Table 2. Incidence rate ratios (IRR with 95% confidence interval) of epithelial ovarian cancer by twin births and sex of the twins, and by sex of children in women with singleton births onlya.

	No. of cases	P–year (× 104)	IRR (95% CI)	P-valueb
Twin birth				0.39
Never (singletons only)	5092	2706.2	1.00 (ref.)
Ever	112	57.2	0.92 (0.76–1.11)
By sex of the twins c				0.10
Two boys	33	19.3	0.82 (0.58–1.16)
Two girls	49	19.4	1.21 (0.91–1.61)
Boy and girl	30	18.5	0.73 (0.51–1.05)
Within age groups (year)
20–44
Two boys	9	12.2	0.83 (0.43–1.60)
Two girls	20	12.3	1.83 (1.18–2.85)
Boy and girl	9	11.2	0.88 (0.46–1.69)
45–54
Two boys	11	41.5	0.76 (0.42–1.38)
Two girls	12	41.5	0.83 (0.47–1.46)
Boy and girl	8	41.7	0.55 (0.27–1.10)
55–74
Two boys	13	29.5	0.88 (0.51–1.51)
Two girls	17	29.4	1.13 (0.70–1.83)
Boy and girl	13	31.6	0.81 (0.47–1.39)
P-value, test for interactiond			0.20
Singleton births, sex of children				0.96
Boys only	1163	662.9	1.00 (ref.)
Girls only	1339	735.7	1.02 (0.94–1.11)
Boys and girls	2590	1298.3	1.00 (0.92–1.08)

^aResults based on Poisson regression analysis of person–years (P–year) at risk, adjusted for attained age, birth-cohort, number of births, and maternal age at first and most recent birth.

^bTest for heterogeneity.

^cWomen with singleton births as reference group.

^dTest for heterogeneity in risk estimates in women above and below age 45 years.