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This chart outlines expected ultrasound measurements (in mm) based on gestational age.

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BPD: biparietal diameter (the diameter between the 2 sides of the head
HC: head circumference

AC: abdominal circumference
FL: femur length

Weeks BPD (mm) HC (mm) AC (mm) FL
(mm)
16 32.3 124.4 99.1 20.5
16.5 34.2 131.2 105.5 22.1
17 36.0 137.9 111.9 23.7
17.5 37.7 144.5 118.2 25.2
18 39.5 151.1 124.5 26.7
18.5 41.3 157.7 130.7 28.3
19 43.0 164.1 136.9 29.8
19.5 44.7 170.5 143.0 31.2
20 46.4 176.8 149.1 32.7
20.5 48.1 183.0 155.1 34.1
21 49.7 189.2 161.1 35.6
21.5 51.4 195.3 167.0 37.0
22 53.0 201.3 172.9 38.4
22.5 54.6 207.2 178.7 39.8
23 56.2 213.0 184.5 41.1
23.5 57.8 218.7 190.2 42.5
24 59.3 224.4 195.9 43.8
24.5 60.8 229.9 201.5 45.1
25 62.3 235.4 207.1 46.4
25.5 63.8 240.8 212.7 47.7
26 65.3 246.0 218.1 48.9
26.5 66.7 251.2 223.6 50.2
27 68.1 256.2 228.9 51.4
27.5 69.5 261.2 234.3 52.6
28 70.8 266.1 239.6 53.8
28.5 72.2 270.8 244.8 55.0
29 73.5 275.5 250.0 56.1
29.5 74.7 280.0 255.1 57.3
30 76.0 284.4 260.2 58.4
30.5 77.2 288.7 265.2 59.5
31 78.4 292.9 270.2 60.6
31.5 79.6 297.0 275.1 61.7
32 80.7 300.9 280.0 62.7
32.5 81.9 304.7 284.8 63.8
33 82.9 308.4 289.6 64.8
33.5 84.0 312.0 294.3 65.8
34 85.0 315.5 299.0 66.8
34.5 86.0 318.8 303.7 67.7
35 87.0 322.0 308.2 68.7
35.5 87.9 325.0 312.8 69.6
36 88.8 327.9 317.3 70.6
36.5 89.7 330.7 321.7 71.5
37 90.5 333.3 326.1 72.3
37.5 91.3 335.8 330.4 73.2
38 92.1 338.2 334.7 74.1
38.5 92.8 340.4 338.9 74.9
39 93.5 342.5 343.1 75.7
39.5 94.2 344.4 347.2 76.5
40 94.8 346.1 351.3 77.3
40.5 95.4 347.7 355.4 78.1
41 95.9 349.2 359.3 78.8
41.5 96.5 350.5 363.3 79.5
42 96.9 351.6 367.2 80.3

Hadlock Formulas: Haddlock Radiology 1984; 152: 497-501

Read More: 
Your Guide to Ultrasound
Fetal Ultrasound Measurements