But if the egg has already been implanted in the uterus, the morning-after pill will not work.
If the government fails, it would clear the way for over-the-counter sales of the morning-after pill to younger girls.
The morning-after pill contains a higher dose of the female progestin hormone than is in regular birth control pills.
They say lowering the age to 15 for purchasers of the morning-after pill makes emergency contraception available for women of all ages.
If a woman already is pregnant, the morning-after pill has no effect.
Overall, about one in five women who had never been married had taken a morning-after pill, compared with just one in 20 married women.
One possibility is a "morning-after pill, " says Helicon Therapeutics' Tim Tully.
Korman's order sparked praise among supporters of the morning-after pill for all ages, such as the Center for Reproductive Rights, which had launched a relevant lawsuit.
Some 11% of sexually active women had used the morning-after pill, up from 4% in 2002, according to the US Centers for Disease Control and Prevention.
Today is the last day for government lawyers to try and stop a New York judge's ruling that would lift age restrictions for buying the morning-after pill.
Use of the morning-after pill in the US is on the rise, a trend driven largely by wider use among younger women, a major government study has found.
Last summer the health department decreed that all new health-insurance policies should cover birth-control services for women, including the morning-after pill (which most pro-lifers consider a form of abortion) and sterilisation.
Any over-the-counter access marks a long-awaited change, but it's not enough, said Dr. Cora Breuner of the American Academy of Pediatrics, which supports nonprescription sale of the morning-after pill for all ages.
The FDA was preparing in 2011 to allow over-the-counter sale of the morning-after pill with no agency limits when Health and Human Services Secretary Kathleen Sebelius overruled her own scientists in an unprecedented move.
Social conservatives had opposed any efforts to loosen restrictions on sale of the morning-after pill, arguing that it was important for parents and medical professionals to be involved in such decisions involving young girls.
Obama and Health and Human Services Secretary Kathleen Sebelius have also questioned whether there's enough data available to show the morning-after pill is safe and appropriate for younger girls, even though physicians groups insist that it is.
U.S. District Court Judge Edward Korman has said that politics was behind efforts by Secretary of Health and Human Services Kathleen Sebelius to block the unrestricted sale of the Plan B One-Step morning-after pill and its generic competitors.
U.S. District Judge Edward Korman took aim at the U.S. Food and Drug Administration's announcement last week that it was loosening restrictions on the Plan B One-Step morning-after pill, saying it can be sold without a prescription to those 15 and older.
The US Food and Drug Administration (FDA) has approved the "morning-after" pill without a prescription for women aged 15 and over.
Evangelicals generally permit the use of birth control, but they object to specific methods such as the morning-after contraceptive pill, which they argue is tantamount to abortion.
This move comes just weeks after a federal judge in Brooklyn, New York, ordered the FDA to make the morning-after birth control pill available to women of any age, without a prescription.
In 2011, the FDA said it had concluded that the "morning after" pill could be safely used by girls of child-bearing age.
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