The overestimate, he says, results in part because early studies did not take into account normal variations in the timing of ovulation during women's menstrual cycles.

The more effective the drug is — that is, the more days it works after intercourse — the more likely it works by preventing implantation rather than fertilization, he says.

Some medical dictionaries define conception as the moment when the sperm enters the egg; others define it as "the implantation of the blastocyst in the endometrium." That second usage is "a deliberate redefinition," argues Stanford, who has been a vocal advocate for the belief that even regular oral contraceptives also sometimes act as abortifacients.

According to studies he has conducted, he says, religious women are more likely to be concerned that a contraceptive method may have a post-fertilization effect. Age, race, income, education and plans for future pregnancy were not as important as religion, he says.

Although proponents of emergency contraceptives say that the pills will cut down on the number of abortions and unwanted pregnancies, Stanford says that studies in Europe, China and the United States prove otherwise.

"In all cases, they found there was no effect on abortion rates and unintended pregnancy rates," he says. It's a "naive view of human nature" to think that people won't indulge in "riskier" behavior if they think they can rely on a morning-after pill, he says.

Stanford served on the FDA's advisory council that examined whether Plan B should be given over-the-counter status. Federal hearings in 2003 were "a mix of science, theater and politics," he says, resulting in a 23 to 4 vote to approve the drug for OTC use. Stanford was one of the four who voted "no."

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