Obstetrics and Gynecology International Table 3: Use of drugs in early pregnancy among women with a previous relatively severely malformed infant compared with women who had no known previous such malformed infant. Odds ratio (OR) with 95% conﬁdence interval (95% CI) adjusted for year of delivery, maternal age, parity, smoking in early pregnancy, and BMI. Drug group Any drug Drugs for stomach ulcer and reﬂux Insulin Multivitamins Folic acid Antihypertensives Thyroid drugs Antibiotics NSAID Opioids Minor analgesics Anticonvulsants Antipsychotics Sedatives/hypnotics Antidepressants Any psychopharmacon Drugs for rhinitis Antiasthmatic drugs Antihistamines Among women with previous malformed infant 2078 53 43 218 276 35 104 186 127 38 449 20 26 32 95 137 67 180 300 Among all women 290480 6062 2614 56200 43780 3147 10728 19036 13047 3974 51043 2037 1992 3301 11714 15558 9944 22702 38689 OR 1.14 1.17 2.46 0.99 1.26 1.39 1.31 1.04 1.27 1.18 0.96 1.44 1.53 1.21 1.26 1.29 0.86 1.07 0.98 7 95% CI 1.08–1.21 0.89–1.55 2.81–3.32 0.86–1.15 1.12–1.43 0.99–1.94 1.07–1.59 0.89–1.20 1.06–1.52 0.85–1.63 0.87–1.06 0.92–2.26 1.03–2.39 0.91–1.83 1.02–1.55 1.08–1.53 0.68–1.10 0.92–1.25 0.87–1.10 A diﬀerent question is if a drug eﬀect diﬀers between cases with and without a known genetic risk for a malfor- mation. By identifying women who already had a pregnancy with the same malformation as in the current pregnancy product, a crude division can be made between genetic high- risk and low-risk fetuses. To explore the impact of genetics on the drug eﬀect a stratiﬁcation of the material can be made into women with and without previous pregnancies with the malformation in question [19]. Unfortunately, numbers are usually so low in the former group that the study does not become informative. One could believe that by excluding infants with a genetic load from the study, the sensitivity for the drug teratogeneic eﬀect should increase. On the other hand, the drug may act only on a genetic background by increasing the penetrance of the genes which contribute to the origin of the malformation. Similarly, studies have been made where cases were divided into presence or absence of speciﬁc genes, associated with the origin of a certain malformation—again numbers have usually been so low that no ﬁrm information was obtained [20]. 3.4.8. Prenatal Diagnosis and Induced Abortion. Some con- genital malformations can be identiﬁed by various methods applied during pregnancy, and if the malformation is regarded as serious this may result in an interruption of the pregnancy. This is a problem in data collection but could also lead to confounding. Use of a drug—or more likely the underlying disease—may aﬀect the probability that a prenatal fetal investigation is carried out or could aﬀect its degree of detail. This has, for instance, been suggested to occur in depressed women. This problem is most important in populations where all pregnant women do not routinely get a sonographic examination which, for instance, is the case in the Scandinavian countries. 3.4.9. Socioeconomy. The importance of parental socioecon- omy varies according to the pregnancy outcome investigated and the population studied. To a large extent, socioeconomic eﬀects can be explained by lifestyle factors, for example, smoking and obesity, the eﬀects of which can be directly controlled when such data are available. For some outcomes, some eﬀects of socioeconomy will remain but it is doubtful to what extent this is true for birth defects. One possible pathway would be via nutrition and food quality. Such eﬀects are probably much stronger in societies with large socioeconomic diﬀerences than in wellfare societies. This may be an explanation that socioeconomy appears to be related to neural tube defects in some countries like Great Britain [21] and USA and is hardly discernible in North European countries like Sweden. Also the impact of socioeconomy on drug usage depends on the wellfare situation in the society. When medical care is free or associated with low costs, the impact will probably be less than in societies where the patients to a large extent have to pay for medical care and drugs. Obviously, there are also socioeconomic diﬀerences in disease rates which aﬀect drug use. Table 4 shows the diﬀerent eﬀects of maternal education level for some drug groups. Even though many show statistically signiﬁcant deviations, the ORs are only