About the Gestational Age Calculator

The equations describing fetal growth are obtained from the polynomial fit to the data published in: "Ultrasonography in Obstetrics and Gynecology" by Peter W Callen, Second Edition, W.B. Saunders Company, Philadel phia 1988.

BPD - Biparietal diameter
The transverse width of the head measured from the leading edge to the leading edge of the bones. The accuracy of gestational age varies during the pregnacy from +/- 1 week at 12 weeks to +/- 4 weeks after 30 weeks.

HC - Head Circumference
The accuracy of gestational age calculated from HC is sligtly better than that derived from BPD.

AC - Abdominal Circumference
AC is measured at the widest point in the abdomen, through the liver at the level of the left portal vein or stomach. AC is determined not only by growing tissues (mainly liver) but also nutrient storage as subcutaneous fat and liver glycogen. It can be significantly decreased in nutritional growth retardation, and decreased in diabetic macrosomia where elevated blood glucose levels lead to increased metabolic storage.

FL - Femur Length
Accuracy of gestational age from FL measurements is relatively independent of nutritional-growth retarding processes. The measurement is reliable, and FL is best measured after 14 weeks