Evidence scattered over the literatures of zoology, psychology, agricultural science, and medicine indicates that diverse stressors will interfere with pregnancy in its early stages. It is probable that the most sensitive period is around the point of intrauterine implantation of fertilized ova. Although there is some indication that conventional "stress" hormones of the pituitary-adrenal axis can inhibit implantation, this evidence is too weak and inconsistent to suggest that these hormones are primary mediators of early pregnancy disruptions. Increasingly, evidence indicates that the balance of ovarian steroids is most important for pregnancy maintenance. It is well known that minute amounts of exogenous estrogens can completely disrupt pregnancy, and some new evidence suggests that endogenous estrogens may be released from the adrenals and/or ovaries during psychological stress.