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Most studies on safety/risk of drugs in pregnancy consider the proportion of births (but not pregnancy terminations) affected by the drug from all exposed infants. Lack of data on pregnancy terminations could bias results. A computerized database for medications dispensed to pregnant women in southern Israel was linked with records from the district hospital; 84 823 deliveries and 998 medical pregnancy terminations took place; 571 of the women were exposed to folic acid antagonists in the first trimester. When only births were examined, there was no association between folic acid antagonists and fetal malformations. When data on pregnancy terminations were examined and births and pregnancy terminations were combined, there was a significant risk (neural tube defects: odds ratio 18.83, 95% confidence interval 9.24-38.37; cardiovascular defects: odds ratio 3.86, 95% confidence interval 1.67-8.88; and neural tube defects: odds ratio 6.30, 95% confidence interval 3.34-9.15; cardiovascular defects: odds ratio 1.76, 95% confidence interval 1.05-2.92, respectively). Inclusion of only birth data in observational studies of drugs in pregnancy constitutes a source of bias toward the null hypothesis.

Little data exist on the safety of H(2)-blockers during pregnancy. A computerized database of medications dispensed from 1998 to 2007 to all women registered in the "Clalit" health maintenance organization, in the Southern District of Israel, was linked with computerized databases containing maternal and infant hospitalization records from the district hospital. The following confounders were controlled for: parity, maternal age, ethnic group, maternal diabetes, smoking, and peripartum fever. Also, therapeutic pregnancy termination data were analyzed. A total of 117 960 infants were born during the study period, 84 823 of them (72%) to women registered at Clalit; 1148 of the latter were exposed to H(2)-blockers during the first trimester of pregnancy. Exposure to H(2)-blockers was not associated with an increased risk for congenital malformations (adjusted odds ratio [OR] = 1.03, 95% confidence interval [CI]: 0.80-1.32); also, no such association was found when therapeutic pregnancy terminations were included in the analysis (adjusted OR = 1.17, 95% CI: 0.93-1.46). Exposure to H(2)-blockers was not associated with perinatal mortality, premature delivery, low birth weight, or low Apgar scores.