Published Mon, 2012-02-13 15:37; updated 4 years ago.

Conception occurs when a man’s sperm fertilises a woman’s egg.

To understand conception and pregnancy, it helps to know about the male and female sexual organs, and to understand the physical process behind a woman’s monthly cycle and conception itself.

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. 

The testes are contained in a bag of skin that hangs outside the body, called the scrotum.

It 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 (testicles) where sperm are made, 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, or the part of the body below the tummy 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:

  • 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.

  • Fallopian tubes: these lead from the ovaries to the womb. Eggs are released from the ovaries into the fallopian tubes each month. 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 in order to let the baby move from the uterus into the vagina.

  • Vagina: the vagina is a tube about three inches (8cm) long. It 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.


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.

The woman’s monthly 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's 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 can't be seen.  


Conception is the process that begins with the fertilisation of an egg and ends with the implantation of this egg into the woman's womb.

A woman is most likely to conceive just after she ovulates, when an egg has been released from one of her ovaries (halfway between her monthly periods).

During sex, sperm are ejaculated from a man’s penis into the woman’s vagina. One ejaculation can contain more than 300 million sperm.

Most of the sperm leak out of the vagina again, but some begin to swim up through the cervix. When a woman is ovulating, the mucus in the cervix is thinner than usual to let the sperm pass through more easily. Sperm swim into the womb and into the fallopian tube. Fertilisation takes place if a sperm joins with an egg and fertilises it in the fallopian tube.

During the week after fertilisation, the fertilised egg (which is now an embryo) moves slowly down the fallopian tube and into the womb.

It's already growing.

The embryo attaches itself firmly to the specially thickened womb lining.

This is called implantation. Hormones released by the embryo and the ovaries prevent the womb from shedding.

This is why women miss their period when they're pregnant. 

The best time to get pregnant

You're most likely to get pregnant if you have sex during the seven days before you ovulate.

This is usually about 12 days after the first day of your last period.

An egg lives for about 12–24 hours after it's released. For pregnancy to happen, the egg must be fertilised by a sperm within this time.

Sperm can live for up to seven days inside a woman's body.

So if you've had sex in the seven 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're practising fertility awareness (see External links).

But in most women, ovulation usually happens 10-16 days before the start of the next period.

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 10-16 days after this, she'll have her next period.

The average cycle takes 28 days, but shorter or longer cycles are normal.

Due for review February 2013

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