The risk of newborn death or disability is highest for the smallest babies, so doctors continually seek better interventions and more precise diagnostic tools to identify and protect the most vulnerable.
In a new study, researchers at UCSF suggest a new cutoff point for assessing risk based on weight for gestational age. Although most clinicians currently consider the smallest 10 percent of fetuses to be at risk, principal investigator Rebecca Smith-Bindman, MD, and colleagues calculated that the risk of adverse neonatal outcomes—either losing the child soon after birth or having an infant with serious health problems—is most profound for only the smallest five percent.
The study involved 1800 pregnant women who were seen for prenatal care at UCSF Medical Center from 1994 through 1997 and who had at least one ultrasound exam. The researchers analyzed medical records, comparing relative risks of small size at birth, low birth weight for gestational age, and adverse neonatal outcomes for small and average-sized fetuses.
The study, “Adverse Birth Outcomes in Relation to Prenatal Sonographic Measurements of Fetal Size,” is published in the April 2003 issue of the Journal of Ultrasound Medicine.
The research team determined that the fetal group representing the lowest 5th percentile (weighing at birth less than what 95 percent of newborns weigh) is at dramatically increased risk for adverse neonatal outcomes. Although previous studies have focused on the risk facing the lowest 10th percentile, this study found that fetuses between the 5th and 20th percentile are all at about the same peril: less risk than the smallest fetuses but greater risk than average-sized fetuses.
The study also found that an ultrasound examination as early as 15 weeks gestation (25 weeks before full term) could predict 29 percent of adverse neonatal outcomes, according to Smith-Bindman, a UCSF assistant professor of radiology, epidemiology and biostatistics.
Compared with average-sized fetuses, the study found those in the lowest 5th percentile were 3-11 times more likely to be born at a low birth weight, 10-12 times more likely to be small for gestational age, and have 2-8 times the risk of preterm birth, extreme preterm birth, long neonatal hospital stays, neonatal ICU admission, stillbirth or neonatal death.
In an earlier study of a 236-patient subset of the same 1800 women, the researchers found that measuring the rate of fetal growth, rather than fetal weight alone, was a strong predictor of adverse neonatal outcomes.
“All this information could help obstetricians focus their attention on the most at-risk pregnancies,” said Smith-Bindman, “and warn those women who need to deliver their babies in a hospital equipped for high risk births.”
The earlier study, “Ultrasound Evaluation of Fetal Growth: Prediction of Neonatal Outomes,” appeared in the April 2002 issue of Radiology.
Ultrasound is routinely used in obstetrics at about 20 weeks gestation to assess whether or not a fetus is an appropriate weight for its gestational age. However, findings from the 2002-published study pointed to a different approach, starting with an ultrasound exam as early as 15 weeks gestation.
In this study, the research team only included those patients who had more than one ultrasound exam. When researchers compared the results of the exams, they found they could distinguish fetuses smaller than average, but developing normally, and fetuses that may be average size but not developing at a healthy rate.
“If physicians have any concern that the fetal weight seems low on the first ultrasound exam, they can evaluate the rate of fetal growth by performing a second exam,” said Smith-Bindman. In addition, growth rate is a much more accurate measure than weight when gestational age is approximated, as happens with the 25 percent of pregnant women who don’t know when conception took place, she said.
“This study will help in two ways. It will help us identify those fetuses that are small and sick, and really do need to be watched more closely. Just as important, it will tell us which ones are small but healthy, so we can reassure the families accordingly,” said Robert Ball, MD, who specializes in high risk pregnancy at the UCSF Center for Mothers and Newborns and the UCSF Fetal Treatment Center. He is a UCSF associate professor of obstetrics, gynecology and reproductive sciences.
According to Ball, low birth weight is most often associated with premature birth (less than 37 weeks gestation), which is now the leading cause of newborn death and a major cause of neonatal health problems, including learning and physical disabilities that last a lifetime. About 20 percent of U.S. births are either low birth weight or preterm and the rate has grown by 27 percent since 1981, he said.
Doctors have long incorporated an understanding of these risks into their practice while studies continue to demonstrate factors that contribute to low birth weight and preterm birth. Current options for intervention, such as counseling mothers on lifestyle factors and prescribing bed rest, are considered inadequate, Ball said.
The researchers point out they have not yet determined the best point in gestation to measure fetal size for predicting an infant’s health.
The study was funded in part by the Mount Zion Health Fund. Co-authors are Philip W. Chu, MS, UCSF Department of Radiology; Jeffrey L. Ecker, MD, Department of Obstetrics and Gynecology, Massachusetts General Hospital and Harvard Medical School; Vickie A. Feldstein, MD, and Roy A. Filly, MD, UCSF Department of Radiology; and Peter Bacchetti, PhD, UCSF Department of Epidemiology and Biostatics.