According to TurboTax, in order to claim a child on a tax return, a Social Security number must be provided. Social Security cards cannot be applied for until a baby is born. Failing to file the number on the tax return results in a fine and investigation.
TurboTax shares that prior to the government requiring Social Security numbers for dependents, the number of dependents claimed were seven million higher than after the law was put into affect. As long as a baby is born during the calendar year for which a person is filing, the child can be claimed as a dependent.