Norplant implants: the mechanism of contraceptive action.

ABSTRACT

PIP: This study sought to determine if fertilization can occur unnoticed among Norplant users who are ovulatory. Serial blood samples were obtained during a 1-month period from women receiving contraceptive service and healthcare at the Center for Research and Services in Human Reproduction and Contraception, Santo Domingo, The Dominican Republic. These women were all sexually active Norplant users who experienced regular menstrual bleeding patterns, and their sequential blood samples were assayed for the presence of human chorionic gonadotropin (hCG) at the Department of Biomedical Sciences at the University of Tampere, Finland. There was also a control group of 20 noncontracepting women included who were trying to conceive. Duration of Norplant was as follows: 4 in the 2nd year of use, 13 in the 3rd year, 11 in the 4th, 3 in the 5th year, and 1 in the 7th year. Pregnancy determination was based on the presence of hCG in the luteal phase by use of a sensitive and specific immunoenzymatic assay that can detect dimeric hCG as early as 7 days postovulation. 9 pregnancies were detected, all in the control group who were trying to conceive. 6 of these advanced to clinical pregnancies and 3 terminated spontaneously at the next menstrual period. None of the Norplant users evidenced and hCG production, whether or not the observed cycles were ovulatory. The probability of finding no pregnancies in the ovulatory months at risk among Norplant users is between 1/50 and 1/150,00. The null hypotheses that Norplant users conceive at a natural rate can be rejected at the 0.05 level. Thus, interruption of early pregnancy (menstrual abortion) does not play a role in the mechanism of action of Norplant contraceptive implants.