Findings reported in the British Medical Journal from a study of 600,000 births found three successive drops in babies born before 37 weeks.
Each occurred after a public smoking ban was introduced but there was no such trend in the period before the bans were put in place.
The latest study was conducted by a team at Hasselt University in Belgium and adds to similar findings from 2012 research in Scotland, though in that work researchers could not fully state the smoking ban was the cause of the change because pre-term births had started to drop before the ban.
However, the Belgian study looked at the rate of premature births after each phase of a smoking ban came into force in that country with a ban in public places and workplaces in 2006, restaurants the following year and bars serving food in 2010.
After each new phase, the rate of premature births fell and overall amounted to a fall of six premature babies in every 1,000 births.