PIP: Indian marriage customs vary by religion, caste, geographic location, and educational background. This study examined the effects of age at marriage and socioeconomic factors on the interval to first birth in Assam and Uttar Pradesh (UP) states. Data were obtained on Bengali-speaking scheduled castes (the lowest castes) in Assam and Hindu-speaking scheduled castes in UP from a 1988-89 survey in Assam and a 1987-88 survey in UP. The Assam survey included 1805 scheduled caste households and the UP survey included 3931 households (1143 scheduled castes). Income is grouped as very poor (under 50 rupees), poor, middle, and rich (over 100 rupees). Cox's proportional hazard models expressed the relative risks of time to first birth. Marriage age underwent a change, with the greatest differences occurring among younger, illiterate, working class Manasudra subclass women in the highest income and middle status groups living in Ghazipur district. The life table methods of Namboordiri and Suchindran were used to calculate median lengths of birth intervals and revealed that Assam women have shorter intervals. The shortest intervals were among Assam working mothers, and the longest intervals were among Assam females of other subcastes. In UP, the shortest birth interval was among literate mothers, and the longest interval (49 months) was among younger mothers. Nearly 7% of women did not give birth within 97 months. As marriage age increased, the birth interval shortened. In Assam, the birth interval was significantly related to marriage ages under 13 years, 14 years, and 15-16 years; the present age of the mother, mother's occupation, other subcaste group; and the very poor and middle income group. In UP, the significant factors explaining birth interval were marriage ages of under 13 years and 14 years, present age of the mother, and mothers living in Ghazipur district. The relative risks of first birth for mothers in marriage age groups under 13 years, 14 years, and 15-16 years was 0.58, 0.77, and 0.88 respectively. Mothers in UP had similar risks in the first two age groups.