Reproductive Risk Factors Associated with Breast Cancer Molecular Subtypes among Young Women in Northern China

Table 2

Multivariate-adjusted ORs and 95% confidence intervals (CIs) for associations between fertility factors and risk of invasive breast cancer by tumor subtype in older women (>40 years of age).

Reproductive characteristics	Controls	Luminal A		Luminal B		HER2 enriched		TNBC
	(%)	(%)	OR (95% CI)	(%)	OR (95% CI)	(%)	OR (95% CI)	(%)	OR (95% CI)
Age at menarche
≤13	1039 (32.5)	368 (34.1)	1.00 (reference)	456 (33.4)	1.00 (reference)	115 (36.4)	1.00 (reference)	104 (33.2)	1.00 (reference)
14	695 (21.7)	212 (19.6)	0.88 (0.72-1.07)	278 (20.4)	0.92 (0.77-1.10)	70 (22.2)	0.90 (0.66-1.23)	69 (22.0)	1.00 (0.73-1.38)
≥15	1465 (45.8)	500 (46.3)	0.99 (0.85-1.17)	632 (46.3)	1.01 (0.87-1.17)	131 (41.5)	0.79 (0.60-1.03)	140 (44.7)	0.96 (0.73-1.25)
			0.985		0.833		0.078		0.743
value for homogeneity of trends					0.854		0.206		0.899
Parity vs. no parity
Never	57 (1.8)	38 (3.5)	1.00 (reference)	41 (3.0)	1.00 (reference)	7 (2.2)	1.00(reference)	10 (3.2)	1.00 (reference)
Ever	3142 (98.2)	1042 (96.5)	0.48 (0.31-0.73)	1325 (97.0)	0.57 (0.38-0.85)	309 (97.8)	0.77 (0.35-1.71)	303 (96.8)	0.55 (0.28-1.08)
1	1522 (47.6)	536 (49.6)	0.52 (0.34-0.79)	666 (48.8)	0.60 (0.40-0.91)	155 (49.1)	0.81 (0.36-1.81)	141 (45.0)	0.53 (0.26-1.06)
≥2	1620 (50.6)	506 (46.9)	0.44 (0.29-0.67)	659 (48.2)	0.54 (0.35-0.81)	154 (48.7)	0.73 (0.33-1.63)	162 (51.8)	0.56 (0.28-1.12)
			<0.001		0.008		0.294		0.815
value for homogeneity of trends					0.401		0.372		0.144
Number of live births
1	1522 (48.4)	536 (51.4)	1.00 (reference)	666 (50.3)	1.00 (reference)	155 (50.2)	1.00 (reference)	141 (46.5)	1.00 (reference)
≥2	1620 (51.6)	506 (48.6)	0.84 (0.73-0.97)	659 (49.7)	0.89 (0.78-1.02)	154 (49.8)	0.90 (0.71-1.14)	162 (53.5)	1.07 (0.84-1.36)
Age at first live birth
<25	1639 (52.2)	485 (46.5)	1.00 (reference)	718 (54.2)	1.00 (reference)	173 (56.0)	1.00 (reference)	159 (52.5)	1.00 (reference)
25-29	1305 (41.5)	467 (44.8)	1.24 (1.07-1.44)	514 (38.8)	0.92 (0.81-1.06)	120 (38.8)	0.88 (0.69-1.12)	129 (42.6)	1.02 (0.80-1.30)
≥30	198 (6.3)	90 (8.6)	1.58 (1.21-2.07)	93 (7.0)	1.09 (0.84-1.42)	16 (5.2)	0.77 (0.45-1.32)	15 (5.0)	0.79 (0.45-1.37)
			<0.001		0.738		0.201		0.671
value for homogeneity of trends					<0.001		0.002		0.021
Breastfeeding (parous women)
Never	178 (5.7)	82 (7.9)	1.00 (reference)	94 (7.1)	1.00 (reference)	18 (5.8)	1.00 (reference)	18 (5.9)	1.00 (reference)
Ever	2964 (94.3)	960 (92.1)	0.72 (0.55-0.95)	1231 (92.9)	0.80 (0.62-1.04)	291 (94.2)	0.99 (0.60-1.64)	285 (94.1)	0.94 (0.57-1.55)
< 6 months	174 (5.5)	57 (5.5)	0.71 (0.47-1.06)	78 (5.9)	0.87 (0.60-1.25)	17 (5.5)	1.01 (0.50-2.03)	18 (5.9)	1.00 (0.50-2.00)
6–11 months	241 (7.7)	93 (8.9)	0.85 (0.60-1.22)	107 (8.1)	0.87 (0.62-1.22)	23 (7.4)	0.98 (0.51-1.87)	20 (6.6)	0.82 (0.42-1.60)
≥ 12 months	2549 (81.1)	810 (77.7)	0.70 (0.53-0.93)	1046 (78.9)	0.79 (0.61-1.02)	251 (81.2)	0.99 (0.60-1.64)	247 (81.5)	0.94 (0.57-1.56)
			0.022		0.059		0.973		0.859
value for homogeneity of trends					0.529		0.193		0.220

All odds ratios (OR) are adjusted for the 5-year age group, year of recruitment (2012, 2013, 2014, 2015, 2016, and 2017), oral contraceptive use (yes, no), BMI (<23, 23–24.99, 25-27.49, and ≥27.5 kg/m²), and family history of breast cancer (yes, no). Breastfeeding history was adjusted for the number of live births.