During the first trimester the crown-rump length of the fetus in mm can be used to fairly accurately determine the gestational age.
|
Menstrual Maturity |
Crown-Rump Length in mm |
||
|---|---|---|---|
|
Weeks |
Days |
Mean |
2 SD |
|
6 |
2 |
7.0 |
3.3 |
|
6 |
3 |
6.5 |
1.4 |
|
6 |
4 |
7.0 |
4.6 |
|
6 |
5 |
6.5 |
4.2 |
|
6 |
6 |
10.0 |
2.6 |
|
7 |
0 |
9.3 |
2.3 |
|
7 |
1 |
10.3 |
8.0 |
|
7 |
2 |
11.8 |
5.7 |
|
7 |
3 |
12.8 |
4.8 |
|
7 |
4 |
13.4 |
6.7 |
|
7 |
5 |
15.4 |
3.6 |
|
7 |
6 |
15.4 |
4.4 |
|
8 |
0 |
17.0 |
4.9 |
|
8 |
1 |
19.5 |
5.7 |
|
8 |
2 |
19.4 |
6.2 |
|
8 |
3 |
20.4 |
5.0 |
|
8 |
4 |
21.3 |
3.8 |
|
8 |
5 |
20.9 |
2.4 |
|
8 |
6 |
23.2 |
3.6 |
|
9 |
0 |
25.8 |
6.0 |
|
9 |
1 |
25.4 |
4.6 |
|
9 |
2 |
26.7 |
4.4 |
|
9 |
3 |
27.0 |
2.8 |
|
9 |
4 |
32.5 |
4.2 |
|
9 |
5 |
30.0 |
10.0 |
|
9 |
6 |
31.3 |
5.5 |
|
10 |
0 |
33.0 |
7.2 |
|
10 |
1 |
33.8 |
7.6 |
|
10 |
2 |
35.2 |
7.3 |
|
10 |
3 |
36.0 |
7.9 |
|
10 |
4 |
37.3 |
9.7 |
|
10 |
5 |
43.4 |
7.7 |
|
10 |
6 |
40.1 |
7.1 |
|
11 |
0 |
46.7 |
6.1 |
|
11 |
1 |
43.6 |
7.2 |
|
11 |
2 |
47.5 |
6.2 |
|
11 |
3 |
48.8 |
5.9 |
|
11 |
4 |
49.0 |
9.5 |
|
11 |
5 |
54.0 |
9.8 |
|
11 |
6 |
56.2 |
9.5 |
|
12 |
0 |
58.3 |
9.4 |
|
12 |
1 |
56.8 |
7.2 |
|
12 |
2 |
59.4 |
6.6 |
|
12 |
3 |
62.6 |
8.6 |
|
12 |
4 |
63.5 |
9.5 |
|
12 |
5 |
67.7 |
6.4 |
|
12 |
6 |
66.5 |
8.2 |
|
13 |
0 |
72.5 |
4.2 |
|
13 |
1 |
69.7 |
8.5 |
|
13 |
2 |
73.0 |
15.1 |
|
13 |
3 |
77.0 |
8.5 |
|
13 |
4 |
NA |
NA |
|
13 |
5 |
NA |
NA |
|
13 |
6 |
76.0 |
5.7 |
|
14 |
0 |
79.6 |
7.8 |
from Robinson (1975)
An equation based on this data is:
days of gestation =
= ((8.052 * ((crown rump length in mm)^(0.5))) + 23.73) +/- 4.7 days
where
• the variation of +/- 4.7 days could be reduced to +/- 2.7 days if three independent measurements were made
• age range is 6 to 14 weeks
Reece (1987) developed the following equation for first trimester pregnancies:
gestational age =
= 5.23 + (2.53 * (crown rump length)) - (0.345 * ((crown rump length) ^2)) + (0.0172 * ((crown rump length) ^3))
The units for the crown-rump length are not given, but the equation only works if the crown rump length is given in cm. Figure 4 shows the units in mm, but the results do not correlate with Robinson's tabular data above. The gestational age range covered is 6-12 weeks.
Limitations:
Fetal positioning may make getting an accurate sagittal section difficult. Variation between different instruments may cause variation in readings.
Specialty: Obstetrics & Gynecology, Pedatrics
ICD-10: ,
