Taking birth control pills: 4 in 10,000 women/year
Pregnant: 6 in 10,000 women/year
This is compared to 2 in 10,000 women/year in those who are healthy, non-smoking, non-pregnant and not taking birth control pills.
Source
Zieman, M. et al. "Patient Information: Hormonal methods of birth control" Up to Date. 20 Nov 2008.