The tiniest and the heaviest fetuses are at much higher risk of being stillborn than those of average weight, new research has found.
Fetuses who are "severely small for gestational age," or weigh below the bottom one percentile of all fetuses, disproportionately account for about six per cent of all stillbirths, according to researchers at St. Michael's Hospital.
Fetuses that are "severely large for gestational age," or weigh above the 99th percentile, account for nearly one per cent of stillbirths.
"In this study of all registered liveborn and stillborn infants in Ontario, extreme underweight and overweight states confer the highest risk of stillbirth," said Drs. Joel Ray and Marcelo Urquia, authors of the paper that appears in the current issue of the Journal of Perinatology.
Stillbirth is traditionally defined as the death of a fetus at more than 23 weeks of gestation weighing 500 grams or more. However, Drs. Ray and Urquia included babies born starting as early as 20 weeks of gestation on the grounds that maternal-fetal 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.
Including those babies provides new information about the degree to which low and high weights are associated with stillbirths, including those before the point of viability, Dr. Ray said. That, in turn, may help doctors better decide at which time point it is better to allow a pregnancy to continue so the fetus can grow, or to deliver a premature baby who might otherwise die in the womb.
The rate of stillbirths in industrialized countries is about six per 1,000, of which half occur after 27 weeks of gestation. In poorer countries, the rate is up to five times higher. Stillbirths are more common than the death of a baby after birth, such as from prematurity or as a result of Sudden Infant Death Syndrome. Yet, stillbirths have largely been ignored by our society, especially in terms of their emotional effect on the mother, her partner and extended family, Dr. Ray said.