A puzzling study of U.S. pregnancies found that women who had miscarriages between 2010 and 2012 were more likely to have had back-to-back annual flu shots that included protection against swine flu. Past studies have found flu vaccines are safe during pregnancy, though there's been little research on impact of flu vaccinations given in the first three months of pregnancy. This study focused only on miscarriages, which occur in the first 19 weeks of pregnancy and are common. The study's authors, two of whom are CDC researchers, saw a big difference when they looked at women who had miscarried within 28 days of getting a shot that included protection against swine flu, but it was only when the women also had had a flu shot the previous season. They found 17 of 485 miscarriages they studied involved women whose vaccinations followed that pattern. Just four of a comparable 485 healthy pregnancies involved women who were vaccinated that way. Some of the same researchers are working on a larger study looking at more recent data to see if a possible link between swine flu vaccine and miscarriage holds up.