Femur length correlates with fetal length and in turn fetal age. The femur length measured by ultrasound is the linear distance between the proximal and distal diaphyseal plates, which can be measured fairly accurately. Since the epiphyseal portions are cartilagenous, they cannot be as readily visualized. It is can be used at 12 weeks of gestation and later (second and third trimesters).
|
menstrual age in weeks |
femur length in cm |
|
12 |
0.7 |
|
12.5 |
0.9 |
|
13 |
1.1 |
|
13.5 |
1.2 |
|
14 |
1.4 |
|
14.5 |
1.6 |
|
15 |
1.7 |
|
15.5 |
1.9 |
|
16 |
2 |
|
16.5 |
2.2 |
|
17 |
2.4 |
|
17.5 |
2.5 |
|
18 |
2.7 |
|
18.5 |
2.8 |
|
19 |
3 |
|
19.5 |
3.1 |
|
20 |
3.3 |
|
20.5 |
3.4 |
|
21 |
3.5 |
|
21.5 |
3.7 |
|
22 |
3.8 |
|
22.5 |
4 |
|
23 |
4.1 |
|
23.5 |
4.2 |
|
24 |
4.4 |
|
24.5 |
4.5 |
|
25 |
4.6 |
|
25.5 |
4.7 |
|
26 |
4.9 |
|
26.5 |
5 |
|
27 |
5.1 |
|
27.5 |
5.2 |
|
28 |
5.4 |
|
28.5 |
5.5 |
|
29 |
5.6 |
|
29.5 |
5.7 |
|
30 |
5.8 |
|
30.5 |
5.9 |
|
31 |
6 |
|
31.5 |
6.1 |
|
32 |
6.2 |
|
32.5 |
6.3 |
|
33 |
6.4 |
|
33.5 |
6.5 |
|
34 |
6.6 |
|
34.5 |
6.7 |
|
35 |
6.8 |
|
35.5 |
6.9 |
|
36 |
7 |
|
36.5 |
7.1 |
|
37 |
7.2 |
|
37.5 |
7.3 |
|
38 |
7.4 |
|
38.5 |
7.4 |
|
39 |
7.5 |
|
39.5 |
7.6 |
|
40 |
7.7 |
from Hadlock (1984)
Equations derived from analysis of this data in JMP:
menstrual age in weeks =
= (0.2181598 * ((femur length in cm) ^ 2) + (2.093776 * (femur length in cm)) + 10.693127
predicted femur length in cm for given fetal age =
= ((-0.003401 * ((menstrual week gestation) ^ 2) + (0.4245474 * (menstrual week gestation)) - 3.873893
Yeh et al (1982) generated the equation:
femur length in cm =
= (0.2127 * (gestational age in weeks) - 0.6809
over the range of 12 to 40 weeks of gestation and 2 to 8 cm of femur length.
If this is rearranged:
gestational age in weeks =
= (((femur length in cm) + 0.6809) / 0.2127)
Limitations:
• Abnormalities in fetal limb development, usually shortened limbs, can result in inaccurate estimates.
Specialty: Obstetrics & Gynecology, Pedatrics
ICD-10: ,
