title: Birth weight ratio as an alternative to birth weight percentile to express infant weight in research and clinical practice: a nationwide cohort study.
abstract: Objective. To compare birth weight ratio and birth weight percentile to express infant weight when assessing pregnancy outcome. Study Design. We performed a national cohort study. Birth weight ratio was calculated as the observed birth weight divided by the median birth weight for gestational age. The discriminative ability of birth weight ratio and birth weight percentile to identify infants at risk of perinatal death (fetal death and neonatal death) or adverse pregnancy outcome (perinatal death + severe neonatal morbidity) was compared using the area under the curve. Outcomes were expressed stratified by gestational age at delivery separate for birth weight ratio and birth weight percentile. Results. We studied 1,299,244 pregnant women, with an overall perinatal death rate of 0.62%. Birth weight ratio and birth weight percentile have equivalent overall discriminative performance for perinatal death and adverse perinatal outcome. In late preterm infants (33(+0)-36(+6) weeks), birth weight ratio has better discriminative ability than birth weight percentile for perinatal death (0.68 versus 0.63, P  0.01) or adverse pregnancy outcome (0.67 versus 0.60, P < 0.001). Conclusion. Birth weight ratio is a potentially valuable instrument to identify infants at risk of perinatal death and adverse pregnancy outcome and provides several advantages for use in research and clinical practice. Moreover, it allows comparison of groups with different average birth weights.
authors.affiliation: Department of Obstetrics and Gynecology, Room H4-232, Meibergdreef 9, Academic Medical Center, 1105 AZ Amsterdam, The Netherlands.
authors.forename: Bart Jan
authors.identifier: 0000-0003-4307-9444
authors.initials: BJ
authors.lastname: Voskamp
authors.affiliation: Department of Obstetrics and Gynecology, Room H4-232, Meibergdreef 9, Academic Medical Center, 1105 AZ Amsterdam, The Netherlands.
authors.forename: Brenda M
authors.initials: BM
authors.lastname: Kazemier
authors.affiliation: Department of Obstetrics and Gynecology, Room H4-232, Meibergdreef 9, Academic Medical Center, 1105 AZ Amsterdam, The Netherlands ; Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Universiteitsweg 100, 3584 CG Utrecht, The Netherlands.
authors.forename: Ewoud
authors.initials: E
authors.lastname: Schuit
authors.affiliation: The Robinson Institute, School of Paediatrics and Reproductive Health, University of Adelaide, Adelaide, SA 5005, Australia.
authors.forename: Ben Willem J
authors.initials: BW
authors.lastname: Mol
authors.affiliation: Department of Obstetrics and Gynecology, Skaraborgs Sjukhus, Skövde, 541 85 Västra Götaland, Sweden.
authors.forename: Maarten
authors.identifier: 0000-0003-3512-8373
authors.initials: M
authors.lastname: Buimer
authors.affiliation: Department of Obstetrics and Gynecology, Room H4-232, Meibergdreef 9, Academic Medical Center, 1105 AZ Amsterdam, The Netherlands.
authors.forename: Eva
authors.initials: E
authors.lastname: Pajkrt
authors.affiliation: Department of Obstetrics and Gynecology, Room H4-232, Meibergdreef 9, Academic Medical Center, 1105 AZ Amsterdam, The Netherlands.
authors.forename: Wessel
authors.initials: W
authors.lastname: Ganzevoort