Revisiting the low birthweight paradox using sibling data with implications for the classification of low birthweight.

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We examined the birthweight threshold for increased odds of neonatal death among second births based on their elder sibling's birthweight category. This population-based cohort study included 190 575 women who delivered their first two non-anomalous singleton live births in Missouri (1989-2005). We examined the birthweight distribution and neonatal mortality curves of second births whose elder sibling had low versus adequate/high birthweight. We determined the optimal cut-off point for the classification of low birthweight among infants in each group based on the Youden index. Infants whose elder sibling had low birthweight had a lower mean birthweight and a higher percentage of low birthweight infants versus those whose elder sibling had adequate/high birthweight, but low birthweight infants in the former group had a lower rate of neonatal mortality. Upon standardizing the birthweight distribution to a Z-scale, neonatal mortality rates became comparable between the two groups at every rescaled birthweight for Z-scores ≥-3.7. The optimal cut-off point for low birthweight was 2500 and 3000 g among infants whose elder sibling had low and adequate/high birthweight, respectively. Using sibling data for the classification of LBW may enable the identification of average-sized infants who may be at increased risk of neonatal mortality.