So, one of the many households I play consists of a single guy. He was the roommate of one of my other Sims while they attended university together; I really liked the Sim, so I ended up moving him in...
Pregnant roommates move out before they have the baby :'( You can't have family units (or babies-teens) as roommates, only single sims.
However (if you have MCCC), you could wait 'till she gives birth, summon and add the baby to your household then invite her over and ask her to be your roommate again.
Edit: added more details
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