I just found out today my wife is pregnant.  she's having lower back pain and not for sure if she can take an OTC pain reliever. Can she?

Acetaminophen is OK. Acetaminophen (commonly referred to by the brand name, tylenol (acetaminophen)) is safe to use in pregnancy for common aches and pains. The maximum daily dose would be 1000mg four times-a-day (equivalent to 2 extra-strength tylenol (acetaminophen) tablets 4 times-a-day).