How do FABMs work? A woman is fertile from a few days before ovulation until after ovulation has occurred. As sperm can survive in the uterus or Fallopian tubes for up to 5-7 days, women are most likely to become pregnant if they have sex in the week before ovulation. FABMs require a woman to calculate and record fertile days by: counting the cycle length over a period of time observing signs that indicate that ovulation (egg release) is about to or has occurred During the fertile days a couple need to abstain from sexual intercourse or use a barrier method. Depending on the individual, sex may need to be avoided for 7-14 days during each menstrual cycle.