Published Mon, 2012-02-13 16:37; updated 1 year 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





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 female sexual 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.


The woman’s internal organs are made up of:



  • 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.









The woman’s monthly cycle



The egg begins to travel slowly down the fallopian tube.



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.


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.


Some time after her period she will ovulate, and then 10-16 days after this, she'll have her next period.


Due for review February 2013

NHS Choices