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How Accurate Are Ultrasounds for Fetal Weight? | Med-Health.net
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How Accurate Are Ultrasounds for Weight?

One reason you get an ultrasound is to estimate the size of your unborn baby. If the fetus is small, it may indicate an issue with the placenta or the baby; if the fetus looks large, the mom can have an issue with gestational diabetes. Traditionally, the midwife or doctor will estimate size by using hands to feel for your fetus and uterus. But this method isn’t reliable. The ultrasound technology is the new way to determine the size. Then how accurate are ultrasounds in measuring weight?

How Accurate Are Ultrasound Exams for Fetal Weight?

Several measurements need to be taken into account to estimate your baby’s weight:

  • Biparietal diameter
  • Femur length
  • Abdominal circumference
  • Head circumference

A mathematical formula uses the above four measurements to calculate your baby’s estimated weight. There are numerous formulas for the estimation of weight and the most accurate ones can still have a 15% or more margin of error in 85% of all babies. This means one in about seven babies will weigh 15% more or less than what was estimated. If you have a 3D ultrasound, there is more accuracy than with a 2D ultrasound.

When you are considering "how accurate are ultrasounds for weight", you need to consider timing, variables and risks.

Timing

When your ultrasound is taken can affect the accuracy of ultrasounds. One study out of Israel showed ultrasounds that were done in the early third trimester (28-34 weeks) didn’t give an accurate estimate of the baby’s weight. It also showed that these weren’t accurate in predicting if the baby was small or large for gestational age. In a Turkish study performed in 2013, it was found that late third trimester ultrasounds were better at predicting how much the baby would weight.

Variables

There are several variables other than timing that can affect how accurate ultrasounds are. The placenta’s position, mother’s body weight, amount of amniotic fluid and the baby’s position can all influence how accurate the weight estimation is. In addition, the technician’s skill can also affect the result.

Risks

There are some risks of estimating your baby’s size before he/she is born. One of these is your doctor pressuring you to have a Cesarean section. In a 2009 study out of Texas, moms who had a baby with birth weight overestimated by 15 percent before labor, had a greater chance of C-section delivery.

How Is Fetal Weight Measured with Ultrasound?

After knowing the answer to "how accurate are ultrasounds for weight", you may wonder how the weight is measured with ultrasound.

  • The first thing is to measure the fetus’s head. This is called a biparietal diameter assessment. It measures the distance between each side of the head. This is usually done in an ultrasound after 13 weeks. The biparietal diameter tends to grow from 2.4 cm to 9.5 cm. It should be noted that some babies will have similar weights but head sizes are different.
  • The next step is to get a measurement on the baby’s femur. This is the longest bone in the body and  is considered to be a key indicator in the growth of your baby. This number tends to grow from 1.5 cm to 7.8 cm.
  • The third step in the process is to get a measurement on the abdomen. This circumference is important in assessing your baby’s weight in late pregnancy.
  • The fourth and final step is to use polynomial equations to calculate the above measurements to give a final estimate of fetal weight. There are several types of computer software that use calculations and charts to determine this number.

Other Mothers' Experience

“At about 37 weeks I had a growth ultrasound and it measured my baby at about 7 pounds 15 ounces. At 39 weeks another ultrasound was performed and it estimated her at 9 pounds and 5 ounces. At this point they wanted to induce me the next day because of the size. In less than 24 hours after that ultrasound I delivered a gorgeous 7 pound 7 ounce baby! The doc told me that amniotic fluid can affect measurements taken late in a pregnancy and the estimate could be off as much as 2 pounds.”

“My 34-week ultrasound predicted my baby to be about 6 and ½ pounds and at 37 weeks he looked to be about 7 ½ pounds. My OB and I thought this baby was looking to be about 8 or 9 pounds. We were wrong! I delivered a baby who was just over 7 pounds. He did have a larger head which could have made the estimation high. If you have a long baby or one with a big head, estimations could be off.”

“At my 40-week appointment they did an ultrasound that predicted my little one to be about 8.5 pounds. My OB got serious and told me he was probably bigger because I was overdue. She also said if I didn’t agree to induction I would likely be a C-section. My hubby and I wanted to let nature take its course so we waited. At 42-weeks and three days I delivered my 7-pound baby.”

Average Fetal Weight for Your Reference

Babies grow differently from the very start; remember this when wondering "how accurate are ultrasounds for weight". Babies are measured from the top (crown) of their head to the bottom (rump) until about 20 weeks. This is due to the fact that babies have their legs curled up against them, making it very hard to get a measurement. After this, they are measured from head to toe.

Gestational Age (weeks)

Length (inches)

Weight (pounds)

8

0.63

0.04

9

0.90

0.07

10

1.22

0.14

11

1.61

0.25

12

2.13

0.49

13

2.91

0.81

14

3.42

1.52

15

3.98

2.47

16

4.57

3.53

17

5.12

4.94

18

5.59

6.70

19

6.02

8.47

20

6.46

10.58

(above is crown to rump)

(below is crown to heel)

20

10.08

10.58

21

10.51

12.70

22

10.94

15.17

23

11.38

1.10

24

11.81

1.32

25

13.62

1.46

26

14.02

1.68

27

14.41

1.93

28

14.80

2.22

29

15.20

2.54

30

15.71

2.91

31

16.18

3.31

32

16.69

3.75

33

17.20

4.23

34

17.72

4.73

35

18.19

5.25

36

18.66

5.78

37

19.13

6.30

38

19.61

6.80

39

19.96

7.25

40

20.16

7.63

41

20.35

7.93

42

20.28

8.12