. 2024 Jan 9;7(1):e2350934. doi: 10.1001/jamanetworkopen.2023.50934

Table 4. Adjusted Associations Between Prepregnancy BMI Category and Twin Delivery and Mediation Analysis by ART Conception.

Prepregnancy BMI	aRR (95 CI)^a			Proportion mediated, % (95% CI)	E-value for total effect^b
Prepregnancy BMI	Total effect	Natural direct effect	Natural indirect effect	Proportion mediated, % (95% CI)	E-value for total effect^b
All twins
Underweight	0.84 (0.74 to 0.95)	0.85 (0.75 to 0.96)	0.99 (0.98 to 1.00)	6 (−2 to 13)	1.67
Normal	1 [Reference]	1 [Reference]	1 [Reference]	NA	NA
Overweight	1.14 (1.07 to 1.21)	1.12 (1.05 to 1.19)	1.01 (1.01 to 1.02)	11 (3 to 18)	1.54
Obesity class I	1.16 (1.06 to 1.27)	1.12 (1.03 to 1.23)	1.03 (1.02 to 1.04)	23 (7 to 39)	1.59
Obesity class II	1.17 (1.02 to 1.34)	1.13 (0.98 to 1.29)	1.04 (1.02 to 1.06)	27 (0 to 54)	1.61
Obesity class III	1.41 (1.19 to 1.66)	1.44 (1.22 to 1.69)	0.98 (0.96 to 1.00)	−7 (−15 to 0)	2.17
Opposite sex twins only
Underweight	0.73 (0.58 to 0.91)	0.74 (0.69 to 0.93)	0.98 (0.96 to 1.01)	5 (−3 to 13)	2.08
Normal	1 [Reference]	1 [Reference]	1 [Reference]	NA	NA
Overweight	1.22 (1.09 to 1.35)	1.19 (1.07 to 1.33)	1.02 (1.01 to 1.03)	11 (3 to 19)	1.73
Obesity class I	1.33 (1.15 to 1.54)	1.27 (1.10 to 1.48)	1.05 (1.03 to 1.07)	18 (7 to 30)	1.99
Obesity class II	1.47 (1.19 to 1.81)	1.39 (1.13 to 1.72)	1.05 (1.02 to 1.08)	16 (5 to 27)	2.30
Obesity class III	1.64 (1.26 to 2.13)	1.68 (1.29 to 2.18)	0.97 (0.95 to 1.00)	−7 (−15 to 2)	2.67

Abbreviations: aRR, adjusted risk ratio; ART, assisted reproductive technology; BMI, body mass index (calculated as weight in kilograms divided by height in meters squared); NA, not applicable.

^{^a}

Effects are estimated from mediation analyses pooled across 20 multiply imputed data sets. aRRs adjusted for maternal height, age, smoking status, parity, and fiscal year.

^{^b}

E-values represent strength of unmeasured confounder (on rate ratio scale) needed to bring the point estimate for total effects to 1.0.