When they are ovulating, which you can track by knowing the days prior to when you normally get your period if you are on a "normal" cycle. The egg will live for 12-24 hours after leaving the ovary before it and your uterine lining are shed through your period, and sperm can live for 48 hours or so inside the body. So that gives you an optimal period of around 3-4 days to get pregnant!
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