Obstetrics and Gynecology International characteristics including age, parity, smoking, country of birth, and (in the next step) BMI removes the apparent protective eﬀect and leaves an OR close to 1.0. As low maternal education is associated with an increased risk for infant ADHD, and high education with a decreased risk, adjustment for maternal education increases the risk estimates but it does not quite reach statistical signiﬁcance (lower CI is 0.99). Also non-cohabitation in early pregnancy is associated with an increased risk for ADHD and when these (relatively few) women are removed from the analysis, the OR increases further and becomes statistically signiﬁcant. What we see here is the opposite eﬀects of some variables on exposure (IVF) and outcome (drug-treated ADHD) which initially results in an apparent protective eﬀect which gradually disappears when such factors are added in the analysis and ﬁnally results in an apparent over-risk. It should be stressed that some of these factors (e.g., low maternal education and not cohabiting) reasonably have no direct eﬀects but may be proxies for two things: socioeconomic level and possibly parental signs of ADHD with a genetic background. So, for instance, some studies indicate that the increased risk for ADHD if the mother smoked is due to the fact that women with genes for ADHD and perhaps have signs of ADHD smoke more than other women [39, 40]. The basic question if infants conceived by IVF do have an increased risk for ADHD is not deﬁnitely solved. This may be a good example of how complex and diﬃcult to handle confounding can be under certain circumstances. 4. Concluding Remarks The problem with confounding is not solved by uncritically adding a number of variables to a logistic regression model. Variables used for adjustment should be carefully selected according to their properties to aﬀect both exposure rate and outcome rate. It should be realized that eﬀorts to adjust for them may be ineﬀective because the available information on the confounders may be too crude or the statistical models used may be inadequate. It should be realized that the major problems in epidemiological studies of the eﬀects of maternal drug use on birth defects are to be found in biased data and low statistical power, not so much in confounding. For studies of other pregnancy outcomes like preterm birth, low birth weight, neonatal and long- term morbidity, confounding plays a more important role. The most diﬃcult confounding refers to confounding by indication, often stated as important but usually without any oﬀer of a good solution for its elimination. Conﬂicts of Interests The author declares no conﬂicts of interest. Ethical Considerations The analyses were performed within the responsibilities of the National Board of Health and Welfare and therefore 15 no ethical approval from outside ethical committees was needed. 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