Mothers were warned yesterday they are putting their children's health at risk after a major new survey found women in Scotland are producing babies heavier than ever before.

In the first significant survey of newborns in a quarter of a century, researchers found that women are producing "alarmingly" heavy babies as the nation's worsening obesity epidemic takes its toll.

They believe the average weight has surged because of the growing numbers of expectant mothers who are obese.

Tam Fry, chairman of The Child Growth Foundation, warned that mothers were putting their children at risk of diabetes and heart disease.

He said: "There are a significant number of mothers who are not only obese but alarmingly so, and they are giving birth to alarmingly large babies. Scotland's obesity problem has obviously caused the average weight of babies to zoom up."

The study, published yesterday, analysed the weights of more than 100,000 Scots children born between 1998 and 2003.

It was conducted by statisticians at Napier University in Edinburgh, funded by the UK chief scientist and published in the online journal BMC Pregnancy and Childbirth.

The average weight of a baby in Scotland is now 7lb 8oz - 4oz heavier than in 1980. Ideally babies should weigh between 5lb 8oz and 7lb 11oz. Since 1980 the proportion weighing more than this has risen from 38% to 44%.

The Prime Minister's wife, Sarah Brown, will next month open a new research centre in Edinburgh where experts will study maternal obesity and the problems it causes.

The centre, at the city's Queen's Medical Research Institute, is set to open on April 10 and is a collaboration between the baby charity Tommy's and Edinburgh University.

Tommy's chief executive Jane Brewin said: "In the UK nearly 50% of women attending antenatal bookings are obese or overweight and obesity in pregnancy causes an increased risk of spontaneous miscarriage, stillbirth and neonatal death rates.

"Emerging evidence also suggests that maternal obesity has long-term consequences for the fetus, including childhood obesity.

"With the healthcare costs of obesity set to top £4.5bn in the next three years, obesity is now a top priority on the government agenda and has been identified as a key area for research."

A parallel study by the British Journal of Obstetrics and Gynaecology found that a fifth of pregnant mothers are now obese. In cases of severe obesity, birth defects may not be detected by ultrasound scans because equipment is not built for such large people.

There is also a greater risk of pre-eclampsia, a condition of the placenta which is responsible for the deaths of about five women and 600 babies in the UK every year.

Growing birth weights over the past 25 years have also been connected with better antenatal care, better maternal health and improved lifestyle of women.

In their recent budget, SNP ministers pledged to spend £15m on tackling obesity over the next three years.

A Scottish Government spokeswoman said: "Birth weight parameters still fall within the normal range for the majority of women. Obese mothers and their babies are prone to a variety of complications during pregnancy, childbirth and postnatally.

"Their care will include careful monitoring of the baby's development in the womb and after birth. Antenatal care addresses these issues and women are given detailed advice on healthy eating during and after pregnancy.

"Tackling Scotland's growing obesity problem will be key to the Scottish Government's plans for health improvement over the next three years.

"Obesity is an increasing problem and poses a serious threat to our nation's health. That is why we have made tackling obesity a key priority.

"The aim will be to tackle the problem from conception, with free fruit for pregnant mothers and pre-school children to adults already coping with health challenges as a result of obesity."