How to Calculate the Weight of a Fetus

Fetal weight estimates are actually one of the few things that become less accurate as your baby grows. Fetal weights are based on length and circumference, which do not necessarily indicate actual weight. It's not unlike adult weights. You can probably assume that someone who is six feet tall weighs more than 100 pounds but you won't know if he weighs 175 pounds or 205 pounds until you get him on the scale.

Things You'll Need

• Ultrasound machine

Instructions

1. Use a chart that incorporates averages based on fetal age (see Resources section below). Plot the age of your fetus with the weights on the chart to get a general idea.

2. Discuss it with your ultrasound technician. Your next ultrasound appointment is a great time to ask an experienced ultrasound technician about your fetus's weight. She will be able to predict based on views and measurements she has taken.

3. Grab a calculator and the ultrasound measurements, and use the formula below.

   Estimated fetal weight = 1,687.47 + (54.1 x femur length in millimeters) + (76.68 x Soft tissue thickness of thigh in millimeters)

4. Ask a midwife. They have a better accuracy rate than doctors when assessing fetal weight.

Tips & Warnings

• Use as many steps as possible to get the most accurate assessment.

• None of these methods are guaranteed to be 100 percent accurate.

Resources

• Fetal Weight Chart Based on Averages