Have you tried seeing what your hospital has available? I have contacted the South Shore Hospital and Harvard Vanguard Medical Assocs. Both are offering prenatal yoga classes.
I also have more south shore places I know of, but that doesn't help you much living in Brookline.
Healthworks in Chestnut Hill 10:45 am on Saturdays
Healthworks Back Bay 4:30 Thursdays
Healthworks Cambridge 4:30 Wednesdays
Drop in or guest rate is either $10 or $20 I can't remember, so I would call ahead to check.
Sports Club LA also opens their classes for non-members, $22 each