Table 3.

Poisson regression analysis of menarche to birth interval and the risk of operative delivery.

	Associations for a 5y increase in menarche to birth interval
	Unadjusted	P=	Fully adjusted†	P=
Risk ratios for operative delivery [95% CI]
All operative delivery	1.26 [1.23, 1.28]	<0.001	1.23 [1.20, 1.25]	<0.001
Cesarean	1.32 [1.27, 1.37]	<0.001	1.29 [1.23, 1.34]	<0.001
Operative vaginal*	1.27 [1.24, 1.30]	<0.001	1.24 [1.21, 1.28]	<0.001

Menarche to birth interval was truncated at 4y and 29y

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

^†Adjusted for maternal height, BMI, smoking status, marital status, onset of labor, week of gestational age, birth weight percentile, infertility treatment, diabetes, educational status, household income, previous miscarriages, pre-eclampsia, and congenital anomalies. Adjustment could not be performed for age at first birth due to the high degree of collinearity between menarche to birth interval and age at first birth (r=0.95).