Doctors looked at data from 1988 to 2007 from Denmark, Finland, Norway and Sweden for more than 92,000 children born through assisted reproduction technology (ART), the term for in-vitro and other methods.
Of them, more than 62,000 were single births, also called singletons, and more than 29,000 were twins.
The health of the babies at birth and in their first year of life was compared to that of children conceived without ART.
“We observed a remarkable decline in the risk of being born pre-term or very preterm,” said Anna-Karina Aaris Henningsen at the University of Copenhagen.
“The proportion of single ART babies born with a low or very low birth weight -– less than 2,500 grammes (5.5 pounds) or 1,500g respectively -– also decreased.
“The rates for stillbirths and death during the first year declined among both singletons and twins, and fewer ART twins were stillborn or died during the first year compared with spontaneously conceived twins,” Henningsen said.
In the period 1988-1992, the rate of pre-term singletons born from ART was 13 percent, compared to five percent among babies that had been spontaneously conceived.
But by 2007, this had fallen to eight percent in the ART group, while the non-ART group stayed at five percent.
Henningsen said several factors contributed to the improvement.
Laboratories became more skilled at culturing fertilised eggs before returning them to the uterus, and hormonal drugs to stimulate ovaries for egg harvesting were milder than before.
The biggest gain, though, was in a policy change to encourage a single embryo implant at a time, not several.
Multiple embryos boost the chance of a live birth, but also raise the odds of having twins or triplets, which can result in lower birthweight and health complications.
From 1989 to 2002, the proportion of ART twins in the four Scandinavian countries was stable at about 23 percent of births, but then began to decline. By 2007, it had halved to 11.6 percent.
The study appears in the journal Human Reproduction.