Table 2.

Poisson regression analysis of age at menarche and the risk of operative delivery

	Associations for a 5y increase in age at menarche
	Adjusted for age at first birth†	P=	Adjusted for menarche to birth interval	P=
Risk ratios for operative delivery [95% CI]
All operative delivery	0.79 [0.74, 0.84]	<0.001	0.99 [0.93, 1.06]	0.81
Cesarean	0.68 [0.60, 0.78]	<0.001	0.89 [0.78, 1.02]	0.10
Operative vaginal*	0.81 [0.75, 0.87]	<0.001	1.03 [0.95, 1.12]	0.48

In all analyses, age at menarche was truncated at 9y and 17y, age at first birth was truncated at 16y and 42y, menarche to birth interval was truncated at 4y and 29y. The correlation between menarche to birth interval and age at menarche was weak (r= -0.22), hence both could be included as covariables in the same model.

^*Analysis of the risk of operative vaginal delivery was confined to women who were not delivered by cesarean.

^†The corresponding risk ratios (95% CI) for a 5y increase in maternal age at the time of the first birth, adjusted for the age at menarche, were 1.26 (1.24, 1,28) for all operative delivery, 1.31 (95% CI 1.25, 1.36) for cesarean section, and 1.27 (1.24, 1.30) for operative vaginal delivery.