Arithmetic Beats Technology
in Predicting Birth Weight
Excerpt By Ross Grant, HealthScoutNews

(HealthScout News) -- A simple, six-step formula is more accurate than ultrasound in predicting the birth weight of an unborn baby, says new Duke University research.

The study, published in the September issue of the Journal of Reproductive Medicine, used the formula to estimate birth weights for 244 babies. On average, the predictions were accurate to within 8 percent. By contrast, ultrasound predictions vary from 8 percent to 16 percent, says study author Dr. Gerard Nahum, an associate clinical professor in the department of obstetrics and gynecology at Duke University Medical Center.

"It's amazing that until the turn of the 21st century, no such formula was developed," he says. "You don't need a calculator. You need a pencil and paper. It's low-tech, but it's more accurate. It's the most accurate method."

Doctors agree that estimating a child's birth weight is an important step in preparing for possible delivery complications, such as the need for a cesarean section. However, they don't appear to be jumping at the chance to trade in their pricey ultrasound machines for Nahum's equation and a pad of paper.

"A dartboard isn't too bad to begin with," says Dr. Rosemary Reiss, co-director of obstetric ultrasound at Brigham and Women's Hospital in Boston.

By simply predicting the birth weight for all babies at eight pounds, most of them will fall within about 10 percent, she says. And other methods -- merely looking at the mother's size or asking her how big she thinks the baby is, for example -- are nearly as accurate.

"People have been trying and trying for 20 years to get a better model, and they all come out about the same," Reiss says.

The problem with ultrasound is that it estimates a baby's birth weight based on only a few measurements, such as the circumference of her head or the length of her femur. While a number of formulas are used to translate the measurements into birth-weight estimates, none of them work well.

"Ultrasounds are limited by what they can see," Nahum says. "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."

His formula is based on six variables: gestational age, maternal height, maternal weight, third-trimester weight gain rate, number of prior children and fetal gender. In a previous study in California, Nahum used the formula to achieve similarly accurate results.

Still, the equation has significant drawbacks. For example, the 244 babies included in the study were picked from 398 babies born between 1998 and 2000 at the Duke University Medical Center. The researchers eliminated cases of non-white babies, twins, babies with diabetes or birth defects and those born before 37 weeks gestation.

To Dr. Terry J. DuBose, director of the sonography at the University of Arkansas for Medical Sciences, those exclusions make the formula useless in practice.

"This works as long as the baby is normal," he says. "The problem is that it's going to miss the abnormalities."

Besides estimating birth weight, ultrasound is used to detect twins, birth defects or complications, such as if the placenta is in the wrong place. And one of the six factors in Nahum's formula is the sex of the baby. This is significant since male babies are heavier than females. However, Nahum would need ultrasound to find that out, or lose 1 percent to 2 percent of his accuracy.

"I am not in any way saying ultrasound isn't useful," Nahum says. "You would be hard-pressed to point to anything that has changed obstetrics as much as ultrasound in the last three decades. But there's a huge amount of infrastructure invested in it. There's been kind of a vested interest in not accepting this formula."

With a patent riding on the equation, Nahum still contends that it's more accurate than ultrasound. Since reporting these findings, he has developed 60 more variations of the formula to account for different ethnicities and birth factors, such as whether the mother smokes.

He intends to create a computer program that would work with ultrasound machinery to "get the best of both."

What To Do

For tips about having a healthy pregnancy, visit the U.S. Centers for Disease Control and Prevention. Or to find an obstetrician or gynecologist in your area, visit the American College of Obstetricians and Gynecologists.

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