Reply

Abstract

We appreciate the external scrutiny and testing of our results 1 that Dr Morales-Roselló et al. have undertaken and reported in their Correspondence. We would like to comment as follows:The results of their analysis actually agree with our results. In their analysis, which was restricted to pregnancies ≥ 32 gestational weeks, they found no sex-related difference in the cerebroplacental ratio (CPR). Our study included a wider gestational age range (19-41 weeks). We found that the difference in CPR between male and female fetuses was more pronounced earlier in gestation (P = 0.006), but, in agreement with the findings of Morales-Roselló et al., the difference was not significant after 33 weeks (P = 0.42), a pattern that is well illustrated in figure 2 of our article.We compared gestational-age specific Z-scores of transformed CPR values. Although we disagree that our finding of higher CPR in male fetuses is caused by an inferior analysis method, we reanalyzed our data, adding sex to the multilevel model which describes the association with gestational age. Adding sex significantly improved the fit to the data overall (P < 0.04, log likelihood test) and for fetuses < 33 gestational weeks (P = 0.03), but not for fetuses ≥ 33 weeks (P > 0.5). Assessing maternal and fetal factors that might influence the association between fetal sex and CPR was beyond the scope of our study, which had set out to provide reference ranges with reliable extreme centiles (reflected in their narrow 95% CI), terms for conditioning to refine longitudinal monitoring for fetuses with undisclosed sex, and, if significant differences between sexes were found, sex-specific ranges and terms.