in A French Lawyer website, I read that in French :

" The mother must sign at the child's birth, a document that certifies that she recognizes that I am the father.

If she did not sign this document, Thailand does not recognize you as the father on the birth certificate, even though your name is registered in the Birth Certificate. "

It's really true and she can do it ?

Edited October 10, 201213 yr by Danielsiam

No, you cannot do it at birth this way. You have to go to court. Only when a child is about 7 years or older do you not have to go to court if other AND child agree. (But for an extension of sty based on the child immirgation might still insist that you go to court and not accpet a legitimisation at the amphur, although it is a perfectly legal way to legitimize a child)

See: http://www.thaivisa.com/forum/topic/381917-how-to-gain-parental-rights-as-a-father/