The study of 600,000 births found three successive drops in babies born before 37 weeks – each occurring after a phase of a public smoking ban was introduced.
There was no such trend in the period before the bans were put in place, the British Medical Journal reported.
The study, by Hasselt University in Belgium, comes after Scottish research in 2012 found a similar pattern.
But experts could not fully state the smoking ban was the cause of the change because pre-term births had started to drop before the ban.
It is already well established that smoking leads to reduced birth weight and an increased risk of premature birth.