Knowing your menstrual cycle — Calculating your fertility

The calendar calculation method is a simple, though not very reliable, way to estimate when you are likely to be most fertile. To use this method, count the days of your cycle for at least 6 months. Subtract 20 from the length of your shortest cycle to estimate the first day of your fertile window. Then subtract 10 from your longest cycle to estimate the last day of your fertile window.

Example: If over 6 cycles, a woman’s shortest cycle was 26 days and her longest was 29 days, her calculation would look like this:

(shortest cycle) 26 – 20 = 6
(longest cycle) 29 – 10 = 19

This suggests her fertile window may be between days 6 and 19 of her next cycle.

Fertility monitoring kits

There are fertility monitoring kits on the market that may be useful in helping to identify the fertile and infertile days of your cycle. These devices tend to be expensive and some women find them difficult to read. But for women who want to gather as much information as possible (whether to get pregnant or avoid getting pregnant) or for women who find it uncomfortable to check their cervix or cervical mucus, these kits may be helpful.

Hormone monitoring devices measure hormone levels in urine to predict ovulation and determine which days you are likely to be fertile. The kit stores this information and builds up a record, or profile, of your personal cycles. The data is then used to help predict fertility.

Hormone monitoring devices are not recommended for women who have short cycles, long cycles, are breastfeeding or have Polycystic Ovarian Syndrome (PCOS). Read the package instructions before buying or using a monitoring kit.

Computerised thermometers and saliva tests are also available, but there is limited evidence about their accuracy.