Equation rivals ultrasound in calculating birth weight

September 26, 2002

Durham, N.C. -- An equation based on maternal characteristics and gestational age is just as accurate as ultrasound for predicting birth weight, while costing less and requiring no additional trained staff, says a new study from Duke University Medical Center and California State University.

The equation also predicts birth weight earlier than ultrasound, allowing doctors more flexibility when monitoring for fetal macrosomia (babies weighing more than 4,000 grams or 8.8 pounds at birth). Excessive fetal size can cause bone fractures and nerve damage to infants during the birth process.

"Ultrasound, the most common way of predicting birth weight, is a complicated and labor-intensive test that can give a mother and physician a false sense of security as far as birth size is concerned," said Gerard Nahum, M.D., lead author and an associate clinical professor in the department of obstetrics and gynecology at Duke University Medical Center. "Ultrasounds are limited by what they can see. If a mother is overweight or the fetus is in an awkward position, this can throw off the accuracy of such a test. We don't encounter those problems with our equation."

Nahum said the equation is also superior to the abdominal palpitation method, in which a doctor feels a pregnant woman's belly to estimate fetal weight. He said this method is subjective and is not as accurate as other methods, particularly if the mother is overweight.

The study, featured in the September 2002 issue of the Journal of Reproductive Medicine, examined 244 nonhypertensive, nondiabetic Caucasian women who delivered full-term, singleton babies at Duke University Medical Center between August 1998 and August 2000. Women were excluded if they delivered prematurely, smoked cigarettes during pregnancy or had medical complications

Characteristics used to estimate fetal birth weight included thegestational age at delivery, maternal height, maternal weight, third-trimester weight gain rate, number of priorchildren and fetal sex. Using these six variables, the researchers could predict birth weight within an average of plus or minus 8.1 percent of actual birth weight. Ultrasound accuracy rates are typically not as good, while abdominal palpitation rates can differ as much as plus or minus 10.3 percent from the actual birth weight, said the researchers.

"When you compare the equation to ultrasound, the advantages are many," said Nahum. "The equation requires no additional testing, trained staffing or equipment, and the information needed can be gleaned from the mother's current medical records. It can be computed by most doctors in about two to three minutes."

Nahum said the equation could also be used three months prior to delivery, which helps in the identification of excessively largefetuses early in pregnancy, allowing mother and physician more time to decide when and if induction of labor is the safest choice for mother and child. "Ultrasound is less accurate at predicting birth weight when it is used before the third trimester, so it's not as helpful in identifying excessive fetal size early enough to intervene in the timing of delivery successfully," he said.

However, the equation does have its limitations, said the researchers. It depends on an accurate gestational age, which must be calculated using an accurate last menstrual period cycle date or an early datingsonogram. Also, the equation has only been validated for babies born between 37 and 42 weeks gestation; the accuracy of the equation for other gestational periods has not been tested yet. However, Nahum said that this does not really affect it as a tool for screening for fetal macrosomia because the great majorityof cases occur at term,between 37 and 42 weeks gestation.

Another limitation is that the equation can currently be used only in Caucasian women who are without major medical problems. Variables such as race, hypertension and diabetes all influence birth weight, and additional research is needed to discover how these factors could be incorporated in the equation for an accurate birth weight prediction for these women, they said.

"This equation is just the foundation, and it is based on a very controlled set of maternal/fetal characteristics and circumstances," said Nahum. "For example, if a mother smokes, that can impact birth weight by 12 to18 grams per cigarette consumed per day during pregnancy, so you have to adjust the equation for that. Even the altitude where the mother lives can impact birth weight. Birth weights decline 10 to14 grams per 100 meters above sea level, so you have to adjust the equation for that also. But we believe this equation is a valid first step in developing simpler, more accurate and cost effective way of estimating birth weight well in advance of delivery."
Harold Stanislaw, Ph.D., of California State University, co-authored the paper.

Duke University Medical Center

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