I totally agree with Richo. There's already millions of unwanted kittens/cats in the world that are being killed every day because they have no homes.
So unless you intend on keeping the entire litter of kittens, or like Richo said... already have loving homes for the entire litter (which could be 6 or more kittens)... then you should NOT purposely let her get pregnant.
Contrary to the myth that cats are healthier once they have at least one litter of kittens before they're fixed, is NOT true. Check with several vets... they'll all tell you they live longer/healthier lives if they never give birth.