Association of a low chest-to-head circumference ratio with breech or transverse lie: the Japan Environment and Children's Study.

Objective: We examined whether the chest-to-head circumference ratio at birth was associated with breech presentation and transverse lie. We also described the obstetric management of such pregnancies in the Japan Environment and Children's Study (JECS).

Methods: We performed a cross-sectional evaluation of data collected between January 2011 and March 2014 in a nationwide prospective birth cohort study, the JECS. We analyzed 83,822 non-anomalous singletons born at 34-41 weeks' gestation to mothers with no history of previous cesareans or uterine surgery. We defined low, normal (reference group), and high chest-to-head circumference ratios as <10th percentile, 10th to 90th percentiles, and >90th percentile, respectively. Logistic regression models were used to estimate odds ratios (ORs) and 95% confidence intervals (95% CIs) for breech presentation and transverse lie. The timing and mode of delivery of such pregnancies were examined.

Results: Breech presentation was recorded in 2.6% and transverse lie in 0.2%. A low chest-to-head circumference ratio was associated with increased rate of breech presentation (5.2%; adjusted OR 2.36, 95% CI: 2.10-2.65) and transverse lie (0.3%; adjusted OR 2.33, 95% CI: 1.50-3.60), whereas a high ratio was linked to reduced breech presentation (1.1%; adjusted OR 0.51, 95% CI: 0.39-0.66). Subgroup analysis of children delivered by cesarean (n = 7971) showed a similar association, albeit with slightly reduced strength for breech presentation. Eighty-three percent of breech births and 46.3% of transverse lie births occurred at 37-38 weeks' gestation. Cesarean section was performed in 96.8% of breech presentations and 63.4% of transverse-lie ones.

Conclusions: These findings imply that the fetal chest-to-head circumference ratio may influence presentation at birth.