My wife and I had a trial separation, since we have gotten back together we have found out she is pregnant. Her period started on th 25th of September and she is very irregular going from 28 days up to nearly 39 days.
We had sex on the 18th of October and since we have sorted our marriage out she has confessed to a one night stand.
I so confused and don't think the baby is mine as ovulation takes place 10-15 days from the beginning of the period. Somebody please help.

Consistent irregular periods would indicate consistent irregualar ovulation... every women is different and there are no hard and fast rules,that being said,the only way that I can think of to determine the father would be a DNA test when the child is born.

Someone else may be able to give a more detailed answer.