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

Table 2 Mutually adjusted associations of MRI percent water in daughters with maternal characteristics and markers of in utero exposures estimated using the complete and imputed data

	Relative change in MRI percent water, geometric mean (95 % CI)
	Complete data^a	Imputed data^b
Maternal characteristics^c	n = 303	n = 490
At participant’s birth only
Mother’s age at menarche (per 1 SD 1.5 years)	0.98 (0.96–1.01)	1.00 (0.98–1.01)
Age mother first used OC (per 1 SD 3.1 years)	1.00 (0.98–1.02)	1.00 (0.98–1.02)
Mother’s height (per 1 SD 6.5 cm)	1.02 (1.00–1.04)	1.03 (1.01–1.05)
Mother’s age at first birth (per 1 SD 4.7 years)	0.99 (0.96–1.03)	0.98 (0.96–1.01)
Mother’s age at participant’s birth (per 1 SD 4.5 years)	1.00 (0.96–1.03)	1.01 (0.98–1.03)
Mother’s pre-pregnancy BMI (per 1 SD 3.0 kg/m²)	1.01 (0.99–1.03)	1.01 (0.99–1.03)
Mother’s parity at participant’s birth
0	1 (ref)	1 (ref)
1	1.04 (0.99–1.10)	1.02 (0.98–1.06)
2+	1.00 (0.93–1.08)	1.00 (0.95–1.06)
Mother had a history of breast cancer
No	1 (ref)	1 (ref)
Yes	1.01 (0.95–1.08)	1.01 (0.96–1.07)
Maternal characteristics at participant’s birth and at mammography^d
Mother’s MPD (per 1 SD 13.4 %)	1.06 (1.01–1.10)	–
In utero exposures^e	n = 107	n = 490
Placental weight (per 1 SD 133.5 g)	1.03 (0.99–1.07)	1.03 (0.99–1.07)
Absolute GWG, week 0 to delivery (per 1 SD 3.9 kg)	0.97 (0.93–1.01)	1.00 (0.98–1.02)
Mother drank alcohol during pregnancy (%)
No	1 (ref)	1 (ref)
Yes	1.01 (0.91–1.11)	1.00 (0.95–1.06)
Mother smoked during pregnancy (%)
No	1 (ref)	1 (ref)
Yes	0.95 (0.89–1.03)	0.99 (0.96–1.04)

Abbreviations: MRI Magnetic resonance imaging, BMI Body mass index, GWG Gestational weight gain, OC Oral contraceptives, MPD Mammographic percent density ref Reference category
MRI breast measurements were log-transformed, and exponentiated estimated regression parameters, with 95 % CIs calculated by exponentiating the original 95 % CIs, are presented. Bold indicates 95 % CI do not cross the null (1.00)
^aAnalysis restricted to those with non-missing data for all variables included in the models
^bSee Statistical methods section in main text
^cMaternal and confounding factors (age, BMI and menstrual phase/hormonal contraceptive use at MRI) were included in the model simultaneously
^dAnalysis restricted to the subset of participants for whose mothers it was possible to retrieve a mammogram (n = 116). Model includes all the maternal characteristics at the participant’s birth listed in the table as well as maternal MPD in later life (mean age at mammography 52.8 years; Table 1), adjusting for the daughters’ age, BMI and menstrual phase at the time of MRI and for the mothers’ age and BMI at the time of mammography
^eIn utero and confounding factors (age, BMI and menstrual phase/hormonal contraceptive use at the time of MRI) were included in the model simultaneously

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