Table 1.

Mean and standard deviation for mother’s age at previous birth and calendar year, person years, and number of mothers in the analysis for second and third birth

	Second birth		Third birth
	Mean	SD	Mean	SD
Age at previous birth	27.68	5.20	29.62	4.42
Calendar year	2001.02	4.23	2002.43	3.50
N mothers	68,217		39,795
N person years	268,594		204,682