Does the efficacy of placing a copper intrauterine device (IUD) for emergency contraception (EC) to prevent pregnancy depend on menstrual cycle timing and timing of unprotected intercourse (UPI)? If the urine pregnancy test is negative prior to IUD placement, the copper IUD is highly effective for EC at any point in the menstrual cycle. The use of the Copper T380A for EC has been encouraged by the failure of oral EC methods to decrease rates of unintended pregnancy and the documented success of the IUD in reducing unintended pregnancies. However scant data exist regarding the efficacy and safety of IUD insertion for EC when accounting for menstrual cycle timing and time since UPI. This is a secondary analysis of data obtained from a previously published prospective cohort study of women who received the Copper T380A IUD for EC between July 1997 and January 2000. We included 1840 participants according to the study inclusion criteria of a known last menstrual period (LMP) and cycle lengths of 2535 days. The original study included women aged between 18 and 44 years who presented for EC at 18 sites throughout China and who had regular menstrual cycles between 24 and 42 days, a known LMP, UPI within 120 h (5 days) and a negative urine pregnancy test (cutoff 25 IU/ml). Women with uncertain LMP dates were excluded. This study included only participants with cycle lengths of 2535 days. Among the 1840 participants with usual cycle lengths of 2535 days, 850 (46.2) had their IUD inserted following UPI in the expected fertile window and 84 (4.6) had the insertion 5 days after the predicted ovulation day and 52 (2.8) had the insertion 5 days after UPI. There were no pregnancies in the first month among the 1771 women who had information available regarding their 1-month follow-up pregnancy test. This was a secondary analysis of an observational study, and thus participants were not randomized to an alternative postcoital method. There were a small number of women who had UPI 5 days after their predicted ovulation day thus limiting the confidence of assuring a low risk of pregnancy in this situation. The ovulation day was calculated based on the LMP prior to IUD insertion and not on the subsequent first day of menses following IUD insertion. If the urine pregnancy test is negative prior to IUD placement, the copper IUD is likely to be effective for EC at almost any point in the menstrual cycle. The original study was funded by the UNDP/UNPFA/WHO/World Bank Special Programme of Research, Development and Research Training in Human Reproduction. The donors and sponsors of the study had no role in the study design, data collection, data analysis, data interpretation, writing of the report or the decision to submit the paper for publication.