Newborn Weight Predictor Equation

Seeing as our baby girl will be delivered early (36 weeks +2 day) for maternal cholestasis, I’ve been thinking a lot about how much she might weigh. I would love it if she she at least passes the 6 pound mark. I found this newborn weight predictor equation on W.ebMD from Duke University Medical Center. They claim that it is about as accurate as an estimated fetal weight via ultrasound (which, I hate to say, usually isn’t too accurate — although they say ultrasounds are within 8-15% of the actual newborn weight). There is an 8% error rate with this equation. All you have to do for this equation is convert some of your info if needed (such as height from inches to centimeters, and weight from pounds to kilograms, etc). and plug it in!

Here’s the equation they posted with some helpful info below (taken from W.ebMD’s website — link below):

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.

(Find the link to the article here.)

It’s kind of a long formula, but if you’re not a math lover don’t worry. You can actually plug in all your numbers and copy and paste it into google’s search bar and it will calculate for you.

The equation predicted that our baby would weigh 6.7 pounds, or just over 6 1/2 pounds. But then I plugged in all the info for our son Josh to see what it predicted his weight to be (which was 6 pounds 13 oz) and it came up with 7 1/2 pounds. So definitely not accurate in our case. But still fun to compute!


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Filed under Baby, Pregnancy

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