I assume you're a US citizen and the child is yours. It's unlikely that your girlfriend can get any US visa in time to give birth in the US. So prepare for your child's US citizenship paperwork by collecting documentary evidence of your physical presence in the US, like your school/employment/medical records. Assuming you qualify to transmit citizenship, your child will be a US citizen from birth, and can be issued CRBA and US passport. Learn more about the CRBA process here -- https://br.usembassy.gov/u-s-citizen-services/citizenship-services/
As to your girlfriend's paperwork, if you're ready to get married, I also recommend going to Brazil as soon as you are able, getting married, and pursuing CR1 visa.