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.