The problem is that Congress is supposed to exercise only the powers enumerated in the Constitution, and those powers don’t include regulating state medical procedures. (The federal government lacks even the power to criminalize murder as such: All federal “murder” statutes punish murdering someone in the course of violating some other federal law because unlike states, the federal government has no general “police power.”)
If you scroll through the powers enumerated to Congress in Article I, Section 8 of the Constitution, you’ll find such things as establishing uniform rules of bankruptcy, raising and supporting armies and navies, and establishing post offices and post roads. What you won’t find is anything that supports congressional power to impose a time limit on abortion.
The standard end run around this is to invoke the Commerce Clause, that is, Congress’ power to regulate commerce “among the several states.” But the performance of an abortion in a local clinic is commerce among the states only if you adopt an unjustifiably expansive reading of the Commerce Clause that supports near-unlimited government power, so the party of small government ought to be reluctant to do so. Some might labor under the misapprehension that the Supreme Court has already upheld Congress’ power to regulate abortion under the Commerce Clause. In fact, in Gonzales v. Carhart, as Justice Clarence Thomas noted, that question was not addressed by either the Supreme Court or any lower court.