Table 1.

Selected characteristics of the participants and their mothers

	n	Mean %	SD	Median	IQR
Participant characteristics at MRI examination
Age, months	491	257.9	11.0	259.0	14.0
BMI, kg/m2	487	23.9	4.4	23.0	5.1
Menstrual cyclea
Luteal phase	70	14.4
Irregular periods	50	10.3
Follicular phase	28	5.8
Use of hormonal contraception	339	69.6
Left-right average breast volume, cm3	490	647.2	461.1	507.8	469.2
Left-right average breast fat volume, cm3	490	406.3	349.5	292.2	327.9
Left-right average breast water volume, cm3	490	240.9	131.2	209.8	172.4
Left-right average breast percent water,b %	491	41.8	10.3	41.7	16.0
Maternal characteristics at participant’s birth
Mother’s age at menarche, years	449	12.9	1.5	13.0	2.0
Mother ever used oral contraceptive pill, %	452	96.7
Age when mother first used contraceptive pill, years	435	18.8	3.1	18.0	3.0
Mother’s height, cm	446	16.6	6.5	165.1	7.6
Mother’s age at first birth, years	463	26.5	4.7	27.0	7.0
Mother’s age at participant’s birth, years	467	29.9	4.5	30.0	6.0
Mother’s parity at participant’s birth
0	223	48.5
1	161	35.0
2+	76	16.5
Mother’s pre-pregnancy BMI, kg/m2	430	22.3	3.0	21.7	3.5
Maternal history of BC when participant was 8 years old, %	355	11.6
Maternal characteristics at mammography
Age, years	176	52.7	3.9	52.0	5.0
BMI,c kg/m2	165	24.3	4.7	23.3	5.4
Left-right average breast area, cm2	176	295.0	141.3	266.8	167.1
Left-right average dense area, cm2	176	63.9	37.3	59.3	37.6
Left-right average percent density, %	176	25.3	13.4	24.8	20.4
In utero exposures
Placental weight, g	121	587.1	132.9	580.0	160.0
Absolute GWG, week 0 to delivery, kg	422	12.1	3.9	12.0	5.0
Mother drank alcohol during pregnancy, %	459	74.1
Mother smoked during pregnancy, %	464	10.6
Participant characteristics at birth
Birthweight, g	460	3395.0	472.6	3400.0	565.0
Birth length, cm	362	50.5	2.3	50.8	2.7
Head circumference, cm	370	34.6	1.2	34.6	1.5
PI,d g/cm3	358	26.4	4.1	26.1	3.2
Gestational age,e weeks
< 39	93	19.9
39	101	21.6
40	132	28.3
≥ 41	141	30.2

Abbreviations: MRI Magnetic resonance imaging, BC Breast cancer, BMI Body mass index, GWG Gestational weight gain, PI Ponderal Index

^aEstimated for women not using hormonal contraception by calculating the number of days since the last menstrual period (date of MRI to start of last menstrual period). Luteal (days 14–17 to 28–31) and follicular (days 0 to 14–17) phases and an ‘irregular period’ (32+ days) were defined using average length of menstrual cycle

^bSections of the breast were missing in the MRI images for one participant; thus, volumetric measurements could not be ascertained, and percent water only was used

^cClinically measured or self-reported BMI. Median time interval between BMI assessment and mammography was 3 years (IQR 1.5 years)

^dPI defined as birthweight (g)/birth length (cm³)

^eData available only as a categorical variable