A woman can get pregnant soon after her menstruation ends because pregnancy is possible during the fertile window, which includes the days leading up to and during ovulation. Ovulation usually occurs around 12 to 14 days after the first day of a typical 28-day cycle, but this timing can vary, especially with shorter or irregular cycles. Sperm can survive up to 5 to 7 days in the reproductive tract, so if ovulation happens soon after the period, pregnancy can occur even from intercourse shortly after menstruation ends. Sometimes, pregnancy can even occur during menstruation if bleeding is not true menstruation but other types of bleeding. In summary:

• Pregnancy can occur shortly after menstruation ends, particularly if the woman ovulates early or has a short menstrual cycle.
• The fertile window spans about 6 to 10 days per cycle, including the 5 days before ovulation and the day of ovulation.
• Ovulation timing is typically 12-14 days after the period starts in average cycles but varies individually.
• Sperm survival in the reproductive tract (up to 7 days) increases the chance of pregnancy from sex soon after menstruation.
• Pregnancy during actual menstruation is less common but possible in some cases.

It's important to note there is no completely "safe" time during the menstrual cycle to avoid pregnancy without contraception due to this variability in ovulation and sperm survival.