Such legislation already exist. How else could a man be forced to pay child support? If he is deemed the "father" at conception, then he HAS to be deemed the "father" CONTINUOUSLY through to the birth. You cannot be a father, then not be a father, then be a father again to your child. A "thing", or a lamp shade, etc cannot possess a "father". Only a child can. This means that child support laws already recognize conception as the beginning of life.
Interesting concept.
But, while it points out the inconsistent and illogical nature of current law, I doubt such a case would be accepted by the supremes as a challenge to Roe in the same way as would a state or federal definition of the beginning of life.