The patients were randomly allocated to 2 treatment groups using computer-generated numbers in sealed envelopes. Treatment was started immediately after the test result was known, and continued until 36 completed weeks of pregnancy or active onset of labor. The patients were randomized to receive either a daily dose of 3 mg warfarin oral tablet or a daily placebo sachet The placebo sachets were especially manufactured to look identical to warfarin tablets. The tablets were placed in sacs and then stored in envelopes numbered from 1 to 200 The envelopes were numbered and randomized according to computer-generated randomization tables to ensure an equal number of patients in each arm. Throughout the trial, access to the randomization code was available only to the pharmacist who manufactured the placebo and packed the envelopes and was not available to any of the treating physicians or patients