Objective: To investigate the conception probability among Chinese women. Design: Prospective observational study. Setting: Clinics in hospitals and family planning institutes in 10 provinces and cities. Patient(s): A total of 851 healthy married women aged 18-35 years with normal menstrual cycles who wish to have babies and with no contraception. Intervention(s): Urinary LH was measured around days of expected ovulation for 7 days. The Barrett and Marshall model was used for calculation of conception probabilities on each cycle day from day -5 to day +1 in women with multiple episodes of intercourse. Main Outcome Measure(s): Pregnancies in 1, 3, and ≥6 months. Result(s): A total of 851 women with 2,055 cycles were analyzed. In 489 cycles there was only one episode of intercourse. A total of 601 pregnancies occurred. The conception probabilities from days in relation to ovulation -5 to +1 for a single episode of intercourse were 0.216, 0.102, 0.236, 0.233, 0.388, 0.293, and 0.386, respectively, and for multiple episodes they were 0.254, 0.271, 0.293, 0.365, 0.315, and 0.284, respectively, with the peak value at day -1. Recalculation of the efficacy of emergency contraception with low-dose mifepristone with the present conception probabilities showed higher efficacy. Conclusion(s): Conception probabilities among Chinese women are different from those in the literature. Further comparative studies are needed to confirm an ethnic difference. Copyright © 2010 American Society for Reproductive Medicine, Published by Elsevier Inc.