I employ a data set with rich family information to explore what factors in addition to the traditional measures - mother’s age at the birth of her first child and the number of siblings - can explain the intergenerational transfer of fertility. I estimate the overall importance of family background on entry into parenthood and completed fertility and whether it changed over time. Furthermore, by adding family-specific factor shared by siblings, I explore what factor explains siblings’ similarity in fertility. Brother and sister correlations in age first birth and final family size were estimated using multi-level linear regression on Swedish longitudinal register data. The overall variation in fertility that can be explained by family of origin is approximately 15%-30% for women and 13%-20% for men. The overall importance of the family of origin has not changed over the approximately twenty-five birth cohorts that were studied (1940-67 for women, 1940-63 for men). The family-specific factors accounts for about 30 percent of siblings’ similarity in age at parenthood and about 25 percent of sibling similarity in the completed fertility. Social background matter the most, followed by socialization. Family complexity accounts for a very small part of the sibling similarity in fertility.