Mothers over 30 are at higher risk of having a stillborn child, small baby or premature birth than those under that age, according to new research that could lead to much younger women being classified as "older mothers".

Women who give birth for the first time between 30 and 34 are as likely to see their baby die as those between 35 and 39, it was found.

The study of almost 1 million first-time mothers in Sweden and Norway between 1990 and 2010 said the risk of a negative outcome increases from age 30.

That challenges the usual view among obstetricians that the risks of older motherhood start at 35.

While the increased risks for individual women aged 30-34 were "small", relative to those aged 25 to 29, researchers stressed, "for society they may be significant as a result of the large number of women who give birth in this age range."

"This study confirms that the risk of very preterm birth [after 22 to 31 weeks of pregnancy], moderately preterm birth, small for gestational age, low Apgar score [a newborn health test], stillbirth and neonatal death all increases with advancing maternal age [and] that risks may increase before the age of 35 years or older, which is the commonly used definition of advanced maternal age", the authors, from Sweden's Karolinska Institute, write in the journal Obstetrics & Gynecology.