Approximate age (weeks) post LMP

5—6 - Gestational sac volume.

7—12 - Crown—rump length.

12—15 - BPD, femoral length.

15—24 - BPD, femoral length, head circumference.

24 - After 24 weeks, individual variation in fetal size and growth render sonographic estimation of fetal age inaccurate. (Foot length may be helpful.)

·         Gestational sac size- GS can be visualized from 4 weeks on TVS and 5—6 weeks at transabdominal US. The sac is usually ovoid, therefore when the sac diameter is calculated an average of AP, transverse and longitudinal diameters is taken. When the sac is round only one measurement is necessary. The measurements are taken from the internal borders — mean sac diameter. Sac volume is calculated as AP x transverse x length x 0.5223.

·         Crown-rump length- This is the most accurate means of dating a pregnancy and is less likely than the GS measurement to be affected by external factors such as uterine compression by the urinary bladder in early pregnancy. It is affected by fetal flexion as the fetus matures. CRL measurement is not affected by maternal age, height, parity or racial origin. The accuracy of CRL measurement/gestational age prediction is ±5—7 days.

·         Biparietal diameter-Once the internal anatomy of the fetus can be clearly identified, standard landmarks can be used to obtain measurements. The fetal BPD diameter is one such measurement and is the most widely used means of assessing gestational age. It is accurate ±5 days.

·         Femoral length- The femoral length may be measured from 12 weeks. It does not replace the BPD entirely satisfactorily but rather should be used to confirm results from BPD measurements. It is of particular value if the fetal skull lies deep in the pelvis and a good section for BPD estimation cannot be obtained. Any of the long bones of the fetus can be measured, the femur is preferred because it is the j longest and easiest to image.