A woman can become pregnant without a period as long as she ovulates, according to the American Pregnancy Association. The organization notes that it is possible to have a bleeding period without ovulation, a condition known as anovulation. It is not possible for a woman to become pregnant if she does not ovulate and release a suitable egg.
Women with certain conditions, such as underweight women, do not have regular periods but still experience ovulation. It is possible for them to become pregnant because their bodies might ovulate at any time, says the American Pregnancy Association. The organization explains that monitoring body temperature, changes in cervical mucus and luteinizing hormone levels gives women with irregular periods the best chances at detecting ovulation and successfully conceiving. However, the American Pregnancy Association warns that women with irregular periods who don't want to become pregnant should always use contraception because they do not have any obvious indicators of ovulation.
In a typical menstrual cycle, ovulation occurs before menstrual bleeding, so it is possible for a young, sexually active girl to become pregnant without experiencing a period, says TeensHealth, a service of Nemours Children's Health System. The site explains that abstinence is the only sure method of preventing pregnancy.