She is telling me she is 16 weeks so working back makes it the 4th of August I did not have sex with here till the first time on the 11th of August how do I wok it out can it be mine

Unfortunately you will not know until the baby is born and a paternity test is done. There is no way to tell during pregnancy.

You can have a prenatal paternity test done before the baby is born.
DNA Diagnostics Center Prenatal Paternity Testing (http://www.dnacenter.com/paternity/prenatal-testing.html)

The only way to know for sure is to have a DNA test performed after the baby is born.

You can have a prenatal paternity test done before the baby is born.
DNA Diagnostics Center Prenatal Paternity Testing (http://www.dnacenter.com/paternity/prenatal-testing.html)

This is EXTREMELY rare due to not only the costs, but also because of the risk factors involved. Risks include ruptured placenta, miscarriage, etc. I would not rely on this as a means of paternity prior to birth. Many doctors will not perform these tests unless Mom is high risk for several reasons.

This should NEVER be considered strictly for paternity.

She had sex with ex on 19 April I had sex on 12th may on 1st July scan came back 8weeks 2days who the dad