OB Measurement

 

  1. Gestational sac diameter

  2. Embryonic crown-rump length

  3. Biparietal diameter

  4. Head circumference

  5. Abdominial circumference

  6. Femur length

Gestational age is calculated from obstetric ultrasound studies using the OB measurement tools. Six methods for calculating gestational age are available.

 

Using the measurement tool, the following information is displayed:

Method Description Source

Gestational Sac Diameter (GSD)

Measurement of the mean GSD. Used between 5 and 12 weeks. The calculation applies a point-to-point measurement in the formula:

Gest. Age (weeks) = (Gest. Sac (mm) + 25.43) / 7.02

Accuracy: +/- 5 days.

 Hellman, L.M., Kobayashi, M, et.al. American Journal of Obstetrics and Gynecology, 1

Embryonic Crown-Rump Length (CRL)

Measured from the top of the fetus' head to its buttocks. Reflects embryonic growth. Used between 5 and 18 weeks. The calculation applies to a point-to-point measurement in the formula:

MA (weeks) = 4.933912 + 2.800701(CRL) - 0.635898(CRL)2 + 0.117067(CRL)3 - 0.011622(CRL)4 + 0.0004622(CRL)5 Accuracy: +/- 3-5 days.

 Robinson, H.P. et.al. A Critical Evaluation of Sonar "Crown-Rump Length" Measurements. Br.JI. of Obstetrics and Gynecology, Sept 1982,707.

Bi-parietal Diameter (BPD)

Measured as the transverse width between the 2 sides of the embryo head (the distance between two skull bones left to right). Used after 12 weeks. The calculation applies to a point-to-point measurement in the formula:

MA (weeks) = 9.54 + 1.482 (BPD) + 0.1676 (BPD)2

Accuracy FL +/- 1.19 weeks 12-18 weeks; +/-1.73 weeks 18-24 weeks; +/-2.18weeks 24-30 weeks; +/-3.08 weeks 30-36 weeks; +/-3.20 weeks 36-42 weeks.

 Hadlock, F.P. et.al. Estimating Fetal Age: computer-assisted analysis of multiple fetal growth parameters. Radiology. 1984 Aug;152(2):497-501.

Head Circumference (HC)

Measured as the perimeter of an ellipse of head. Used after 12 weeks. The calculation applies to the length of the perimeter of an ellipse in the formula:

MA (weeks) = 8.96 + 0.540 (HC) + 0.0003 (HC)3

Accuracy FL +/- 1.19 weeks 12-18 weeks; +/-1.48 weeks 18-24 weeks; +/-2.06 weeks 24-30 weeks; +/-2.98 weeks 30-36 weeks; +/-2.70 weeks 36-42 weeks.

 Hadlock, F.P. et.al. Estimating Fetal Age: computer-assisted analysis of multiple fetal growth parameters. Radiology. 1984 Aug;152(2):497-501

Abdominal Circumference (AC)

Measured as the perimeter of an ellipse of abdomen. Used at 34 - 36 weeks. Applicable after 12 weeks. The calculation applies to the length of the perimeter of an ellipse in the formula:

MA (weeks) = 8.14 + 0.753 (AC) + 0.0036 (AC)^3

Accuracy FL +/- 1.66 weeks 12-18 weeks; +/-2.06 weeks 18-24 weeks; +/-2.18 weeks 24-30 weeks; +/-2.96 weeks 30-36 weeks; +/-3.04 weeks 36-42 weeks.

 Hadlock, F.P. et.al. Estimating Fetal Age: computer-assisted analysis of multiple fetal growth parameters. Radiology. 1984 Aug;152(2):497-501

Femur Length (FL)

Measured as the length of the baby's femur. Used after 12 weeks. The calculation applies to the point-to-point measurement in the formula:

FL MA = 10.35 + 2.460 (FL) + 0.170 (FL)2

Accuracy FL +/-1.38 weeks 12-18 weeks; +/-1.80 weeks 18-24 weeks; +/-2.08 weeks 24-30 weeks; +/-2.96 weeks 30-36 weeks; +/-3.12 weeks 36-42 weeks.

 Hadlock, F.P. et.al. Estimating Fetal Age: computer-assisted analysis of multiple fetal growth parameters. Radiology. 1984 Aug;152(2):497-501