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. At that point, you may qualify for a larger premium tax credit. (For example, in 2021, if you and your spouse together earned $35,000, as a household of two you would be required to contribute 6.2 percent of your household income toward the premium, or $158/month, for the benchmark plan in the marketplace. This is because your income is at least 200 percent of the federal poverty level, but less than 250 percent of the federal poverty level. Once the baby is born and you are a household of three, you would be required to contribute 0 percent of your income, or $0/month). With your income and the expansion of your household, your household will be less than 150 percent of the federal poverty level.
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. 2018-34).