A couple having intercourse during the periovulatory period has no more than a one in five chance of conceiving a pregnancy that will result in the delivery of a mature infant. Since fertilization occurs in at least four of five cycles, this is an apparently poor rate of success. However, with regard to early reproductive loss, failure is more appropriately viewed as a natural fail-safe mechanism to eliminate abnormal gametes and embryos. Spontaneous abortions serve as the process for the selection of normal and near-normal embryos and fetuses. Pregnancies are aborted with decreasing frequency as pregnancy progresses; approximately 70% of fertilized oocytes will be rejected before a pregnancy is recognized by the usual clinical criteria. After the first trimester of pregnancy, the majority of abnormal pregnancies have been eliminated; pregnancy loss after 14 weeks of gestation is negligible.