Has my wife cheated me?

I has sex with my wife in July 1st week without protection. Then on August 5-6 with protection. She left thereafter 2 days later. She came to know that she is pregnant when she went to doctor for some illness and he told that she shud see gyn as she had symptoms pf pregnancy. Lady doc confirmed she is pregnant. This happened between October 10-20. She had left me after home fight on August 7. When she knew of pregnancy she wished to abort the child saying that she didnt wanted it ie. my child. But parents stopped her. She finally delivered on June 3, 2011. I suspect child is not mine. Before i go for DNA and complicate the relation, I want to know even if we assume contraception failed on August 6, whatever is her mensural cycle, is it possible for her to know of her being pregnant so late ie after 60 days from August 10 (date of last sex) to October 10.