Should I get a DNA test before or after my son is born?

[Jon]

my gf and i are now seperated. She is due on march 11th. We found out we were expecting on july 5th and the conception date was june 19th. She says she was with no other man while we were togehter but I worked day shift that week and dont rember if we slept together or not during the week but most likely only the weekend and there was one night were she went drinking with her friend and I couldnt get ahold of her in the morning when I called cause she told me to call and wake her. But she started to give me weird feeling about our relationship about a month later and then on october 1st she broke it up with me saying she doesnt have the same feelings for me anymore. Now she is texting one of her exs and I say him send her a video of him playing with himself and another one of his privates. Should I ask her for a dna test before the baby is born or wait til after. I asked her once and she got really pissed off and told me to get out of the house. She swears its mine but the things after we broke up lead me to assume its not. Should I ask her for one now and risk not beign able to go to the appts and maybe delivery?