IMO, I think police found the cement blocks with legs attached. I read they used a tibia bone for DNA. Since most articles state the body was missing the legs, they would have had to find the legs in order to do this.
If they found the cement blocks and they have the molds, they could prove the blocks were made from that mold. I could be wrong, but I seem to recall that leg bones were used (muscle & bone marrow)from both the mother & baby.