any possible way that it could be my ex's?

Since you are using your period as the way to determine the age of your baby, it is impossible that this method will give you any other date than with your current boyfriend (you had your period after having sex with you ex.

Your info is indicating that you got pregnant in the first few days of December from sex in the last week of November or the first week in December.

Normally an early ultrasound scan will give you a better estimation.

Take care.

Thank you. According to my last ultrasound which was done April 10, 2012 they said I was 19 weeks and 4 days according to the size of the baby. My original due date was August 24th. Now it is August 29th. I am just worried it may not be my current boyfriends. Is there any way that it could be the ex's?

Someone please reply so my mind can be at ease. Thanks.