The 51-year-old is thought to have poured flammable liquid over his 33-year-old wife, from whom he was separated, before setting her alight in the North Rhine-Westphalian city on Sunday evening.
Guests at the cafe managed to put the flames out, the Westdeutsche Allgemeine Zeitung reported on Monday, but the woman was seriously injured and rushed to a nearby hospital. A spokesman confirmed that neither she nor her baby were in a life threatening condition.
A police spokesman added that they did not know why the man attacked his wife but that they believed it was attempted murder.
As of yet, the Dortmund state prosecutor has not given out any further information and the suspect is being held in custody.