The position effect of embryos in trisomy-1-producing mice

Size and weight of embryos can differ according to the location in the uterine horn. In these experiments the source of teratogenesis was within the individual trisomy-1 embryos, producing either euploid or trisomic embryos in a given position in the uterus. We therefore used the embryo and not the litter as a statistical sampling unit. In our opinion a nested analysis of variance is the most appropriate method to assess a position effect of embryos in this model. We were unable to show a position effect on gestational days 13 and 14. There was a significant difference though, among litters. We were able to show that further investigations of trisomy-1 embryos, in particular morphometric studies, are not complicated by a position effect. A position effect as described by several authors should be assessed for every new teratogenic model. The methods described here provide a reasonable vehicle for similar studies.