. Author manuscript; available in PMC: 2022 May 1.

Published in final edited form as: Birth Defects Res. 2021 Feb 19;113(8):633–643. doi: 10.1002/bdr2.1877

TABLE 2.

Selection probabilities for case and control mothers given self-reported body mass index under three assumptions about occurrence of selection bias in the study

	Assumption 1^a		Assumption 2^b			Assumption 3^c
Self-reported body mass index category	Case mothers	Control mothers	Case mothers (LB, SB)	Case mothers(TOP)^d	Control mothers	Cases mothers (LB, SB)	Cases mothers(TOP)^d	Control mothers
Underweight	0.67	0.65	0.67	0.67*p	0.65	0.67	0.67*p	0.65*0.84

Normal weight	0.67	0.65	0.67	0.67*p	0.65	0.67	0.67*p	0.65*1.04

Overweight	0.67	0.65	0.67	0.67*p	0.65	0.67	0.67*p	0.65*0.96

Obesity	0.67	0.65	0.67	0.67*p	0.65	0.67	0.67*p	0.65*0.89

Missing	0.67	0.65	0.67	0.67*p	0.65	0.67	0.67*p	0.65*1.00

Abbreviations: LB, live birth; SB, stillbirth; TOP, termination of pregnancy.

Study participation rates are the only source of selection bias.

Study participation rates and underascertainment of cases contribute to selection bias.

Study participation rates and underascertainment of cases contribute to selection bias and study participation rates vary by case–control status and prepregnancy body mass index.

p is the selection probability for mothers in sites and years with and without active ascertainment of cases among prenatal diagnoses: p = .22 among mothers enrolled under active ascertainment; p = .14 among mothers who were not.