Background: School-based pregnancy prevention programs should optimally be offered while students are still engaged in school since early disengagement is strongly associated with risk of a teen birth. Methods: We used linked New York City birth and enrollment data (2005-2013), a sample of 6,809 teen mothers (mean age conception = 16.2 years). We measured preconception disengagement using monthly absences, age 12 until conception, and identified five attendance trajectories using group-based trajectory modeling. Results: We identified five attendance trajectories that were heterogeneous with respect to age of onset and rate of increase of absenteeism. In two groups, 80% were chronically absent (CA) [2-<4 absences/month] or severely chronically absent (SCA) [4+ absences/month] at age 12, and over 90% by age 13, when they averaged 3.8-5.1 absences/month. Despite heterogeneity, 37% of teen mothers were CA or SCA at age 12, increasing to 56% by age 14. By early high school, age 15, 63% of teen mothers had absenteeism problems; 26% were CA and 37% SCA. Conclusions: Most teen mothers were disengaged before high school, years before conception. School-based pregnancy prevention programs should be offered in middle school or earlier when at-risk students are not missing significant amounts of school and may be more receptive to prevention messages.