If you are applying for marketplace coverage, you and your spouse will apply as a household of two. When the baby is born, you must update your family information with the marketplace within 30 days to reflect that you have become a household of three. The birth of a baby is a qualifying life event that entitles you to a special enrollment period to add your child to your plan. At that point, you may qualify for a larger premium tax credit based on your income and the expansion of your household.
If your household size including the unborn child puts your income eligibility at or below 138 percent of the federal poverty level then you should apply for Medicaid coverage. This is because Medicaid and CHIP eligibility counts pregnancy when calculating household size, so that even without an income change your eligibility for Medicaid could change. (26 C.F.R. § 1.36B-3; 45 C.F.R. § 155.330; IRS, Rev. Proc. 2022-34).