I can't make my Sim pregnant anymore on the Sims 2?

I have spawned the tombstone of L and D, and had the parents woohoo to try for a baby, and so far have had seven kids. I need my Sim to have 3 more, for her life long aspiration (yeah, what a freak), but she won't get pregnant anymore. I have had them drink the life potion stuff (green hourgl aspiration award), and I guess she's already too old or something. I really want her to have 3 more kids. please help me!