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A Guide to Fertility

When are women & men most fertile?

There is only a specific time in each menstrual cycle when it’s possible to get pregnant. This ‘fertile window’ is once a month, generally close to the time of ovulation when the woman ovulates and releases an egg approximately 2 weeks before the next period is due. Men do not have a ‘fertile window’ because sperm is continually formed and stored in the testicles, ready to be used at any time.

The days in the cycle with the highest chance of getting pregnant

Adapted from Colombo & Masarotto (2000). Daily fecundability: First results from a new database. Demographic Research. Retrieved from

Graphic 1 shows when pregnancy is most likely to happen in people having sexual intercourse without contraception. Day ‘0’ is the day of ovulation when the egg is released. The pink section in graphic 1 shows that pregnancy is most likely to happen when sex takes place in the 3 days before ovulation. For example, the chance of pregnancy if people have sex -2 days before ovulation is 26% compared to 1% if they have sex +1 day after ovulation.

Because most women do not know on which day of the month they ovulate, contraception is recommended to avoid pregnancy.

There are ways to help predict ovulation but these methods are not a reliable from of contraception.

More Information


This guide gives you information about fertility and infertility. To help understand this information we’ve included an explanation of the main medical terms used.

  • eggs and sperm

    Cells that are produced by the ovary (eggs, oocytes, ova) and testicles (sperm) and that combine after sex to produce a pregnancy. Women produce eggs and men produce sperm. A healthy sperm is motile, which means it has the ability to move. This movement is what makes it possible for sperm to reach the egg.

  • ejaculation

    Semen is the fluid produced by the male sexual organs to protect and carry sperm. The process of discharging this fluid from the penis is called ejaculation.

  • insemination

    Treatment that involves directly inserting sperm into a woman’s womb.

  • menopause

    The menopause is the time when menstrual periods stop permanently, and women are no longer able to have children. For most women this happens at about 51 years. The age a woman will reach menopause generally be similar to the age at which her mother reached menopause.

  • menstrual cycle

    The monthly changes that occur in the female reproductive system (specifically the uterus and ovaries) which make pregnancy possible. The length of the menstrual cycle is calculated as the time from the first day of a woman’s period (bleeding) to the day before her next period or bleeding. The average time between two periods for women is about 28 days but in teenagers it could be longer (up to 45 days) and sometimes 2 to 3 months, becoming shorter as the teenager gets older. There are events that occur during the menstrual cycle which are repeated each month. These are: development of the egg (phase 1), release of the egg from one of the ovaries (phase 2), preparation of the uterus for a pregnancy (phase 3), and menstruation or bleeding (phase 4). The next period then happens if there is no pregnancy. Young women should have regular periods within 3 years of the rst period occurring. Women could have some spotting in early pregnancy.

  • ovaries

    The two oval-shaped organs located in the lower abdomen (right and left side) that produce the female eggs.

  • ovulation

    Is the release of the oocyte (mature egg, sometimes called ovum) from the ovaries, ready for fertilization. Ovulation occurs about two weeks before the next period is due, for example around day 14 of a 28-day cycle or day 21 of a 35-day cycle. The actual day of release could differ between cycles and between women, and is a affected by many factors (e.g. lifestyle).

  • testicles also called testes or balls

    Oval-shaped organs that sit in a sac that hangs behind the penis. A main job of the testicles is to make and store sperm.

‘A Guide to Fertility’ was developed and evaluated by Prof J Boivin, Cardiff University.