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If your partners periods are regular, ovulation days can be determined by calculation method of family planning. Alternatively basal body temperature
recorded everyday can be used to determine ovulation days.
Unprotected sexual intercourse without during the days of ovulation are most likely to be fruitful. Avoid unprotected intercourse
during these days.
If you are unable to determine your partner's ovulation days, then in general unprotected sexual intercourse during period days and few days before and after periods are least likely to cause pregnancy
Hope it is clear to you.