In a typical 28-day cycle, a woman ovulates on day 14 (on average), a process that lasts about 12 to 48 hours. But that doesn’t mean conception can’t occur outside that window.
“Theoretically, if she is bleeding for days 1–7, then it makes sense to think that she will not be able to get pregnant if she has sex while she is still bleeding,” says Dr. Alexis Melnick, assistant attending obstetrician and gynecologist at NewYork-Presbyterian/Weill Cornell Medical Center and assistant professor of reproductive medicine and obstetrics and gynecology at Weill Cornell Medicine. “However, this is not always the case. Not all cycles are the same length, so even if a woman gets a period once per month, she may have a 25-day cycle one month, a 27-day cycle the next, and a 26-day the month after — and may actually ovulate earlier than day 14. Furthermore, sperm can live for three to five days in the female genital tract. So even if ovulation happens after bleeding stops, if the sperm are still present, conception can occur.”
What’s more, Dr. Melnick says, “as women get older, menstrual cycles typically shorten and ovulation can happen within the first few days of a cycle, even while bleeding is still occurring. Lastly, many women will have mid-cycle bleeding, often around the time of ovulation, which can be mistaken for a period. Bottom line, if a woman does not want to get pregnant, she needs to be using contraception throughout all days of her cycle.”