Pre-natal exposures and breast tissue composition: findings from a British pre-birth cohort of young women and a systematic review

Table 3 Associations between participant’s size at birth and MRI percent water

			Relative change in MRI percent water, geometric means^a (95 % CI)
			Complete data^b	Imputed data^c (n = 491)
Absolute size vs. rate of growth			n = 455
Model 1	Birthweight (per 1 SD 472.6 g)		1.03 (1.02–1.05)	1.03 (1.02–1.05)
Model 2	Birthweight (per 1 SD 472.6 g)		1.04 (1.02–1.06)	1.04 (1.02–1.06)
	Gestational age (weeks)	<39	1 (ref)	1 (ref)
		39	0.97 (0.92–1.02)	0.97 (0.92–1.02)
		40	0.97 (0.92–1.02)	0.97 (0.92–1.02)
		41+	0.96 (0.92–1.01)	0.96 (0.92–1.01)
LR test/Wald test p value^d			0.519	0.477
Which measure best captures linear (skeletal) growth?			n = 356
Birth length (per 1 SD 2.3 cm)			1.00 (0.98–1.02)	1.01 (0.99–1.03)
Head circumference (per 1 SD 1.2 cm)			1.02 (1.00–1.05)	1.02 (1.00–1.04)
Linear growth vs. adiposity			n = 361
Model 1	Birthweight (per 1 SD 472.6 g)		1.03 (1.01–1.05)	1.03 (1.02-1.05)
Model 2	Birthweight (per 1 SD 472.6 g)		1.03 (1.00–1.06)	1.03 (1.01–1.06)
	Head circumference (per 1 SD 1.2 cm)		1.01 (0.98–1.03)	1.00 (0.97–1.03)
LR test/Wald test p value^d (n = 353)			0.671	0.917
Model 1	Birthweight (per 1 SD 472.6 g)		1.03 (1.01–1.05)	1.03 (1.02–1.05)
Model 2	Birthweight (per 1 SD 472.6 g)		1.03 (1.01–1.05)	1.03 (1.02–1.05)
	Ponderal Index (per 1 SD 4.1 g/cm³)		1.01 (0.99–1.02)	1.00 (0.99–1.02)
LR test/Wald test p value^d			0.577	0.654

Abbreviations: MRI Magnetic resonance imaging, LR Likelihood ratio test, ref Reference category
^aMRI percent water was log-transformed for the analysis, and exponentiated estimated regression parameters, with 95 % CIs calculated by exponentiating the original 95 % CIs, are presented. Models adjusted for age, BMI z-score and menstrual phase/hormonal contraceptive use at the time of MRI scan. Bold indicates 95 % CI do not cross the null (1.00)
^bAnalysis restricted to those with non-missing data for all variables included in each model
^cSee Statistical methods section of main text
^dLR test performed on the complete record data, while a Wald test was performed on the imputed data (and summarised using Rubin’s rule), to test the null hypothesis that the inclusion of the additional variable in model 2 did not improve the fit to the data

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