Physicians in Denmark also reported assisted conceptions, multiple pregnancies and elective deliveries increased during the same period and were associated with early birth.

British doctors are warning the impact for society will be considerable if the reports are correct.

Preterm deliveries account for fewer than 1 in 10 births, but result in 75 percent of neonatal deaths and most neonatal intensive care admissions, said Andrew Shennan and Susan Bewley of London’s St Thomas’ Hospital.

Preterm birth also has considerable impact on long-term future health. For instance, the researchers say 1 in 4 survivors born at less than 25 weeks’ gestation have severe mental or physical disability. Even beyond 32 weeks, 1 in 3 children have educational and behavioral problems by age 7.

Shennan and Bewley say untangling the causative factors may be difficult, but general health measures involving smoking, teenage and middle age pregnancy, prevention of sexually transmitted diseases, obesity and social inequities would be a good place to start.

The study appears in British Medical Journal.