Short answer: A birth, adoption, or placement for adoption is a qualifying event, so you generally have a special enrollment window, usually 30 days on an employer plan (60 days on the marketplace), to add the child, with coverage retroactive to the date of birth or placement.
Adding a new child is one of the most time-sensitive enrollment tasks. On an employer group plan, you typically have 30 days from the birth, adoption, or placement to enroll the child, and coverage is usually effective back to that date so the newborn’s early care is covered. On the ACA marketplace the window is generally 60 days. Don’t wait; missing the window usually means waiting until the next open enrollment, leaving the child uninsured in between. Submit the enrollment change with any required documentation (such as a birth certificate or placement papers) as soon as possible.