Many fathers unwittingly sign the "acknowledgement of paternity" form when their baby is born, not realizing that this form then puts a time limit of 60 days (generally) as to when they must request a DNA test.
60 days - or be prepared to prove fraud (or some other ground)
Generally, when a new father signs the acknowledgement of paternity, he generally has up to 60 days to move to revoke the acknowledgement by asking for a DNA test. If he does not, then he must generally prove "fraud, duress, or material mistake of fact." This requirement is usually not satisfied by claiming "she told me the child is mine & now I have doubts."
If you have even a 1% doubt you are not the father, don't sign the acknowledgement
Many hospitals push both forms in front of the new parents & ask the father to sign both. Sometimes nurses mistakenly explain that the father must sign the acknowledgement of paternity in order to have his name placed on the birth certificate. This isn't true! The mother can put down anyone's name as the father on the child's birth certificate - placing the name on the birth certificate (in itself) does not establish paternity rights. Thus, if you have even a small doubt as to whether you are or are not the father, don't sign the acknowledgement of paternity. Just have the mother list your name on the borth certificate (if you both want it) & then go back to celebrating. You can always sign the acknowlgement of paternity at a later time.