Non-Political Description
Opposed to abortion services and legal access to them.

Political Description
Receiving or performing abortion services is an illegal and immoral act that that results in the murder of a child prior to its birth. Pro-Life supporters generally advocate legal protection of all stages of human development after conception. Supporters of the Pro-Life agenda do not necessarily reject abortion in all circumstances, such as cases of rape, incest, or endangerment of the mother. The core tenet is that abortion should never be performed as a matter of convenience.

Political Benefits
Infers that those who are not “Pro-Life” are by definition “Anti-Life” or “Pro-Death”
Invokes the idea murder occurs if one is not “Pro-Life.”
Avoids referencing the word abortion which is the concept at the heart of the issue. Avoids being “Anti-” something, which carries a negative connotation.

Reference Point
Self and others who share same political view. Unsurprisingly, this euphemism is not used by Pro-Life opponents who instead refer to Pro-Life advocates as Anti-Choice.

History
I was unable to confirm the origins of the term. The best information available is the euphemism was first used shortly after Roe v. Wade, the 1973 United States Supreme Court decision that held abortion to be a constitutional right.

Example Usage
From the National Review Online

June 29th marks the 15th anniversary of the Supreme Court’s decision in Planned Parenthood of Southeastern, Pennsylvania v. Casey. At the time, the decision was a major disappointment for the right to life movement. The Supreme Court refused to overturn Roe v. Wade despite the fact that in the previous 12 years, presidents elected with the enthusiastic support of much of the pro-life movement had appointed five new justices to the Supreme Court. However, the news was not all bad for pro-lifers that day. In fact, the Casey decision granted pro-life movement the ability to make greater incremental progress at the state level, progress that continues to this day.