To figure out your best window of opportunity for conceiving, use this formula: Record the length of your menstrual cycle for several months (i.e. 28 days, 31 days, etc…), then take the average. Subtract 14 from that number. Now, for each month, the best time to conceive is that number of days after the start of your period. So if your period comes every 23 days, subtracting 14 yields 9-so the 9th day after your period begins is the best day to make love if the goal is to get pregnant. But you should get a head start since sperm can live for three to five days inside a woman's cervical mucous. You can also use your basal body temperature to pinpoint ovulation (more on that in a moment).
If you're trying to increase your chances of conception, your best chance happens one to two days before ovulation. To maximize sperm quantity and quality, have sex every two or three days starting five days before ovulation (because sperm can live for at least 48 hours in the vagina, you'll be covered). The challenge is that the actual day of ovulation can change from cycle to cycle.
More Answers from Michael Roizen, MD