Ondansetron (Category B1) is safe in pregnancy and one of the alternative medications to manage morning sickness. It isn't Category A because you are not ethically permitted to test drugs on pregnant women. However, it is well recognised within the literature as an option.

A 2004 study states "This drug does not appear (although the sample size is limited) to be associated with an increased risk for major malformations above baseline"

BJOG. 2004 Sep;111(9):940-3.
The safety of ondansetron for nausea and vomiting of pregnancy: a prospective comparative study.
Einarson A, Maltepe C, Navioz Y, Kennedy D, Tan MP, Koren G.

This article Australian Family Physician Vol. 36, No. 9, September 2007 may give you alternative options for management too.