Obstetrics and Gynecology International an analysis, for instance, of the possible eﬀect of hormonal treatment on hypospadias risk. For severe kidney malforma- tions, a reduced risk is seen for infants of women with a high education. It can be noted that no eﬀect of socioeconomic level is seen on the risk for an infant with a chromosome anomaly which indicates no diﬀerence in prenatal detection rate according to maternal education, and there is no eﬀect on (sub)luxation of the hip, a condition sensitive to variable diagnostic and reporting completeness. The classical eﬀect of socioeconomy on neural tube defect risk is not seen in this material. Maternal education is one way to evaluate socioeconomic conditions. In some societies, the man’s education plays a more important role than the female’s for the socioeconomy of the family. Formal social group classiﬁcations exist in some countries. Family income may be a useful variable when known, but in many wellfare societies with extensive social security systems and usually also high tax rates, this measure may be less sensitive. A further variable which can be used is if the woman is cohabiting or not with the man at the beginning of preg- nancy. In Sweden only 3.3% of the women who give birth are not cohabiting in early pregnancy—this may be the result of the abortion law which permits abortions without any restrictions before week 12. Table 5 demonstrates the weak eﬀect of this variable on malformation risk—a “protective” eﬀect is seen on (sub)luxation of hip which may be a result of multiple testing. When other outcomes than congenital malformations are studied, like preterm birth and intrauterine growth retardation, socioeconomic level can have a stronger eﬀect. 3.4.10. Race/Ethnicity and Country of Birth. In many popula- tions, race or ethnicity is an important confounder in reproduction epidemiology. Other populations are rather homogeneous from a racial point of view. In some societies information on race is not politically possible to record (e.g., in Sweden). In such areas, sometimes country of birth can give an idea of these factors but noticed eﬀects can also be related to the status of being an immigrant. Analyses of the eﬀect of maternal country of birth on pregnancy outcome in Sweden indicated that only few groups may deviate from Swedish-born women, among them women from Sub-Saharan Africa [22]. They will make up a rather small proportion of the studied population. Table 6 shows that the country of birth of the woman sometimes slightly aﬀects the risk for an infant with a relatively severe malformation. Due to the large numbers involved, some odds ratios reach statistical signiﬁcance even though the magnitude of the deviation is small. Both women born in the other Nordic countries and women born in non-Nordic countries have a slightly decreased risk for a malformation in the infant. These women have moved to Sweden for various reasons. One group (often from Asia) were adopted as children and have lived most of their life in the Swedish society and have Swedish as their native language but carry the genetic load of their country of origin. Some women were born by Swedish parents who at that time were 9 living abroad. Other women have immigrated as refugees from catastrophe or war areas; some have immigrated for purpose of searching work (often highly educated) or because they had a relationship with a Swedish man. The weak tendency to a malformation risk slightly below that for Swedish-born women could perhaps be explained by a “healthy immigrant” eﬀect, that at least some of the reasons listed above will favour women without chronic diseases. The genetic composition will vary between geographical areas why speciﬁc conditions may be present for speciﬁc malformations. As an example diﬀerent rates of neural tube defects in USA are seen when white, black, and Hispanic populations are compared [23]. The pattern of drug use during pregnancy often diﬀers according to country of birth [2]. As a confounder, country of birth will be rather weak but when in doubt it may be wise to repeat an analysis based only on nonimmigrant women. 3.4.11. Confounding by Concomitant Use of Other Drugs. Women often use combinations of drugs, and in studies of one speciﬁc drug, other drugs may act as confounders, if they are used more often together with than without the drug under study and if they in themselves increase the risk for instance of a birth defect. In Table 7 some examples are given based on maternal use of antidepressants [24]. Many drug groups are used in excess by these women. The strongest relationship is seen with oral contraceptives and psychoac- tive drugs: opioids, antipsychotics, and sedatives/hypnotics. Weaker but statistically signiﬁcant relationships are seen with drugs for stomach ulcer and reﬂux, systemic corticosteroids, thyroid drugs, NSAIDs, antiasthmatic drugs, and antihis- tamines. Much of these associations can be explained by known comorbidity. On the other hand no associations is seen with some drug groups, and some are used less often by women taking antidepressants than by other women: multivitamins, folic acid, and minor analgesics. It can be debated if this mirror an actually lower use or is an eﬀect of the fact that women may concentrate on reporting the use of potentially harmful drugs and neglect common and apparently harmless drugs. Coexposure for diﬀerent drug categories can have dif- ferent results. In order to act as confounders, the codrug must in itself aﬀect outcome. This may be true for some of the drugs listed but probably not for other, for example, drugs for stomach ulcer or reﬂux and antihistamines. The use of a nonteratogenic drug will not appear as a confounder in studies of birth defects: even if it is associated with the exposure under study as it does not associate with the outcome under study. The number of women who have been exposed to a putative teratogenic drug by coexposure is usually low, and the exclusion of such cases in the analysis may be the easiest way to deal with the problem. There is another complication which can occur due to exposure for two drug categories. Neither of the two drug groups may have a noticeable teratogenic activity but when used together they may—they would act synergistically. This, for instance, was suggested in a study [25] indicating that neither SSRI drugs nor benzodiazepines had an observable