A certified birth certificate is mandatory proof that the US citizen spouse must submit. This certificate is different from just a photocopy of a birth certificate. You can typically identify if the birth certificate is a certified copy by looking for a stamp (ink) or something that clearly suggests the copy is certified.

If you don't have this, most states allow you to request a copy online through their respective secretary of state or department of health website. This is a key basic piece of evidence and not submitting this makes it very likely that a RFE (request for evidence) may be sent by USCIS.