When my son was born on 12/31 I of course couldn't have had the number fast enough. I was told to put down. "applied for" that first year. After that, I had to use the actual number, of course. The worst that would happen, is the IRS might require proof of birth to show you were entitled to that deduction. But you should call/write the IRS --or see what H&R Block people say--to be sure.