From IRS Pub 501 page 13, https://www.irs.gov/pub/irs-pdf/p501.pdf
Death or birth of child
A child who was born or died during the year is treated as having lived with you more than half the year if your home was the child's home more than half the time the child was alive during the year. The same is true if the child lived with you more than half the year except for any required hospital stay following birth.
Child born alive
You may be able to claim as a dependent a child born alive during the year, even if the child lived only for a moment. State or local law must treat the child as having been born alive. There must be proof of a live birth shown by an official document, such as a birth certificate. The child must be your qualifying child or qualifying relative, and all the other tests to claim the child as a dependent must be met.
