After decades of research the answer is not absolutely clear.
Two family-owned companies, Oklahoma-based arts-and-crafts retailer Hobby Lobby, controlled by evangelical Christians, and Pennsylvania-based cabinet-manufacturer Conestoga Wood Specialties Corp, owned by Mennonites, object on religious grounds to a requirement of President Barack Obama's healthcare law: that employer-sponsored insurance cover contraception.
The companies say they have no objection to covering forms of birth control that prevent conception, the fertilization of an egg by a sperm.
What concerns them are after-intercourse products, so-called emergency contraception such as the "morning-after" pill, which prevent pregnancy.
Anti-abortion groups contend the products act after fertilization, destroying embryos.
"For us, the issue is the life-ending mechanisms that some emergency contraceptives can have," said Anna Franzonello, an attorney at Americans United for Life, an anti-abortion legal group that has filed a brief for seven Catholic and other anti-abortion groups siding with the companies.
Mainstream scientific and medical organizations, as well as abortion-rights supporters, counter by citing research showing that the vast majority of emergency contraceptives prevent fertilization.
While the Supreme Court will not be ruling on the science, and has never defined pregnancy, many groups have filed friend-of-the-court briefs offering their view of how emergency contraceptives work.
Image: Birth control pills, via Shutterstock
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