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Health & Pregnancy

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You Can Predict Your Newborn's Weight

Equation Just as Good as Ultrasound, Easier to Use


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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