Is the morning-after pill considered an abortion in Halachic terms or does it have another status, and, if the latter, may it be used?
As heard from Rabbi Weiner, a student of Rabbi Elyashiv and a bio-halachist:
When the morning-after pill is administered, we do not know if there is an embryo present. Hence it is permissible in the case of a rape. (He wouldn't go so far as to give it carte blanche for, say, the happily-married couple who are trying to space their pregnancies a bit better.)
"The right to life is a basic halachic right beginning from the 40th day after conception.
According to this understanding, that "life" begins 40 days after conception, taking a pill "the morning after", which prevents embryo attachment to the uterus, would not be halachically considered an abortion.
There may be other reasons why it shouldn't be relied on as a primary method of birth control, but it seems from these sources that the heter to use this pill would extend beyond cases of rape.
Even if a young observant teenage girl made a mistake and had unprotected sex willingly, taking this pill is a far better option than forcing her to endure the humiliation of a teenage pregnancy. While such things may be relatively normal in the secular world, if a teenage girl in a frum community got pregnant, it would embarrass her family, deny her any decent shidduch in the future, and effectively ban her from all aspects of community participation.
I'm NOT saying that the consequences above are a good enough reason to have an ABORTION. They aren't. However, since taking the morning after pill is not an abortion, the above consequences are more than enough of a heter to use it.