We appreciate the letter and interest in our article from Lisonkova et al. We understand the primary concern to be the methodology behind our composite fetal/infant mortality risk calculation and we hope we can clarify any misunderstanding that readers may have.

The composite mortality calculation aims to quantify the fetal and infant mortality associated with an additional week of pregnancy at each gestational age. Of particular concern was the denominator used in this calculation. It is important to note that the denominator is different for the 2 components. The stillbirth component of the calculation utilizes ongoing number of fetuses at risk minus half the number of fetuses delivered during the week in question to account for the deliveries that may have already taken place. The infant death calculation uses a denominator of live births at each gestational age, and thus only examines the impact on those who survived the fetal period up until birth. The summation of these rates leads to a composite measure of the mortality of 1 week of expectant management.

Also of concern was the fact that by using a summation of these values the mortality rate will be weighted in favor of infant death at earlier gestational ages. It is true, in the extreme example given at 21 weeks of gestation, one would expect uniform mortality, which is the whole point about including infant death in this calculation. If one only considered stillbirth and sought to minimize that risk, one would deliver all pregnancies at 21 weeks of gestation to avoid stillbirths. This is why we rarely choose to deliver pregnancies for fetal indications at periviable gestations and in most pregnancies such an option does not become a consideration until the late preterm or term gestations. By incorporating both components and comparing the risk of fetal/infant mortality to that of infant mortality (ie, immediate delivery) by each week of gestation, one arrives at a more appropriate comparison. By doing this we were able to determine at what gestational age the mortality risk was minimized. We believe this method to be useful in patient care, particularly in cohorts at increased risk of stillbirth such as twin gestations. We hope that this clarifies our approach and that our work will add to the body of literature regarding the timing of delivery in the late preterm and term periods.