For some women this appears to happen quickly, whilst for others it may take some time.

Out of every 100 couples trying for a baby, 80 to 90 will get pregnant (conceive) 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 menstrual cycle and periods work.

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 before the first day of your next period. An egg is viable for about 12-24 hours after it's released.

For a  pregnancy to begin, an egg  and sperm must fuse within this time. If you want to get pregnant, having sex every couple of days will increase the chances of an egg (ovum) and many thousands of sperm cells coming into contact in the Fallopian tube where conception usually occurs.

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 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 and function normally. 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 – now all together called semen. 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. Semen travels 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 umbilicus (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 being born.
• Womb or uterus –  the uterus 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.
• Fallopian tubes – these lead from the ovaries to the uterus. Eggs are released from the ovaries into the fallopian tubes (called ovulation), and this is where fertilisation takes place. Typically this occurs most months of a woman’s life whilst she menstruates (but not when taking the pill, even though there may still be menstruation)
• Ovaries – there are two ovaries, each about the size of an almond, and they produce the eggs, or ‘ova’.
• Cervix – this is the neck of the uterus. It is normally almost closed, with just a small opening through which blood passes during the monthly period. During labour, the cervix dilates (opens) in order 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 birth.

Menstruation (periods)

A majority of women ovulate 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 uterus 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 there has been recent sexual intercourse with ejaculation by the man, it is in the Fallopian tube that the highest chance of successful fertilisation occurs. The lining of the uterus is now thick enough for the egg to be implanted in it after fertilisation.

If the egg is not fertilised, it passes out of the body during the woman's monthly period, along with the lining of the uterus, which is also shed. The egg is so small that it cannot be seen to the naked eye except under a microscope.

Hormones

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. In women, the sex hormones, oestrogen and progesterone, control many of the events of the menstrual cycle, such as the release of the egg from the ovary, the thickening of the uterus lining, and receptivity of the uterus for implanting of the an embryo.

A pregnancy test measures these hormones especially those that rise after conception.

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 uterus lining to build up, the blood supply to your uterus and breasts to increase and the muscles of your uterus to relax to make room for the growing baby.

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 pair with the 23 from the mother, making 46 in total.

X and Y chromosomes

Chromosomes are tiny threadlike structures made from DNA, which each carry human 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 a woman’s egg is always the same and is known as the X chromosome. But the sex chromosome from a man’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).

Healthy lifestyle

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. Seeking health advice from a health professional in helping you prepare for getting pregnant may assist.