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.
The information provided on eHealth Forum is designed to improve, not replace, the relationship between a patient and his/her own physician.
Personal consultation(s) with a qualified medical professional is the proper means for diagnosing any medical condition.