Objective: To help identify the optimal timing for provision of pregnancy decision-aids, this paper examines temporal patterns in women's preference for mode of birth after previous cesarean, prior to a decision-aid intervention. Methods: Pregnant women (. n=. 212) with one prior cesarean responded to surveys regarding their preference for elective repeat cesarean delivery (ERCD) or trial of labor (TOL) at 12-18 weeks and again at 28 weeks gestation. Patterns of adherence or change in preference were examined. Results: Women's preferences for birth were not set in early pregnancy. There was evidence of increasing uncertainty about preferred mode of birth during the first two trimesters of pregnancy (McNemar value. =. 4.41, p=. 0.04), decrease in preference for TOL (McNemar value. =. 3.79, p=. 0.05) and stability in preference for ERCD (McNemar value. =. 0.31, p=. 0.58). Adherence to early pregnancy choice was associated with previous birth experience, maternal country of birth, emotional state and hospital site. Conclusion: Women's growing uncertainty about mode of birth prior to 28 weeks indicates potential readiness for a decision-aid earlier in pregnancy. Practice implications: Pregnancy decision-aids affecting mode of birth could be provided early in pregnancy to increase women's opportunity to improve knowledge, clarify personal values and reduce decision uncertainty.