Table 2.

Association of number of births among women born after 1965 and diagnosed with primary invasive breast cancer between 2009 and 2016 in Scotland by molecular subtypes adjusted for age at diagnosis with and without an interaction term for age at diagnosis

	Luminal B-like (HER2−) (n = 998)		Luminal B-like (HER2+) (n = 629)		HER2-overexpressed (n = 214)		TNBC (n = 617)
	OR	95% CI	OR	95% CI	OR	95% CI	OR	95% CI
Number of births
0	1.00	(reference)	1.00	(reference)	1.00	(reference)	1.00	(reference)
1	1.03	0.82–1.28	1.12	0.87–1.45	1.66	1.09–2.54	1.45	1.11–1.89
2	1.16	0.95–1.42	1.10	0.87–1.39	1.78	1.20–2.63	1.50	1.18–1.92
≥ 3	1.11	0.87–1.42	0.98	0.73–1.32	1.87	1.18–2.96	1.44	1.07–1.94
Age at diagnosis (years)
20–35	1.00	(reference)	1.00	(reference)	1.00	(reference)	1.00	(reference)
36–40	0.76	0.56–1.03	0.62	0.45–0.86	0.60	0.37–0.96	0.52	0.37–0.71
41–45	0.50	0.38–0.66	0.40	0.30–0.54	0.32	0.21–0.50	0.30	0.23–0.41
46–50	0.39	0.29–0.52	0.27	0.20–0.38	0.27	0.17–0.44	0.23	0.17–0.32
Models with an interaction term between number of births and age at diagnosis
Number of births
0	1.00	(reference)	1.00	(reference)	1.00	(reference)	1.00	(reference)
1	1.46	0.83–2.56	1.23	0.69–2.22	2.00	0.83–4.82	1.65	0.92–2.95
2	2.72	1.57–4.71	1.87	1.05–3.34	2.48	1.02–5.98	2.46	1.38–4.40
≥ 3	1.74	0.95–3.91	0.95	0.48–1.85	2.59	0.99–6.74	1.91	1.00–3.64
Age at diagnosis (years)
20–35	1.00	(reference)	1.00	(reference)	1.00	(reference)	1.00	(reference)
36–40	0.94	0.58–1.54	0.64	0.38–1.08	0.66	0.28–1.53	0.54	0.31–0.92
41–45	0.89	0.57–1.40	0.57	0.38–1.08	0.38	0.17–0.87	0.41	0.25–0.68
46–50	0.51	0.31–0.81	0.24	0.14–0.41	0.33	0.14–0.77	0.27	0.16–0.46

^*Significant at 5% level

Referent subtype used here is luminal A-like breast cancer models included an interaction term between age at diagnosis and number of births