After menstrual without Sex wife got pregnant????

I had done sex with my wife before 2 days(26/6/08) of her MC(28/6/08). she got her menstrual timely. after that i have not done made relationship with her but now she said that she is 5 week pregnant on 2nd Aug 08. How it can possible. Can sperm live in women body through MC and can survive and can make her pregnant. Please help me what is this? How it is possible? I m too much tensed. Please help me. She undergone through intra vaginal USG and in report doctor said she is 5 week pregnant The size of GS is 44 to 48 MM.secondary sac is seen there but no heartbeat is there he suggest us to come after 1 week to check heartbeat. If we count 28 jun as Last MP than she is 5 week pregnant,whose child she is carrying bcos i have not made any relationship in this month due to i was out of home. if on 28 jun that was not MC than it should be more than 7-8 weeks. Please suggest me what i have to do to clear this