fetal biometry

Fetal Biometry and Why it is Important

  • Updated
  • 3 mins read
  • Published

Fetal Biometry and Why it is Important

What is fetal biometry?

Fetal biometry, conducted through ultrasounds during pregnancy, is a test that assesses the baby’s size. It involves various measurements that provide insight to your healthcare provider. During ultrasounds, the ultrasound technician or doctor will measure the baby’s head, body (crown to rump), and thigh bone.  This test aids in identifying fetal growth disorders, which, if left undetected, could result in complications during the pregnancy.

The report may include:

  • AC (abdominal circumference), the length going around the baby’s belly
  • BPD (biparietal diameter), the diameter of the baby’s head
  • CRL (crown-rump length), the length from the top of the head to the baby’s bottom
  • FL (femur length), the length of the femur bone in the baby’s leg
  • HC (head circumference), the length around the baby’s head

All pregnant women get a routine fetal biometry. Most women get about three ultrasounds when they’re pregnant, however, surrogates tend to have a few more ultrasounds.

Why is fetal biometry important?

Fetal biometry is a component of prenatal ultrasound used to assess if the fetus’s size aligns with its estimated gestational age, therefore indicating how far along the pregnancy is in weeks.

When the fetus appears smaller or larger than expected for its age, further tests may be needed to investigate potential reasons behind the discrepancy. This could be linked to health issues in the birth parent, placental problems, or congenital abnormalities in the fetus. It is important to note that discrepancies in fetal biometry results do not necessarily indicate a problem. Such variations can be a normal occurrence.

How accurate is fetal biometry?

Fetal biometry is highly precise in the early stages of pregnancy, serving as a dependable method for healthcare providers to track fetal growth. These measurements are usually taken during regular ultrasounds in the initial and middle stages of pregnancy. If there are any pregnancy complications or if you are expecting multiple births, your provider will monitor these measurements more frequently throughout the pregnancy. However, fetal biometry tends to be less precise after the second trimester.

Crown to Rump Measurement Chart

Gestational Age Length (inches) Weight (oz/lb) Length (cm) Mass (g)
8 weeks 0.63 0.04 oz 1.6 1
9 weeks .9 0.07 oz 2.3 2
10 weeks 1.22 0.14 oz 3.1 4
11 weeks 1.61 0.25 oz 4.1 7
12 weeks 2.13 0.49 oz 5.4 14
13 weeks 2.19 0.81 oz 7.4 23
14 weeks 3.42 1.52 oz 8.7 43
15 weeks 3.98 2.47 oz 10.1 70
16 weeks 4.57 3.53 oz 11.6 100
17 weeks 5.12 4.94 oz 13 140
18 weeks 5.59 6.70 oz 14.2 190
19 weeks 6.02 8.47 oz 15.3 240
20 weeks 6.46 10.58 oz 16.4 300
21 weeks 10.51 12.70 oz 26.7 360
22 weeks 10.94 15.17 oz 27.8 430
23 weeks 11.38 1.10 lb 28.9 501
24 weeks 11.81 1.32 lb 30 600
25 weeks 13.62 1.46 lb 34.6 660
26 weeks 14.02 1.68 lb 35.6 760
27 weeks 14.41 1.93 lb 36.6 875
28 weeks 14.80 2.22 lb 37.6 1005
29 weeks 15.2 2.54 lb 38.6 1153
30 weeks 15.71 2.91 lb 39.9 1319
31 weeks 16.18 3.31 lb 41.1 1502
32 weeks 16.19 3.75 lb 42.4 1702
33 weeks 17.20 4.23 lb 43.7 1918
34 weeks 17.72 4.73 lb 45 2146
35 weeks 18.19 5.25 lb 46.2 2383
36 weeks 18.66 5.78 lb 47.4 2622
37 weeks 19.13 6.30 lb 48.6 2859
38 weeks 19.61 6.80 lb 49.8 3083
39 weeks 19.96 7.25 lb 50.7 3288
40 weeks 20.16 7.63 lb 51.2 3462
41 weeks 20.35 7.93 lb 51.7 3597
42 weeks 20.28 8.12 lb 51.5 3685
43 weeks 20.20 8.19 lb 51.3 3717

 

 

Sources:

https://my.clevelandclinic.org/health/diagnostics/25087-fetal-biometry

https://www.bmus.org/static/uploads/resources/Aug_2009_Fetal_Measurements_D3NApK5.pdf

https://babyyourbaby.org/pregnancy/during-pregnancy/fetal-chart/