Understanding ovulation and the fertile window

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Produced by The Fertility Society of Australia: Pre-Conception Health Special Interest Group and Your Fertility
Last updated November 2015

When you want to have a baby you can improve your chance of getting pregnant if you know about ovulation and the ‘fertile window’ in the menstrual cycle. Knowing when you are most fertile and making sure you have sex during the right time could even save you a trip to the fertility clinic-why have IVF treatment if you can do it yourself?

Facts about the menstrual cycle

The menstrual cycle is regulated by hormones. It has three phases: the follicular phase, ovulation, and the luteal phase. The pituitary gland in the brain produces two hormones which are vital for the development and release of eggs; the follicle stimulating hormone (FSA) matures follicles which contain eggs and the luteinising hormone (LH) stimulates the release of a mature egg from the follicle.

The follicular phase begins on the first day of a period (cycle day 1). During this phase there is an outpouring of FSA from the pituitary, which stimulates a cluster of follicles containing eggs to begin to grow. One of these becomes the leading follicle and when the egg inside it is mature and ready to be fertilised, a spike in LH from the pituitary causes the follicle to rupture and the egg is released into the Fallopian tube. This release is ovulation. The two weeks after ovulation is the luteal phase. It is during this time that an egg can be fertilised and form an embryo which implants in the uterus and grows into a fetus. If the egg is not fertilised, the lining in the uterus sheds and a period starts.

The fertile window

Knowing when ovulation happens is critical when you want to get pregnant because the window of opportunity to conceive is fairly small every month. As shown in the graph, conception is only possible from about five days before ovulation through to the day of ovulation. These six days are the “fertile window” in a woman’s cycle and reflect the lifespan of sperm (five days) and the lifespan of the egg (24 hours). But the likelihood of conceiving is dramatically increased if sex occurs in the three days leading up to and including ovulation. If a woman has sex six or more days before she ovulates, the chance she will get pregnant is virtually zero. Then, the probability of pregnancy rises steadily and is 27-33% in the three days leading up to and including ovulation. From that point, the probability of pregnancy declines rapidly. Twelve to 24 hours after ovulation, a woman is no longer able to get pregnant during that cycle.

If all this seems too complicated, an alternative is to have sex every two to three days. That way all bases are covered without getting too technical about when the chance of conceiving is greatest.

How do I know when I ovulate?

Knowing how long your cycles are and the physical changes to watch for will help you pinpoint this. A cycle is counted from the first day of one period to the first day of the next period. Some women have shorter and some have longer cycles and timing of ovulation depends on the length of the cycle. If on average you have a period every 28 days you ovulate around day 14 and your best chance of conceiving is between days 11 and 14. But if you have a shorter interval between periods, say 24 days, ovulation happens around day 10 and your ‘fertile window’ is between days 7 and 10. If on the other hand you have 35 days between periods, you should focus your baby making efforts between days 18 and 21.

On our timing webpage you can find an ovulation calculator to help you work out when you are likely to ovulate. Knowing your body and how it changes when ovulation approaches is also important. A few days before ovulation the vaginal mucus changes and becomes clear and slippery; a bit like egg white and perfect for sperm to swim along! Mucus changes provide an early and useful cue that ovulation is approaching.


Using an ovulation kit

If you want to reassure yourself about getting your timing right you can use ovulation predictor kits which are available at any pharmacy or supermarket. A few days before you think you will be ovulating you start testing your urine each day. You can expect to ovulate 24-36 hours after the test turns positive so make sure you and your partner are ready then!