More than 88 percent of the 1.2 million abortions performed each year in the United States occur in the first trimester of pregnancy. Most doctors will not perform them beyond 22 or 24 weeks. Barely 1 percent of abortions — about 15,000 each year — are done after 21 weeks. A full-term pregnancy lasts about 40 weeks from the first day of the last menstrual period, according to the American College of Obstetricians and Gynecologists.

But exactly how many abortions are performed late in pregnancy, when in a pregnancy they are performed, by whom and under what circumstances is unclear. The government does not collect detailed data, and doctors who perform them provide little public information.

Although hospitals nationwide perform abortions later in pregnancy, many will not. Those that do refer patients to doctors who specialize in the procedures when cases are complicated.

Rob Stein