Hi, and thanks for your medical question on eHealth forum!
If your LMP was May 1st 2009, and you have an average cycle of 28 days, then you would have conceived around 15th of May. 21st of May would be too late. However if the cycle is more than 30 days then it is a possibility.
Please discuss this with your gynecologist once. Hope this helps. Take care!
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Personal consultation(s) with a qualified medical professional is the proper means for diagnosing any medical condition.