We thank the authors for their interest and for recognizing the importance of the observation linking repeated (more than one) episodes of reduced fetal movement to adverse fetal outcomes in pregnancy. They are quite right to seek clarity in the study methods to ascertain the robustness of our analysis and subsequent interpretation of our data.

The authors question the validity of the logistic regression analysis because of a lack of knowledge of the nature of the mathematic relationship between the variables, notably between mean uterine artery pulsatility index (PI) and birthweight. We are able to reassure them that we have demonstrated, in a previous publication, the existence of a linear relationship between these variables. The authors also suggested that the relationship between maternal age and small-for-gestational-age may be “U” shaped; however in our study, there was no evidence of significant relationship between the 2, which led to the conclusion that maternal age is a mere proxy for more direct and objective markers of placental dysfunction, such as PAPP-A concentration and uterine artery PI.

Laas et al argue that caucasian ethnicity should be the reference group. The choice of a reference group is immaterial because ethnicity has no significant contribution to the prediction of either repeated episodes of reduced fetal movements or for small-for-gestational-age. Furthermore, the confounding variables that were used (maternal age, body mass index, and ethnicity) were chosen because of known associations to both placental dysfunction and small-for-gestational-age birth. The latter method is well-accepted and previously published by ours and other research active groups.