The rising incidence of premarital pregnancy is largely an indication of the rapid change of the attitudes and practices about sex and marriage in China．Pregnancies ended with abortion also imply new threats to women’ s health and development．Based on discussing the data bias and theoretical shortages in past research，we propose new frames to measure and analyze this problem．Using data from the 2013 National Floating Population Dynamic Monitoring Survey we examine age，peri- od，and cohort trends in premarital pregnancy，and test hypotheses about the effect of migration．Re- sults show that younger cohorts have higher incidence of premarital pregnancy promoted by migration， especially for the rural migrants and those with lower education．We suggest to collect more data on premarital sex behavior and pregnancy，and to offer family planning services for the youth and adults who have not yet married．