Sheikh K
Br J Ind Med. 1987 Apr;44(4):244-9. doi: 10.1136/oem.44.4.244.
Cohort studies of adverse reproductive effects of occupational exposures that compare the outcome of exposed pregnancies with non-exposed pregnancies among the same group of mothers (internal controls) and among a different group of mothers (external controls) have been reviewed. Confounding of the hypothesised associations by parental characteristics, the outcome of previous pregnancies, and other exposures is not adequately assessed in these studies. The risk of an adverse outcome appears to be consistently higher among the external control group of pregnancies than among the internal controls. If the differences between the two control groups are real, comparisons based on external controls underestimate the relative or attributable risk. Alternative explanation for this phenomenon are discussed. A likely explanation is a misclassification of exposure or a high prevalence of risk factors other than the exposure of interest among the external controls. Non-exposed pregnancies among exposed mothers may be used for comparison in preference to pregnancies among non-exposed mothers.
已对职业暴露的不良生殖影响进行了队列研究，这些研究在同一组母亲（内部对照）和不同组母亲（外部对照）中比较了暴露妊娠与未暴露妊娠的结果。在这些研究中，未充分评估父母特征、既往妊娠结果和其他暴露对假设关联的混杂作用。外部对照妊娠组中不良结局的风险似乎始终高于内部对照。如果两个对照组之间的差异是真实的，基于外部对照的比较会低估相对风险或归因风险。文中讨论了对此现象的其他解释。一种可能的解释是暴露的错误分类，或者外部对照中除感兴趣的暴露因素之外的风险因素患病率较高。暴露母亲中的未暴露妊娠可优先用于与未暴露母亲中的妊娠进行比较。
