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In case, you have a 28 day cycle, and your LMP was on 2/20/12, then the possible ovulation would be around 3/5/12 and the date of conception can be from 5th to 7th of March. This information is based on extrapolation from the data provided.
In case, of doubts, you might consult with your doctor and get a Ultrasound test, which can give much accurate dates, for your convenience.
I Hope this helps.