When does ovulation happen?
It's difficult to pinpoint exactly when ovulation happens, unless you're practising fertility awareness (FPA guide to natural family planning). However, ovulation usually happens 10-16 days before the start of a woman's next period.
The menstrual cycle is counted from the first day of a womans period. This is day one. Some time after her period she will ovulate, then 10-16 days after this she will have her next period. The average cycle takes 28 days, but shorter or longer cycles are normal.
Its not accurate to say that women are fertile on day 14 of the menstrual cycle, says Toni Belfield, a specialist in sexual health information and a trained fertility awareness teacher. Being fertile on day 14 might be true for women who have a regular 28-day cycle, but it wont apply to all women.
If your cycle is 35 days long, you're not going to be fertile on day 14 of your cycle. Youre more likely to be fertile around days 19 to 25. If you have a shorter cycle for example, 23 days you might ovulate around days 7 to 13."
Sperm can survive in a woman's body for several days after sex. If you want to get pregnant, having sex every couple of days will mean there are always sperm waiting in the fallopian tubes to meet the egg when it's released.
Source:
http://www.nhs.uk/Livewell/menstrualcycle/Pages/WhencanIgetpregnant.aspx