EMERGENCY when did She conceive

So long story short, A friend of mine had sex on February 8 and the 28th with a partner.  Then again on March 7 with a different partner.  The first partner was unprotected and he  finished inside of her and the one in March did not.  I just found out she is pregnant 4-5 weeks.  Which time frame did She concieve? Please no stupid remarks. 

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