New year guide to new life
In this New Year guide, Teen AIDS explains one of the fundamental facts of life - how humans come to be born.
During adolescence, boys and girls undergo a biological development of the sex organs.
A male's testes mature inside the scrotum and produce sperm, while in females, the ovary discharges mature eggs about once a month.
When a male has sexual intercourse, seminal fluid containing sperm and a mixed secretion of the scrotum and other sex glands is ejaculated into the female's sex organ via the urethra and penis.
During sexual intercourse, the male's penis becomes erect and the female vagina extends and produces secreta to make it easier for the penis to penetrate.
In the female body during sexual intercourse, eggs move downwards along the oviduct. An egg will become fertilised if combined with one of the millions of sperm contained in the seminal fluid.
Once an egg is fertilised, it moves along the oviduct and continually splits and grows. It reaches the uterus and hides in the endometrium, where it becomes an embryo, absorbing food from the mother.
After 40 weeks, the uterus contracts severely, opening the uterine cervix, where the penis entered during sex.
The embryo, now a foetus, moves towards the vagina and is delivered as a newborn.