The Crown-rump Length
This measurement can be made between 7 to 13 weeks and gives very accurate estimation of the gestational age. Dating with the CRL can be within 3-4 days of the last menstrual period. An important point to note is that when the due date has been set by an accurately measured CRL, it should not be changed by a subsequent scan.
The Biparietal Diameter
The diameter between the 2 sides of the head. This is measured after 13 weeks. It increases from about 2.4 cm at 13 weeks to about 9.5 cm at term. Different babies of the same weight can have different head size; therefore dating in the later part of pregnancy is generally considered unreliable. Dating using the BPD should be done as early as is feasible.
The Femur Length
Measures the longest bone in the body and reflects the longitudinal growth of the fetus. Its usefulness is similar to the BPD. It increases from about 1.5 cm at 14 weeks to about 7.8 cm at term. Similar to the BPD, dating using the FL should be done as early as is feasible.
The Abdominal Circumference
The single most important measurement to make in late pregnancy. It reflects more of fetal size and weight rather than age. Serial measurements are useful in monitoring the growth of the fetus.