Conception is when a woman's egg is fertilised by a man's sperm, and then implants itself into the woman's womb. Most couples don't have a problem getting pregnant.
For most couples, regular unprotected sex is all it takes to conceive a child.
If you're trying for a baby or thinking of doing so in the future, knowing the basic facts about fertility can be helpful.
The monthly cycle
Every month, hormonal changes in a woman's body cause the ovaries to release a single egg. This egg passes into the fallopian tubes, which link the ovaries to the uterus (womb).
At the same time, the lining of the womb thickens. This is to prepare it for the possibility of receiving a fertilised egg.
If fertilisation does not occur, the womb lining will break down and will be shed through the vagina. This is a woman's period. The period is made up of the womb lining and a small amount of blood.
Women of childbearing age have a period approximately every 28 days, although the length of the cycle can vary and between 24 and 35 days is common.
If a woman has unprotected sex with a man around the time of her egg being released, sperm from her partner may fertilise her egg while it is in the fallopian tube. The fertilised egg will then travel to the womb and become embedded in its lining, where it will start to grow.
Find out more about periods and the menstrual cycle.
How conception happens
An egg can be fertilised by sperm during the 12 to 24 hours after it has been released from the ovaries.
Sperm can survive in the fallopian tubes for up to seven days, so fertilisation can occur even if sperm entered the fallopian tubes before an egg was released.
During conception, a single sperm from a man penetrates the egg of a woman. The sperm carries the father's genes, while the mother's genes are contained in the egg. Once the egg has been fertilised by a single sperm, no more sperm can enter.
The fertilised egg, called a zygote, continues to move down the fallopian tubes, until it reaches the womb. Here, it will implant itself into the lining of the womb (about 6-10 days after ovulation), where it begins to grow.
Until eight weeks after conception, the implanted zygote is called an embryo. After this it is called a foetus.
For most women, the first sign that they are pregnant is a missed period. A few days after that missed period, a urine test can confirm the pregnancy.
Urine tests for pregnancy are available through your GP or family planning clinic. You can buy a test to do at home at your local pharmacy or supermarket.
Read more about doing a pregnancy test.
Article provided by NHS Choices