There are 2 issues at play here:
1. Personal exemptions - $4,050 (currently, adjusts for inflation) per taxpayer/dependent included on your return, deducted from taxable income. This is not prorated if a person is part of the family for only part of the year - that is, whether you adopt on Jan. 1 or Dec. 30, you still get $4,050.
2. Adoption tax credit - a credit to your tax liability equal to a portion of your adoption expenses. This is not a refundable credit - meaning that if you owe $10k in tax but are eligible for a credit of $15k, you will only get $10k of benefit.
All that being said, I would keep your withholding higher to make sure you can maximize the benefit of the adoption credit or to cover you if (God forbid) things don't work out. Keep in mind that what you put on your W-4 is not required to match what you actually claim on your return. Definitely worth a conversation with your tax guy.
*I am a CPA, but not your CPA. Verify info with your own provider.*
Edit to mention that both of the items above have been brought up in tax reform discussions, so you may want to wait to see how that plays out before making any major changes to your withholding structure.
