Newly prego: exercise question!

Mommy • Trying to conceive baby #2!
Hey girls! I usually do workout classes at my gym about 4 days per week. Today I told my instructor I was prego (in private of course!) and she said I shouldn't do any core work or ab exercises. I thought that sounded strange. Does anyone know if continuing to workout the way you always have before pregnancy is ok? Even core exercises?