Ans. A. Crown - rump lengthThe most sensitive indicator of symmetric and asymmetric IUGR is the abdominal circumference, which has a sensitivity of over 95 percent if the measurement is below the 2.5th percentile. Head/abdomen ratio is very useful in suspected IUGR when sparing head growth is observed (asymmetrical IUGR) and ratio is increased. Between 20 and 36 weeks of gestation, the HC/AC ratio normally drops almost linearly from 1.2 to 1.0. The ratio is normal in the fetus with symmetric growth restriction and elevated in the infant with asymmetric growth restriction.1. First trimestercrown-rump length is most accurate. The image should be obtained in a sagittal plane and include neither the yolk sac nor a limb bud. If carefully performed, it has a variation of only 3 to 5 days.2. Second trimesterBetween 14 and 26 weeks, the biparietal diameter (BPD) is usually the most accurate parameter, with a variation of 7 to 10 days. By convention, the BPD is measured from the outer edge of the proximal skull to the inner edge of the distal skull, at the level of the thalami and cavum Septi pellucidi.The head circumference (HC) also is measured. If the head shape is flattened (dolichocephaly) or rounded (brachycephaly), this measurement is more reliable than the BPD.The femur length (FL) correlates well with both BPD and gestational age. The femur is measured with the beam perpendicular to the long axis of the shaft, excluding the epiphysis, and has a variation of 7 to 11 days in the second trimester.The abdominal circumference (AC) is the parameter with the widest variation of 2 to 3 weeks. This is because the AC measurement involves soft tissue rather than bone and is also the parameter most affected by fetal growth. The AC is measured at the skin line in a transverse view of the fetus at the level of the fetal stomach and umbilical vein.3. Third trimesterThe variability of gestational age estimation increases as pregnancy advances. By the third trimester, all individual measurements become less accurate. Estimates are improved by taking an average of the various parameters the BPD, HC, AC, and FL. Also, each measurement should be assessed individually; if one measurement is significantly different from the others, it can be excluded from the calculation. The outlier measurement could result from poor visibility, or it could indicate a fetal growth abnormality. If third-trimester ultrasound measurements must be used to determine gestational age, accuracy can be improved by performing serial examinations and documenting normal interval fetal growth