No, deductions do not begin at conception.

In Cassman v. U.S., 31 Fed.Cl. 121 (Fed. Cl. 1994), Plaintiffs were denied a 'dependent' tax deduction for an unborn child:

Plaintiffs argue that Jonathan Cassman was a resident of the United States prior to his birth because his mother was a resident, and they ask the court ot take judicial notice of the fact that it would have been physically impossible for the mother to be a resident and her unborn child not to be a resident. This argument is without merit. The court cannot justify viewing an unborn child as "residing" anywhere.