This is an hard to answer question as every women is different. A woman should know how many days are her cycles and retract approx. 14 days and that would be her ovulation day. Now a sperm can last up to 72 hours and an ovum can last at best 24 hours. So the best time for intercourse would be 3 days prior to ovulation and 24 hours after the ovulation. Now, this is not true for me as I have a 26 to 28 day cycles and I always ovulate around day 16..see what I mean? A good thing to do is to buy an ovulation predictor kit and follow instructions, it should help her determine her time of ovulation. Hope this helps.. :)
If the woman begins to count, with the first day of her menses being "Day 1," she can learn how long it is until the next time the menses begins. For many women, this is about 28 days (however as poupoune said, every woman is different). Let us say this is a typical woman and the length of one cycle is 28 days from start of menses to the start of the next menses. In this typical woman, a woman's egg is often produced around Day 14. And it is viable for about a day.
Sperm can last in ideal conditions about 4 days. Therefore, meeting at Day 10, and then Day 12, and then Day 14 would cover a lot of opportunities for the egg.
And as poupoune said, buying an ovulation test kit and using it will give a much more specific answer, not based on averages.
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