There are different factors that affect your fertility, which include the timing of ovulation, how long the egg lasts, and the presence and duration of the sperm. A woman who has regular periods can easily calculate when ovulation occurs. The menstrual cycle of a woman is typically 28 days on the average. Menstruation occurs when an egg fails to get fertilized. Thus, it comes about 12 to 14 days after the egg is produced. If you have a consistent 28-day cycle every month, your egg is produced around day 14. In this case, the most fertile time is day 12 to day 16. If your menstrual cycle is different from 28 days on average, you need to adjust the calculations. For instance, if you have a 30-day cycle, you can expect to ovulate 16 days after the last period, which is 14 days before the next one. The bottom line is that it is important to remember that your egg is almost always produced about 14 days before the next period starts. However, if your menstrual cycle is irregular, you cannot predict when the ovulation will occur to help in planning a pregnancy effectively. You may need to use ovulation predictor kits that detect the rise in hormones occurring before the egg is released. Sperm can survive 48 hours in the vagina or the uterus. Your egg can last approximately 24 hours after ovulation so sperm need to be available during this 24 hour window. But you don't have to have intercourse within the 24 hour window because sperm can live inside you for days waiting for your egg to be released so you can get pregnant anywhere from several days before ovulation to a day after ovulation. I would suggest that you have a consultation with an OBGYN if you have any difficulty with conception or concerns about your fertility.