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

Table 3

Multivariate-adjusted ORs and 95% confidence intervals (CIs) for associations between fertility factors and risk of invasive breast cancer by tumor subtype in the younger group (≤40 years).

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	466 (47.4)	42 (48.8)	1.00 (reference)	204 (48.6)	1.00 (reference)	38 (50.7)	1.00 (reference)	68 (50.0)	1.00 (reference)
14	236 (24.0)	21 (24.4)	0.99 (0.57-1.71)	92 (21.9)	0.93 (0.69-1.26)	17 (22.7)	0.93 (0.51-1.69)	25 (18.4)	0.74 (0.46-1.21)
≥15	281 (28.6)	23 (26.7)	0.91 (0.54-1.56)	124 (29.5)	1.03 (0.79-1.35)	20 (26.7)	0.93 (0.53-1.64)	43 (31.6)	1.08 (0.72-1.64)
			0.750		0.883		0.782		0.836
value for homogeneity of trends					0.766		0.888		0.755
Parity vs. no parity
Never	52 (5.3)	10 (11.6)	1.00 (reference)	47 (11.2)	1.00 (reference)	5 (6.7)	1.00 (reference)	10 (7.4)	1.00 (reference)
Ever	931 (94.7)	76 (88.4)	0.40 (0.18-0.88)	373 (88.8)	0.40 (0.25-0.62)	70 (93.3)	0.76 (0.27-2.11)	126 (92.6)	0.85 (0.40-1.81)
1	599 (60.9)	54 (62.8)	0.44 (0.20-0.98)	237 (56.4)	0.41 (0.26-0.65)	37 (49.3)	0.68 (0.24-1.92)	88 (64.7)	0.93 (0.43-1.99)
≥2	332 (33.8)	22 (25.6)	0.32 (0.14-0.77)	136 (32.4)	0.38 (0.23-0.61)	33 (44.0)	0.92 (0.32-2.65)	38 (27.9)	0.69 (0.31-1.55)
			0.029		0.005		0.484		0.178
value for homogeneity of trends					0.316		0.015		0.431
Number of live births
1	599 (64.3)	54 (71.1)	1.00 (reference)	237 (63.5)	1.00 (reference)	37 (52.9)	1.00 (reference)	88 (69.8)	1.00 (reference)
≥2	332 (35.7)	22 (28.9)	0.73 (0.44-1.23)	136 (36.5)	0.93 (0.72-1.20)	33 (47.1)	1.40 (0.85-2.30)	38 (30.2)	0.74 (0.49-1.11)
Age at first live birth
<25	348 (37.4)	37 (48.7)	1.00 (reference)	157 (42.1)	1.00 (reference)	28 (40.0)	1.00 (reference)	52 (41.3)	1.00 (reference)
25-29	488 (52.4)	34 (44.7)	0.65 (0.40-1.05)	183 (49.1)	0.88 (0.68-1.14)	35 (50.0)	0.99 (0.59-1.67)	63 (50.0)	0.92 (0.62-1.36)
≥30	95 (10.2)	5 (6.6)	0.48 (0.18-1.27)	33 (8.8)	0.78 (0.50-1.22)	7 (10.0)	1.05 (0.44-2.54)	11 (8.7)	0.86 (0.43-1.73)
			0.043		0.209		0.949		0.598
value for homogeneity of trends					0.263		0.243		0.293
Breastfeeding (parous women)
Never	66 (7.1)	7 (9.2)	1.00 (reference)	41 (11.0)	1.00 (reference)	6 (8.6)	1.00 (reference)	14 (11.1)	1.00 (reference)
Ever	865 (92.9)	69 (90.8)	0.75 (0.33-1.70)	332 (89.0)	0.58 (0.38-0.88)	64 (91.4)	0.80 (0.33-1.94)	112 (88.9)	0.62 (0.34-1.15)
<6 months	59 (6.3)	5 (6.6)	0.75 (0.22-2.50)	30 (8.0)	0.69 (0.38-1.25)	5 (7.1)	0.83 (0.24-2.91)	15 (11.9)	1.11 (0.49-2.52)
6–11 months	141 (15.1)	13 (17.1)	0.82 (0.31-2.17)	59 (15.8)	0.67 (0.40-1.10)	12 (17.1)	1.06 (0.38-3.00)	25 (19.8)	0.84 (0.41-1.72)
≥12 months	665 (71.4)	51 (67.1)	0.73 (0.32-1.68)	243 (65.1)	0.55 (0.36-0.84)	47 (67.1)	0.75 (0.30-1.83)	72 (57.1)	0.52 (0.28-0.99)
			0.492		0.005		0.428		0.004
value for homogeneity of trends					0.580		0.962		0.209

All odds ratios (OR) are adjusted for 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.