Strengths and limitations
The strengths of the study are: first, large sample size with
prospectively collected data, second, focus on placental dysfunction
related stillbirths, rather that treating all stillbirths as a
homogeneous condition, and third, comparison of the predictive
performance of two of our models that were previously internally
validated1,7. We acknowledge the prerequisite for
external validation to support generalization of our results and wide
implementation of our model. Such external validation would require a
large prospective multicenter study.
It is possible that in some cases the birthweight of the stillborn
babies is lower than the weight at the time of death because there is a
relationship between intrauterine retention interval and reduction in
birthweight34. In our cases we did not have
information on this interval and therefore the incidence of placental
dysfunction related stillbirths maybe overestimated.
A Some of the risk factors included in the RCOG guideline for the
prediction of SGA were not included in the competing risks model for SGA
because we did not have such risk factors for any or some of our
patients. For example, we did not have data on low fruit intake before
pregnancy, paternal SGA, daily vigorous exercise, heavy bleeding similar
to menses, or notching of the uterine artery Doppler waveforms, but
these factors may well suffer from subjectivity or information bias.
Similarly, we did not have available data on PAPP-A for all of our
patients and did not use the criterion of <0.4 MoM for
assessment of risk; in a previous study we reported that inclusion of
PAPP-A as a binary variable (<0.4 MoMs) increases the screen
positive rate without any significant improvement in the detection
rate.33