BOSTON — A single dose of Plan B (levonorgestrel) alters the timing and duration of the menstrual period that immediately follows emergency contraception, according to a new study. Taken early in the cycle, it causes the menstrual period to commence earlier than usual. Taken late in the cycle, it prolongs the next menstrual period.
But intermenstrual bleeding is uncommon, and menstrual timing resumes its normal pattern with the second cycle said coauthor James Trussell, Ph.D., professor and director of the Office of Population Research at Princeton University. Dr. Trussell presented the research at a conference on contraceptive technology sponsored by Contemporary Forums. The paper was simultaneously published in the journal Contraception (2006;73:376–81).
The study may help to settle an ongoing debate about the clinical significance of bleeding after the use of emergency contraceptive pills (ECPs). Several previous studies have suggested that ECPs provoke intermenstrual bleeding. The new study's message—that Plan B merely hastens the end of a cycle and that this effect is short-lived—will be reassuring to many women.
For women with late menses, however, the study suggests a pregnancy test may be in order.
Researcher Dr. Elizabeth G. Raymond, of Family Health International in Research Triangle Park, N.C., with Dr. Trussell and colleagues asked 113 women to maintain daily bleeding diaries for 10 weeks after receiving a single dose of 1.5 mg of levonorgestrel.
The women, who were patients at a Planned Parenthood clinic in Boston, had no known risk factors for bleeding abnormalities.
Though Plan B dosing approved by the Food and Drug Administration consists of two 0.75-mg pills taken 12 hours apart, the International Consortium for Emergency Contraception currently recommends the 1.5-mg single-dose regimen. Research since Plan B's 1999 approval suggests that the single-dose regimen is safe and effective (Lancet 2002;360:1803–10; Contraception 2002;66:269–73).
To control for menstrual irregularities that frequently arise in untreated women, researchers compared bleeding patterns in the women taking Plan B with an external comparison group of 843 women who, as subjects in an unrelated trial, kept daily bleeding diaries.
In addition, the researchers compared each Plan B users' bleeding pattern with her usual bleeding pattern reported at study entry. A menstrual period was defined as bleeding of at least 3 days' duration preceded by at least 2 days without bleeding.
Plan B taken in the first 3 weeks of a menstrual cycle significantly shortened the cycle, compared with the women's own usual cycle and the cycles in the external control group. The earlier in the cycle Plan B was taken, the sooner menstrual bleeding commenced.
Plan B taken in week 3 or later in the cycle significantly increased the number of bleeding days in the next cycle but did not affect the timing of the cycle.
The length of the second cycle after taking levonorgestrel returned to normal.
Though uncommon, bleeding between menstrual periods in either cycle one or cycle two occurred in significantly more among women taking Plan B (13%) than among controls (7%).
The new study is the first specifically designed to evaluate bleeding patterns after Plan B emergency contraception, said Dr. Trussell. It is also the first to study bleeding patterns in the second menstrual cycle after Plan B use, he added.
Potentially, factors other than Plan B could account for some of the study's results. For example, body weight—which can affect hormone secretion and uterine bleeding—was not measured in the external comparison group.
And since the subjects did not undergo routine pregnancy testing, it is possible that undetected, lost pregnancies could account for some bleeding irregularities.
Despite these limitations, the authors say in their published report, “An early menstrual period … will likely be welcome to many women using ECPs, who are eager for evidence that they are not pregnant.”
The study was supported in part by Barr Laboratories.