My wife is pregnant. is it safe for her to have a ppd test for tb?

Yes: The majority of sources indicate that it is safe but there does seems to be some disagreement. The cdc and world health organization both indicate that the risk of missing the disease is much worse than the risk of testing. See: http://www.Cdc.Gov/tb/publications/factsheets/specpop/pregnancy.Htm.

Yes: A PPD test (purified protein derivative) is a subdermal skin test which is very safe. If she has tested positive in the past, she doesn't need a repeat. Most women are tested in pregnancy especially if you live in an area where it's endemic, traveled to areas where it's common, or if you've been exposed to someone with tb. It's also ok to have a shielded chest-ray after 20 weeks if needed.