Figure 1.

Directed acyclic graph representing the role of gestational age. (A) Without stratifying or (B) stratifying on gestational age. Stratifying on gestational age as a ‘collider’ opens the path ‘Early pregnancy body mass index (BMI)—Unknown or unmeasured confounders—Severe asphyxia related complications’ and may result in biased estimates of associations between early-pregnancy BMI and severe asphyxia-related complications. Gestational age is known as a ‘collider’, as it can be affected by both early-pregnancy BMI and unknown or unmeasured confounders in the association between gestational age and severe asphyxia-related complications.