2.2 Fetal Ultrasonic Measurements
Ultrasound examinations were performed every 3-4 weeks between 14 and 32 weeks, then every 2 weeks until delivery in singleton and DCDA twins; while for MCDA twins, every 2 weeks between 14 and 32 weeks and weekly beyond 32 weeks of gestation. At each visit, the fetal biparietal diameter (BPD), head circumference (HC), femur length (FL), humerus length (HL), anteropostero trunk diameter (APTD) and transverse trunk diameter (TTD) for each fetus were measured and recorded. The BPD was measured from the outer to the inner edge of the fetal skull at the level of the thalami. The HC was measured as an ellipse around the perimeter of fetal skull. The FL was measured from one end of the femoral diaphysis to the other, not including the distal femoral epiphysis. The HL was measured from one end of the humeral diaphysis to the other with the borders clearly visible. Both APTD and TTD extended from the outer aspects of the lateral abdominal wall, and TTD usually was perpendicular to APTD. Estimated fetal weight (EFW) was calculated based on BPD, APTD, TTD and FL according to Hadlock formulas26, 27. Abdominal circumference (AC) was calculated based on the mean arithmetic diameters of APTD and TTD28.