pad of tissue attached to the wall of the uterus supplies nutrients to the foetus and removes waste.
The baby is growing fast now and maturing in preparation for its journey into the outside world.