Pregnancy can only occur at the time of ovulation. It's very important that sperm is in the reproductive tract before ovulation occurs or on the day of ovulation. Sperm introduction into the reproductive tract after ovulation will not result in pregnancy.
In order for a woman to determine when she's ovulating, there are various means. One is simple calendar check. Woman who know they have, for example, a regular 28-day cycle should always believe that their ovulation is going to occur approximately 14 days prior to their next period. So a woman who has a 30-day cycle will probably be ovulating around day 16. A woman with a 32-day cycle, ovulating approximately day 18.
Other ways to check are slightly more complicated, but very useful. One is what is called a basal body temperature chart. Women can start checking their temperatures a few days before their presumed ovulation, and when there is a sustained rise in temperature, that usually indicates that ovulation has occurred. This is a great method for determining ovulation patterns, but not for that particular month. Once the temperature rises, it's over. Ovulation has already occurred.
Other ways include using pharmacy ovulation prediction kits, or are called OPKs, ovulation prediction kits. These kits are used to determine when the ovulatory event is about to happen. They pick up a certain hormone, called LH, in the urine shortly before ovulation occurs. Once that color change or double line is indicated on the kit, ovulation is likely to occur 24 to 36 hours later.
Other ways to determine ovulation include ultrasound examinations. A follicle is shown to be released by ultrasound, and the timing of ovulation can therefore be timed. Also, biopsies of the uterine lining are used in certain cases to determine if ovulation has occurred. The only true proof of ovulation, however, is pregnancy.