Norplant implants: the mechanism of contraceptive action.

OBJECTIVE

To determine if fertilization occurs unnoticed among Norplant users who are ovulatory.

DESIGN

Serial blood samples were obtained during 1 month from sexually active Norplant users experiencing regular menstrual bleeding patterns and a control group of noncontracepting women trying to conceive. The sequential blood samples were assayed for the presence of human chorionic gonadotropin (hCG).

SETTING

All samples were obtained from women receiving contraceptive service and health care at the Center for Research and Services in Human Reproduction and Contraception, Santo Domingo, The Dominican Republic. Assays for hCG were performed at the Department of Biomedical Sciences, University of Tampere, Finland.

PATIENTS, PARTICIPANTS: A total of 32 women using Norplant implants were enrolled in the treatment group, and 20 women of proven fertility who were attempting to conceive served as a control group.

INTERVENTIONS

Duration of Norplant use was as follows: 4 in the 2nd year of use, 13 in the 3rd year, 11 in the 4th year, 3 in the 5th year, and 1 in the 7th year.

MAIN OUTCOME MEASURE

The determination of pregnancy was based on the presence of hCG in the luteal phase, using a sensitive and specific immunoenzymatic assay that can detect dimeric hCG as early as 7 days after ovulation.

RESULTS

Nine pregnancies were detected. All were in the control group trying to conceive. Six of these advanced to clinical pregnancies, and three did not proceed beyond the next expected menses. None of the Norplant users had evidence of hCG production, whether the observed cycles were anovulatory or ovulatory. The probability of finding no pregnancies in the ovulatory months at risk among Norplant users is between 1 in 50 and 1 in 150,000. The null hypothesis that Norplant users conceive at a natural rate can be rejected at the 0.05 level.

CONCLUSION

Interruption of early pregnancy (menstrual abortion) does not play a role in the mechanism of action of Norplant contraceptive implants.