This paper validates the fertility histories of the German Generations and Gender Survey (GGS). Focusing on the cohorts 1930-69 of West German women, the total number of children, the parity distribution and the parity progression ratios are compared to external sources. One major result from this validation is that the German GGS understates the fertility for the older cohorts and overstates it for the younger ones. We presume that two mechanisms are responsible for this pattern in the German GGS: On the one hand, children who have left parental home are underreported in the retrospective fertility histories. On the other hand, women with small children are easier to reach by the interviewer. These two mechanisms taken together produce too low numbers of children for the older and too high ones for the younger cohorts. Extending the validation to marital histories has revealed a similar bias. Our general conclusion from this investigation is that the German GGS may not be used for statistical analyses of cohort fertility and marriage trends. For subsequent surveys, we suggest integrating simple control questions in questionnaires with complex retrospective fertility and union histories.