Birthweight correlates to pubo-femoral distances and angles in hip ultrasound of newborns at 6 weeks of age: a retrospective cohort study

Background and purpose - There is inconsistency in the literature regarding the relationship between increased birthweight and risk of developmental dysplasia of the hip (DDH). We aimed to investigate the correlation between birthweight and pubo-femoral distance (PFD), as well as Graf's alpha angle in newborns undergoing hip ultrasound examination at 6 weeks of age. Patients and methods - Basic newborn characteristics and ultrasound measurements were retrospectively collected during a 1 -year study period. We excluded multiple births, newborns born at less than 37 gestational weeks, and incomplete information. Simple and multiple linear regression analyses were performed to evaluate the correlation of birthweight and PFD, and, second, birthweight and alpha angles including a stratified regression analysis investigating the potential effect modification of sex. Results - 707 newborns (1,414 hips) were included. Mean birthweight was significantly higher for male newborns (P < 0.001). Increased birthweight was positively correlated to PFD values (crude coefficient 0.21, 95% confidence interval [CI] 0.10-0.32) and the correlation was still present after adjusting for sex, family history, and breech presentation (adjusted coefficient 0.18, CI 0.07-0.29). The stratified alpha angle model for the males was significant for both the crude coefficient (-0.73, CI -1.28 to -0.19) and the adjusted (-0.59, CI -1.15 to -0.03), and also for the females (crude coefficient -1.14, CI -1.98 to -0.31 and adjusted coefficient -1.15, CI -1.99 to -0.31). Conclusion - We found that increased birthweight positively correlated to PFD, and negatively correlated to alpha angle, but this was not of clinical significance.