The biparietal diameter (width of cranium from parietal to parietal lobes) is measured from a transverse plane through the skull at the level of the thalamus and cavum septi pellucidi, calculating the distance from the external surface of the cranial side near the transducer to the internal surface of the cranial side away from the transducer.
It is most useful in the period of 10 to 28 weeks; later in pregnancy it becomes a less reliable predictor of development. Biparietal diameter can be affected by variation in head shape; therefore it is best to use estimates based on the biparietal diameter in conjunction with other measures of fetal development.
|
menstrual age in weeks |
biparietal diameter in cm |
|
12 |
1.7 |
|
12.5 |
1.9 |
|
13 |
2.1 |
|
13.5 |
2.3 |
|
14 |
2.5 |
|
14.5 |
2.7 |
|
15 |
2.9 |
|
15.5 |
3.1 |
|
16 |
3.2 |
|
16.5 |
3.4 |
|
17 |
3.5 |
|
17.5 |
3.8 |
|
18 |
3.9 |
|
18.5 |
4.1 |
|
19 |
4.3 |
|
19.5 |
4.5 |
|
20 |
4.6 |
|
20.5 |
4.8 |
|
21 |
5 |
|
21.5 |
5.1 |
|
22 |
5.3 |
|
22.5 |
5.5 |
|
23 |
5.6 |
|
23.5 |
5.8 |
|
24 |
5.9 |
|
24.5 |
6.1 |
|
25 |
6.2 |
|
25.5 |
6.4 |
|
26 |
6.5 |
|
26.5 |
6.7 |
|
27 |
6.8 |
|
27.5 |
6.9 |
|
28 |
7.1 |
|
28.5 |
7.2 |
|
29 |
7.3 |
|
29.5 |
7.5 |
|
30 |
7.6 |
|
30.5 |
7.7 |
|
31 |
7.8 |
|
31.5 |
7.9 |
|
32 |
8.1 |
|
32.5 |
8.2 |
|
33 |
8.3 |
|
33.5 |
8.4 |
|
34 |
8.5 |
|
34.5 |
8.6 |
|
35 |
8.7 |
|
35.5 |
8.8 |
|
36 |
8.9 |
|
36.5 |
8.9 |
|
37 |
9 |
|
37.5 |
9.1 |
|
38 |
9.2 |
|
38.5 |
9.2 |
|
39 |
9.3 |
|
39.5 |
9.4 |
|
40 |
9.4 |
from Hadlock (1984)
Equations derived from analysis of this data in JMP:
menstrual age in weeks =
= (0.0357155 * ((biparietal diameter in cm) ^ 3) - (0.404291* ((biparietal diameter in cm) ^ 2) + (4.246243 * (biparietal diameter in cm)) + 5.4570605
predicted biparietal diameter in cm for given fetal age =
= ((-0.004823 * ((menstrual week gestation) ^ 2) + (0.5310366 * (menstrual week gestation)) - 4.02343
Limitations:
• Measurements in a plane higher than that specified results in underestimation of the biparietal diameter, and thus the estimated gestational age and fetal weight.
• Variation in head shape can result in imprecise measurements.
• Hydrocephalus makes calculations invalid.
• Brachycephalic (abnormality in skull contour with shortening along the anterior-posterior axis, making the contour more round) skulls have an increase width in the biparietal diameter measurement relative to overall fetal size, resulting in overestimates of fetal weights or gestational age.
• Dolichocephalic (abnormality in skull contour with elongation along the anterior-posterior axis, narrowing in the coronal plane and flattening of the vault) skulls have a decrease in the biparietal diameter measurement relative to overall fetal size, resulting in underestimates of fetal weight or gestational age.
• Biparietal diameter measurements tend to be less affected in states of intrauterine growth retardation than abdominal circumference.
• If the head is engaged, then the biparietal diameter measurement may be inaccurate.
Purpose: To correlate fetal ultrasound measurements with those predicted for menstrual age assuming normal development.
Specialty: Obstetrics & Gynecology, Pedatrics
Objective: imaging studies
ICD-10: P05-P08,
