New Birth-Weight Reference Is More Accurate

By Will Boggs MD

June 18, 2019

NEW YORK (Reuters Health) - A new U.S. reference for singleton-birth-weight percentiles uses obstetric estimates of gestation and thereby better identifies outliers, researchers report.

"Given the concerns regarding the validity of previous birth-weight references based on maternal reports of last menstrual period, the need for an obstetric-estimate-based reference has become increasingly appreciated," said Dr. Izzuddin M. Aris of Harvard Medical School, in Boston, and the National University of Singapore.

"Thus, we hope that our new updated reference will allow clinicians (and researchers) to weigh its appropriateness against their specific needs," he told Reuters Health by email.

Birth-weight-for-gestational-age is an important clinical indicator of perinatal morbidity, survival and long-term health outcomes in children and their mothers, Dr. Aris and colleagues note in Pediatrics, online June 14.

The team used data on more than 3.8 million live births from the 2017 U.S. natality files to create an updated birth-weight-for-gestational-age reference and used two smoothing techniques to develop percentile curves.

The resulting z-score calculator, which provides sex-and parity-specific z-scores and percentiles, is available as an interactive Web application at:

The researchers also provide gestational age-specific cutpoints at the 3rd, 10th, 25th, 50th, 75th, 90th, and 97th percentiles for male and female infants as well as for first-, second- and third-or-more-born infants.

Cutpoints derived from these data identified 9.8% to 10.2% of births as <=10th percentile >=90th percentile and 2.6% to 3.3% of births as <=3rd percentile or >=97th percentile across preterm, term and post-term gestational-age categories.

In contrast, cutpoints from U.S. references based on data from 1999 to 2009 yielded a much larger variation in proportions of 2017 births at these thresholds, especially for preterm and post-term gestational-age categories.

"We expect clinicians to be able to use the percentile thresholds from our updated reference to identify at-risk infants who may have had restricted or excessive fetal growth," Dr. Aris said. "Researchers may also use this reference to derive continuous measures of birth size for studies examining predictors of fetal growth or associations of fetal growth with later health outcomes."

"Furthermore," he said, "our reference is an update of a 2003 birth-weight reference by Oken et al. (published in BMC Pediatrics), which is not only highly cited for research, but also has been readily adopted in clinical settings. For example, Massachusetts General Hospital in Boston currently uses the 'Oken reference' as a growth standard for all fetal ultrasounds. We anticipate that our updated reference will have similar uptake."

Epidemiologist Dr. Nicole M. Talge from Michigan State University, in East Lansing, who was not involved in the research, previously published a U.S. birth-weight reference corrected for implausible gestational-age estimates using 2009-2010 U.S. birth records. She told Reuters Health by email, "This birth size reference can be used to identify infants who are small- or large-for-gestational-age, based upon an infant’s sex and whether or not it is the mother's first child. It reflects the current sociodemographic characteristics of the United States live birth population and is based upon the most accurate estimates of gestational age available from vital records data."

"This reference may help identify babies who are in need of additional monitoring or care," she said.

"Because preterm infants are smaller than babies who are still in-utero at the same gestational week, any reference generated from live births will likely underestimate growth restriction in this gestational age range," Dr. Talge said. "Ultrasound-based standards get around this issue, but are generated from much smaller samples and commonly exclude complicated pregnancies. It will be important to determine how these various methods for characterizing birth size relate to infants’ short- and long-term health outcomes."


Pediatrics 2019.