This article was updated at 6 p.m. ET

On Tuesday (Nov. 8), Mississippi voters will decide whether fertilized eggs qualify as "persons" under the law from the moment when sperm and egg meet. But while the law is designed to challenge Roe v. Wade and outlaw abortion, doctors say that the wording is also likely to outlaw common methods of birth control, including the birth control pill.

That's because some of those methods may work, in part at least, by making the uterus inhospitable to implantation by an fertilized egg, said Pittsburgh family physician Deborah Gilboa. That could mean that some eggs become fertilized and are flushed out in women taking a birth control pill, using an intrauterine device (IUD) or taking "Plan B," the morning-after pill.

"If you have anything that makes the lining of the uterus not hospitable and [the fertilized egg] doesn't attach, the woman won't ever know, because it's tiny, just microscopic," Gilboa told LiveScience.

Fertilization and birth control

After a sperm and egg meet, the resulting cell is known as a zygote. This zygote divides and re-divides as it drifts toward the uterus, becoming a blastocyst on the fifth day after fertilization. The blastocyst then implants on the uterine wall, where it receives the nutrients it needs to continue developing. This implantation process, after which pregnancy becomes medically detectable, starts a week or so after conception.

Proposition 26, Mississippi's anti-abortion amendment, pushes legal personhood back to the moment of fertilization, a point that is not generally medically detectable. Proponents of the new law saw that it will not ban the use of hormonal contraceptives. However, "Yes on 26" members also write on their website that the group is opposed to birth control methods, "which act to prevent implantation of the newly formed human into the lining of the womb." They include in this category some forms of the pill and other hormonal drugs, as well as IUDs.

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In fact, Gilboa said, if 26 defines personhood from the moment when sperm meets egg, any birth control method that theoretically allows sperm and egg to meet could be outlawed. [Read: 7 Surprising Facts About the Pill]

The birth control pill, for example, prevents pregnancy in three ways: The pill thickens the cervical mucus to make it more difficult for sperm to reach the egg; it suppresses ovulation by mimicking pregnancy-level hormones in the body, preventing eggs from being released from the ovaries; and finally, as a fail-safe, the pill makes the lining of the uterus inhospitable to any fertilized egg that might slip through. The time between fertilization and implantation (when a pregnancy becomes medically detectable) usually takes about a week.

IUDs likewise may prevent fertilization by suppressing ovulation or killing sperm. But IUDs may also prevent implantation of a fertilized egg by irritating the uterine lining, Gilboa said.

The morning-after pill, sold under brand names such as "Plan B" and "Ella," are essentially heavy doses of regular birth control pills that can potentially prevent pregnancy by preventing implantation. After implantation, the drugs have no effect.

It's difficult to pinpoint how often any of these methods actually result in the flushing of a fertilized egg, but whenever intercourse occurs without barriers there is always a chance, Gilboa said. The result, she said, is that most birth control methods other than condoms, diaphragms, cervical caps or other barriers could be seen as breaking the law under Proposition 26.

"Pretty much every method is [potentially] post-fertilization," Gilboa said. "The only methods that are pre-fertilization are barrier methods or abstinence."

Popular methods

Even taking birth control out of the equation, however, does not ensure that implantation will occur successfully, resulting in a natural abortion of sorts. In a 1988 study published in the New England Journal of Medicine, sexually active women took daily urine tests to measure for traces of hormones that indicate fertilization. The study found that about 25 percent of fertilized eggs failed to survive past six weeks — so early that most of the women had no idea they had conceived. About 95 percent of the participants who lost pregnancies before they knew they had conceived were reproductively healthy and went on to have successful pregnancies within the next two years.

If Proposition 26 passes, most legal experts expect a court challenge that could end up at the Supreme Court, because the law floutes rules laid down in 1973's Roe v. Wade decision affirming the right to abortion. How contraceptive debates are likely to play into the proceedings are yet to be known, but if 26 does end up targeting contraception, it could be on a collision course with the practicies of a large number of women.

According to 2010 data from the Guttmacher Institute, a reproductive health research organization, 28 percent of women who used contraceptives between 2006 and 2008 opted for the pill. Another 12 percent used IUDs or other hormonal alternatives, including implants and patches.

Another Guttmacher report, this one published in 2011, found that religious teachings often play little role in women's day-to-day birth control choices. The report found that among American Catholic women, whose church prohibits birth control other than avoiding sex during fertile periods, 98 percent had at some point used another, non-church-approved method of birth control.

Of Catholic women, 68 percent used a hormonal method of birth control, the IUD or sterilization, Guttmacher found, joining 73 percent of their mainline Protestant sisters and 74 percent of evangelical women.

You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.