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.