Getting pregnant (conception) happens when a man’s sperm fertilises a woman’s egg. For some women, this happens quickly, but for others it can take longer. Out of every 100 couples trying for a baby, 80 to 90 will get pregnant within one year. The rest will take longer, or may need help to conceive.
To understand conception and pregnancy, it helps to know about the male and female sexual organs, and to understand how a woman’s monthly menstrual cycle and periods work.
The menstrual cycle is counted from the first day of a woman’s period (day one). Some time after her period she will ovulate, and then around 12-14 days after this she'll have her next period. The average cycle takes 28 days, but shorter or longer cycles are normal.
The best time to get pregnant
You're most likely to get pregnant if you have sex within a day or so of ovulation (releasing an egg from the ovary). This is usually about 14 days after the first day of your last period, if your cycle is around 28 days long.
An egg lives for about 12-24 hours after being released. For pregnancy to happen, the egg must be fertilised by a sperm within this time. If you want to get pregnant, having sex every couple of days will mean there are always sperm waiting in the fallopian tubes to meet the egg when it is released.
Sperm can live for up to seven days inside a woman's body. So if you've had sex in the days before ovulation, the sperm will have had time to travel up the fallopian tubes to "wait" for the egg to be released. It's difficult to know exactly when ovulation happens, unless you are practising natural family planning, or fertility awareness.
The male sexual organs
The penis is made of erectile tissue. This tissue acts like a sponge and, when it becomes filled with blood, the penis becomes hard and erect. Men have two testes (testicles), which are glands where sperm are made and stored. The testes are contained in a bag of skin that hangs outside the body, called the scrotum.
The scrotum helps to keep the testes at a constant temperature, just below the temperature of the rest of the body. This is necessary for the sperm to be produced. When it's warm, the scrotum hangs down, away from the body, to help keep the testes cool. When it's cold, the scrotum draws up, closer to the body for warmth.
Two tubes, called the vas deferens, carry sperm from the testes to the prostate and other glands. These glands add secretions that are ejaculated along with the sperm. The urethra is a tube that runs down the length of the penis from the bladder, through the prostate gland to an opening at the tip of the penis. Sperm travel down this tube to be ejaculated.
The female sexual organs
A woman's reproductive system is made up of both external and internal organs. These are found in what is usually referred to as the pelvic area, the part of the body below the belly button.
The external organs are known as the vulva. This includes the opening of the vagina, the inner and outer lips (labia) and the clitoris.
The woman’s internal organs are made up of:
The pelvis: this is the bony structure around the hip area, which the baby will pass through when he or she is born.
Womb or uterus: the womb is about the size and shape of a small, upside-down pear. It is made of muscle and grows in size as the baby grows within it.
Fallopian tubes: these lead from the ovaries to the womb. Eggs are released from the ovaries into the fallopian tubes each month, and this is where fertilisation takes place.
Ovaries: there are two ovaries, each about the size of an almond; they produce the eggs, or ova.
Cervix: this is the neck of the womb. It is normally almost closed, with just a small opening through which blood passes during the monthly period. During labour, the cervix dilates (opens) to let the baby move from the uterus into the vagina.
Vagina: the vagina is a tube about three inches (8cm) long, which leads from the cervix down to the vulva, where it opens between the legs. The vagina is very elastic, so it can easily stretch around a man’s penis, or around a baby during labour.
The woman’s monthly cycle
The video below shows what happens during the menstrual cycle. Ovulation occurs each month when an egg is released from one of the ovaries. Occasionally, more than one egg is released, usually within 24 hours of the first egg. At the same time, the lining of the womb begins to thicken and the mucus in the cervix becomes thinner, so that sperm can swim through it more easily.
The egg begins to travel slowly down the fallopian tube. If a man and a woman have recently had sex, the egg may be fertilised here by the man's sperm. The lining of the womb is now thick enough for the egg to be implanted in it after it has been fertilised.
If the egg is not fertilised, it passes out of the body during the woman's monthly period, along with the lining of the womb, which is also shed. The egg is so small that it cannot be seen.
Hormones are chemicals that circulate in the blood of both men and women. They carry messages to different parts of the body, regulating certain activities and causing certain changes to take place. The female hormones, which include oestrogen and progesterone, control many of the events of a woman's monthly cycle, such as the release of the egg from the ovary and the thickening of the womb lining.
During pregnancy, your hormone levels change. As soon as you have conceived, the amount of oestrogen and progesterone in your blood increases. This causes the womb lining to build up, the blood supply to your womb and breasts to increase, and the muscles of your womb to relax to make room for the growing baby.
The increased hormone levels can affect how you feel. You may have mood swings, feel tearful or be easily irritated. For a while, you may feel that you can't control your emotions, but these symptoms should ease after the first three months of your pregnancy.
Read more about what happens in pregnancy week by week
Will it be a boy or a girl?
Both the man’s sperm and the woman’s egg play a part in determining the gender of a baby. Every normal human cell contains 46 chromosomes (23 pairs), except for the male sperm and female eggs. They contain 23 chromosomes each. When a sperm fertilises an egg, the 23 chromosomes from the father join with the 23 from the mother, making 46 in total.
X and Y chromosomes
Chromosomes are tiny threadlike structures that each carry about 2,000 genes. Genes determine a baby’s inherited characteristics, such as hair and eye colour, blood group, height and build.
A fertilised egg contains one sex chromosome from its mother and one from its father. The sex chromosome from the mother’s egg is always the same and is known as the X chromosome, but the sex chromosome from the father’s sperm may be an X or a Y chromosome.
If the egg is fertilised by a sperm containing an X chromosome, the baby will be a girl (XX). If the sperm contains a Y chromosome, the baby will be a boy (XY).
Find out about early signs of pregnancy, and where to get help if you're having problems getting pregnant.
If you’ve decided to have a baby, you and your partner should make sure you’re both as healthy as possible. This includes eating a healthy balanced diet, stopping smoking and, for the woman, taking a folic acid supplement. You should also know about the risks of alcohol in pregnancy.