BACKGROUND: A(H1N1)v2009 influenza vaccination of pregnant women was a challenge 
for health care providers, as little safety data were available.
METHODS: We prospectively followed the pregnancies of women who were vaccinated 
at any time during pregnancy or ≤ 4 weeks prior to conception and compared these 
outcomes to a control cohort matched by the estimated date of birth. Primary 
endpoints: rate of spontaneous abortion and major malformations. Secondary 
endpoints: preeclampsia, gestational age at birth, and birth weight.
RESULTS: Pregnancy outcome of 323 women immunized with adjuvanted or 
non-adjuvanted A(H1N1)v2009 influenza vaccines from 2009-09-28 to 2010-03-31 
were compared to 1329 control subjects. The risk for spontaneous abortions (HR 
0.89; 95% CI 0.36-2.19) and the rate of major malformations (all trimesters: OR 
0.87; 95% CI 0.38-1.77; preconception and first trimester exposure: OR 0.79; 95% 
CI 0.13-2.64) did not vary between the two cohorts. Furthermore, there was no 
increase in preeclampsia, prematurity, and intrauterine growth retardation in 
the vaccinated cohort.
CONCLUSION: The results of our study do not indicate a risk for the pregnant 
woman and the developing embryo/fetus after H1N1 vaccination. We provide and 
apply methods novel in observational studies on pregnancy outcome, especially if 
a single dose exposure is investigated.