Is emend safe to take while pregnant or breastfeeding?

Yes. Likely so, as it is related to Zofran (ondansetron) which is considered largely safe. I've never heard of a pregnant woman using emend, however. It is a long acting 5ht3 antagonist typically used for prevention of nausea in women undergoing chemotherapy.
Pregnancy category B. Fda gas considered pregnancy category b. Animal reproduction studies have failed to demonstrate a risk to the fetus and there are no adequate well controlled studies in pregnant woomen/.