BACKGROUND This study was conducted to explore the effect of gestational age (GA) on the induction-to-abortion interval of mifepristone-misoprostol midtrimester termination of pregnancy (TOP) regimen. METHODS This study involved a consecutive series of 270 pregnancies between 12.0 and 22.6 weeks that have undergone legal TOP from April 2006 to June 2009. All women received a single oral dose of 200 mg mifepristone and, 36-48 h later, a course of misoprostol (an initial vaginal dose of 800 mcg plus four oral doses of 400 mcg at 3-hourly intervals). Treatment was considered to be a failure if abortion did not occur within 24 h. The impact of GA, parity and maternal age on the induction-to-abortion interval was assessed by means of Cox regression. RESULTS Overall, the mean GA at TOP was 18.0 weeks. The mean induction-to-abortion interval was 9.8 h (SD=8.2 h; range=1-50 h), and 246 women (91%) aborted successfully within 24 h. GA at TOP and parity were the only two variables independently associated with the induction-to-abortion interval. The mean induction-to-abortion interval was increased by about 50% in patients undergoing TOP between 20.0 and 22.6 weeks (12.9 h, SD=8.9), as compared with those at 16.0-19.6 weeks (7.8 h, SD=5.9) and 12.0-15.6 weeks (8.2 h, SD=8.3) (p<.001). The effect of parity on the induction-to-abortion interval was more modest, with a 20% increase in induction-to-abortion interval in nulliparous (10.1 h, SD=9.1), as compared with women with a previous live birth (8.1 h, SD=6.7). CONCLUSIONS The mean induction-to-abortion interval increases by 4 h after 20 weeks GA. This information may be relevant for counseling and planning of the procedure.