All bets are off right now. The new tax law for 2018 eliminates personal exemptions, including exemptions for dependents, so having a child will not change your tax. The new W-4 form for 2018 is not available yet. The IRS has said that they will provide some guidance for 2018 withholding sometime in January. Your best course right now is to do nothing and wait. Don't make any changes to your W-4s. You'll have to make some adjustments after the new withholding information is available. See the following link on the IRS web site for more information.
https://www.irs.gov/newsroom/irs-statement-withholding-for-2018
