Time frame: From the date of registration until the date of documentation of pregnancy outcome (up to 10 months i.e. 28 days prior to conception till the month of estimated date of delivery)
Participants were followed from registration upon exposure to Twinrix within 28 days prior to conception until data on pregnancy outcome was obtained. Follow-up via telephone or a form mailed to the contact was sought in the month of the estimated date of delivery.
Pregnancy outcomes were stratified by the trimester of exposure, with an additional stratum for preconception exposure with no subsequent administration during pregnancy; multiple exposures were classified by the earliest trimester of exposure. Gestational weeks were counted from the date of the last menstrual period. The second trimester was considered to begin at week 14, and the third trimester beginning at week 28.
Pregnancy outcomes were dichotomized according to the presence or absence of birth defects and further categorized as: Live births, Spontaneous abortions (i.e., pregnancy losses), Induced abortions and Fetal deaths.