The blast near the northern city of Mosul also wounded the driver of the ambulance, the sources said, adding that the dead were a relative of the woman and a medic.
The woman was in labor at the time of the incident but the fate of the baby was not immediately clear.
Nineveh province, of which Mosul is the capital, is one of the most dangerous areas of Iraq, with militants carrying out near-daily attacks.
Gunmen also killed a tribal sheikh and another man in Mosul on Monday.
Militants have a major presence in Mosul and are said to extort money from shop owners in the city.
With the latest violence, 590 people have been killed this month and over 4,400 since the beginning of the year, according to AFP figures based on security and medical sources.