Maternal and pathological pregnancy characteristics in customised birthweight centiles and identification of at‐risk small‐for‐gestational‐age infants: a retrospective cohort study

OBJECTIVE: To regenerate coefficients for the New Zealand customised birthweight centile calculator using an updated birth cohort, and compare the identification of at-risk small-for-gestational-age (SGA) infants between full customisation (including maternal characteristics) and an ultrasound-based fetal weight and infant gender partial customisation. DESIGN: Retrospective cohort study of prospectively collected maternity data. SETTING: National Women's Health Auckland, New Zealand. POPULATION: Singleton pregnancies in the period 2006-2009; n = 24,176. METHODS: Multiple linear regression analysis was performed for full customisation (adjusted for gestation, infant gender, maternal characteristics and pathological variables) and ultrasound-and-gender customisation (adjusted for gestation and infant gender). MAIN OUTCOME MEASURES: Risks of SGA-related perinatal death were compared between models. RESULTS: Changes occurred in some ethnicity coefficients, including Chinese (-135 g), Tongan (-101 g) and Samoan (-89 g), and ten ethnicities were added. Overall, full customisation identified SGA infants with higher odds of perinatal death (OR 5.6, 95% CI 3.6-8.7) than infants classed as SGA by ultrasound-and-gender customisation (OR 2.1, 95% CI 1.4-3.3) (P = 0.02). In subgroup analyses, infants classed as SGA by full but not ultrasound-and-gender customisation (n = 888, 3.4%) had an increased risk of perinatal death (RR 4.7, 95% CI 2.7-7.9); however, those identified as SGA by ultrasound-and-gender customisation alone were not at an increased risk (n = 676, 2.6%, RR 1.1, 95% CI 0.4-3.6). The population attributable risk (PAR) of SGA-related perinatal death was higher for full (49.8%) than for ultrasound-and-gender (43.0%) customisation. CONCLUSIONS: Updating the New Zealand customised birthweight centile calculator resulted in revised coefficients that better reflect a contemporary birth cohort. Inclusion of maternal characteristics in a birthweight customisation model increases the detection of SGA infants at risk of perinatal death.