Public Release: 

Study develops new equation for estimating gestational age

Society for Maternal-Fetal Medicine

ATLANTA (Feb. 1, 2016)--In a study to be presented on Feb. 6 at 8:45 a.m. EST, at the Society for Maternal-Fetal Medicine's annual meeting, The Pregnancy Meeting™, in Atlanta, researchers will present findings from a study titled, The NICHD Fetal Growth Studies: Development of a contemporary formula for estimating gestational age from ultrasound fetal biometrics.

Accurate assessment of gestational age is an important variable affecting perinatal morbidity and mortality. The most commonly used formula for estimating gestational age has been Hadlock's formula which uses biparietal diameter, head circumference, femur length and abdominal circumference. If gestational age is not accurately estimated, induction of labor may be performed inappropriately. A smaller premature fetus may be thought to have fetal growth restriction and undergo induction of labor, which can produce prematurity. A fetus wrongly thought to be post term may also undergo induction of labor, which is an unnecessary intervention. It is important to accurately estimate gestational age.

Researchers used fetal biometric data from the National Institute of Child Health and Human Development Fetal Growth Studies. They sought to develop and validate a new gestational age estimation equation and compare its accuracy to Hadlock formula created in 1984.

Healthy women from four racial/ethnic groups comprised of 614 (26%) non-Hispanic whites, 611 (26%) non-Hispanic blacks, 649 (28%) Hispanics and 460 (20%) Asians. All were low-risk for altered fetal growth and reported a sure last menstrual period, underwent serial ultrasound every four weeks starting at an average of 19.7 weeks.

Biparietal diameter (BPD) which is one of the basic biometric parameters to assess fetal size, abdominal circumference (AC), femur length (FL) and head circumference (HC) were used to develop a formula for estimating gestational age. The formula was validated using 50% training and test set paradigm; a 50% random sample was used to develop the predictor and the remaining 50% was used to evaluate predictive accuracy. This procedure was run one thousand times and the predictive accuracy measures averaged. Comparative formula accuracies were assessed using the standard deviation of prediction derived from the predicted versus actual population gestational ages.

Daniel W. Skupski, M.D., one of the researchers of the study who is with New York Presbyterian Queens in Flushing, N.Y. and will present the findings said, "We have developed and validated a new equation for estimating gestational age from fetal biometrics measured between 14 and 22 weeks gestational age using a multi-racial/ethnic population, certified sonographers and modern ultrasound units." The study shows a slight improvement in this newly developed formula over the traditional Hadlock with accuracy of less than nine days versus less than 10 days for Hadlock. It also validates the establishment of this new formula in a large, high-quality multi-center study.

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A copy of the abstract is available at http://www.smfmnewsroom.org and below. For interviews please contact Vicki Bendure at Vicki@bendurepr.com 202-374-9259 (cell).

The Society for Maternal-Fetal Medicine (est. 1977) is the premiere membership organization for obstetricians/gynecologists who have additional formal education and training in maternal-fetal medicine. The society is devoted to reducing high-risk pregnancy complications by sharing expertise through continuing education to its 2,000 members on the latest pregnancy assessment and treatment methods. It also serves as an advocate for improving public policy, and expanding research funding and opportunities for maternal-fetal medicine. The group hosts an annual meeting in which groundbreaking new ideas and research in the area of maternal-fetal medicine are shared and discussed. For more information visit http://www.smfm.org.

Abstract 105 The NICHD Fetal Growth Studies: Development of a contemporary formula for estimating gestational age from ultrasound fetal biometrics

Authors: Daniel W. Skupski1, John Owen2, Sungduk Kim3, Paul Albert3, Katherine Laughon Grantz3
1New York Presbyterian Queens, Flushing, NY, 2University of Alabama at Birmingham, Birmingham, AL, 3Division of Intramural Population Health Research Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, MD

Objective: Using fetal biometric data from the NICHD Fetal Growth Studies, we sought to develop and validate a new gestational age (GA) estimation equation and compare its accuracy to a commonly used formula (Hadlock 1984).

Study Design: Healthy women from 4 racial/ethnic groups who were low-risk for altered fetal growth and reported a sure LMP confirmed by CRL at <14+0 weeks, underwent serial ultrasound every 4 weeks, starting at an average of 19.7 weeks, using study-certified sonographers. Project EDC was based on the LMP date+280 days. Linear regression with BPD, AC, FL and HC and product terms were used to develop a formula for estimating GA. The formula was validated using 50% training and test set paradigm; a 50% random sample was used to develop the predictor and the remaining 50% was used to evaluate predictive accuracy. This procedure was run 1,000 times and the predictive accuracy measures averaged. Comparative formula accuracies were assessed using the standard deviation (SD) of prediction derived from the predicted versus actual population GA's.

Results: The study population comprised: 614 (26%) non-Hispanic Whites, 611 (26%) non-Hispanic Blacks, 649 (28%) Hispanics, and 460 (20%) Asians. The best-fit formula was: GA=7.85-0.127*BPD+0.07304*HC+0.00638*AC+0.122*FL+0.000685*BPD*AC-0.00015*HC*AC; validation confirmed a SD of 4.57 days (2SDs=9.14 days). The SD of the Hadlock 1984 formula was 5.06 days (2 SDs=10.12 days). Figure 1 shows the observed versus the predicted GA for the best-fit formula.

Conclusion: We have developed and validated a new equation for estimating GA from fetal biometrics measured between 14 and 22 weeks' gestational age using a multi-racial/ethnic population, certified sonographers and modern ultrasound units. Sonography confirms the best GA with accuracy of ? 9 days. Our study shows a slight improvement in our newly developed formula over the traditional Hadlock formula (accuracy ? 10 days) and validates this established formula in a large high-quality multi-center study.

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