Fecundity refers to the biological capacity to procreate, or the probability that a woman will conceive in any given month. Fecundity is typically measured by the time to pregnancy (TTP) or the length of time that it takes for a couple to conceive. TTP is used to calculate the probability of conception or the fecundability odds ratio (FOR). Direct biological markers of fecundity are not easily isolated from the social, environmental, and behavioral factors that promote or reduce the odds of conception and the sustained implantation of the fertilized egg. Some of these factors are the age at which women and men try to conceive, their overall health status, reproductive health (e.g., menstrual cycle regularity, endometriosis, sexually transmitted infections), disease and medication history (e.g., diabetes, ...