Toronto, Jun 28 (IANS): The lightest and the heaviest foetuses are at much higher risk of being stillborn than those of average weight, new research has found.

Stillbirth is traditionally defined as the death of a foetus at more than 23 weeks of gestation weighing 500 grams or more. Foetuses which are "severely small for gestational age," disproportionately account for about six percent of all stillbirths, according to researchers of St. Michael's Hospital.

Foetuses that are "severely large for gestational age," account for nearly one percent of stillbirths, reports the Journal of Perinatology.

"In this study, of all registered liveborn and stillborn infants in Ontario, extreme underweight and overweight states confer the highest risk of stillbirth," said a St. Michael's statement quoting study co-authors Joel Ray and Marcelo Urquia.

Ray and Urquia included babies born starting as early as 20 weeks of gestation on the grounds that maternal-foetal bonding is well established at that point, since most mothers-to-be have undergone a Level 2 ultrasound detailing the unborn baby's developing bones and organs.

Researchers examined records of all 767,016 live births and all 4,697 stillbirths between 2002 and 2007. They said that because they looked at so many births, they could also make statistically precise estimates of the impact of extremely low birth weight on the risk of stillbirth.