You Can Predict Your Newborn's Weight

Equation Just as Good as Ultrasound, Easier to Use

From the WebMD Archives

Sept. 26, 2002 - A new baby's gender, name, time of birth, and birth weight are nice information for a birth announcement, but birth weight is especially important for an obstetrician. A large size at delivery has long been associated with an increased risk of injuries to a newborn and its mom. So the better a doctor can predict birth weight, the easier the delivery may be.

Most of the time, birth weight is predicted with ultrasound. Now researchers say a mathematic equation is just as good a predictor and can predict birth weight earlier than ultrasound for some newborns.

Duke University Medical Center doctors tested the accuracy of an equation that calculated birth weight based on stage of pregnancy, mother's height and weight, third-trimester weight gain rate, the number of other children, and fetal sex.

When they used it for 244 women who gave birth to single babies, the equationaccurately predicted birth weight within an average of 8% of actual birth weight. For 87% of the births, weight was predicted within 15% of the actual weight. Ultrasound is estimated to predict within 8-15% of true birth weight.

It was also easier for doctors to identify babies who might be bigger than 8.8 pounds, a weight at which the risk of delivery problems begin to increase.

The researchers say the equation is easier to use and more efficient than ultrasound. "The equation requires no additional testing, trained staffing, or equipment, and the information needed can be gleaned from the mother's current medical records," says Gerard Nahum, MD, of Duke's department of obstetrics and gynecology. "It can be computed by most doctors in about two to three minutes," he says in a news release.

The mathematical formula does have its limitations. The study looked at only white nonsmokers who didn't have high blood pressure or diabetes and who gave birth to a single baby after 37-42 weeks of pregnancy. Researchers say others were excluded because differences in racial groups are linked to differences in birth weight. And high blood pressure and premature birth are linked to low birth weight while diabetes is linked to high birth weight.

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A key to the equation's accuracy is knowing exactly -- or nearly exactly -- how long a woman has been pregnant, which is calculated using the last day of the her last period or by analyzing an early ultrasound.

Nahum says the equation also can be used early in the third trimester, since most women see their doctors every two weeks by that time. Third-trimester weight gain can be calculated once the woman has been weighed twice during the third trimester.

For those of you who have a thing for math, here's the equation:

Birth weight (g) = gestational age (days) x (9.38 + 0.264 x fetal sex + 0.000233 x maternal height [cm] x maternal weight at 26.0 weeks [kg] + 4.62 x 3rd-trimester maternal weight gain rate [kg/d]] x [number of previous births + 1]).

Pointers for performing the calculations:

  • Perform all multiplications before the additions.
  • Gestational age is the stage of your pregnancy in days, or days since the onset of your last normal menstrual period plus 14.
  • For fetal sex, use 1 for a boy, -1 for a girl, or 0 if sex is unknown.
  • For mother's height, inches multiplied by 2.54 gives you centimeters.
  • For the mother's weight, pounds divided by 2.2 gives you kilograms.
  • For the baby's birth weight, take the number from your calculation and divide by 453 to get your baby's estimated weight in pounds.

If you don't have a thing for math, you might want to show this to your obstetrician. -->

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