Table 4. Unadjusted odds ratios for breast cancer by parity, age at first birth, CMV IgG and EBV IgG seroconversion.

Characteristic	Number of cases	Number of controls	Odds ratio for breast cancer	(95% CI)
Nulliparity
No	343	344	1.0
Yes	56	55	1.0	(0.6–1.5)
Number of children
None	56	55	1.0
1	55	45	1.1	(0.7–2.0)
2	132	124	1.1	(0.7–1.7)
3	103	92	1.0	(0.6–1.7)
4	39	46	0.8	(0.4–1.5)
5+	14	37	0.3	(0.2–0.7)
Age at first child (for parous women only) (years) a
0–20	63	72	1.0
21–22	68	57	1.4	(0.9–2.4)
23–24	47	66	0.9	(0.5–1.5)
25–27	59	61	1.2	(0.7–2.0)
28+	64	45	1.8	(1.0–3.3)
BMI b
⩽22	83	81	1.0
21.1–24	67	63	1.1	(0.6–1.7)
24.1–26	46	57	0.8	(0.4–1.3)
>26	81	76	1.0	(0.6–1.6)
Seroconversion for CMV IgG between samples
No	388	396	1.0
Yes	11	3	3.7	(1.0–13.0)
Seroconversion for CMV IgG between samples in parous women a
No	333	342	1.0
Yes	10	2	9.0	(1.1–71.0)
Seroconversion for EBV IgG between samples
No	397	396	1.0
Yes	2	3	0.7	(0.1–4.0)

Abbreviations: BMI=body mass index; CI=confidence interval; CMV=cytomegalovirus; EBV=Epstein–Barr virus; IgG=immunoglobulin G.

^aRestricted to 301 cases and 301-matched controls with at least one live birth.

^bRestricted to 277 cases and 277-matched controls with BMI available.