Why are there so many pro-life billboards in Minnesota?

I did an impromptu road trip through the Midwest this week and I think I saw about two dozen pro-life billboards and road signs, but none that were pro-choice. Anyway, about half of them were in Minnesota. I think I got an entire lesson on fetal development while driving east on US 2 and south on I-35 in the Gopher State. Is Minnesota considered a "swing state" by the pro-life movement? Is that why there are so many billboards?