The original purpose of the Early Pregnancy Study—carried out in 1982-86— was to determine the extent of pregnancy loss that occurs before women know they are pregnant. To accomplish this, 221 healthy women who were planning to become pregnant were enrolled at the time they discontinued all methods of birth control. Study participants collected daily urine specimens and kept daily diaries of their intercourse and menstrual bleeding. Urine samples were assayed using a highly sensitive assay for human chorionic gonadotropin (hCG), the key hormone marker of pregnancy. About 25% of pregnancies detectable by hCG did not survive to become clinically recognized, being lost in bleeding that was mistaken as menstrual bleeding.
Further hormonal assays for estrogen and progesterone metabolites in urine made it possible to estimate the day of ovulation in each cycle. Once this key benchmark of the cycle was determined, a number of basic observations about normal pregnancy and its variations have been possible. These include the first accurate descriptions of the fertile days of the menstrual cycle (i.e., the five days before ovulation and the day of ovulation), the time of implantation (six to twelve days after conception) and the incidence of vaginal bleeding in the early days of healthy pregnancies. These data continue to be a rich source of descriptive data on the events of early pregnancy.