Farhat J. Jumshyd ( Lady Dufferm Hospital, Karachi. )
The following methods are used for estimation of foetal at different stages of pregnancy:
1. Gestational sac this is seen from 5-6 weeks gestation tifi its time of disappearance around 12 weeks (Fig. 1).
2. Grown Rump length (CR1) this estimation is accurate uptil 12-13 weeks (Sanders) and is not reliable after 22-23 weeks (Fig 2).
Second And Third Trimester:
1. Femur length this measurement is accurate uptill 23 weeks (Fig 3).
2. Biparietal Diameter (BPD) one of the most reliable parameters from 13-30 weeks with accuracy of ± 10 days BPD is shown in the scan and should show oval shape of skull with arrow shaped at the level of third vertricle. In third trimester, BPD can be used to determine the following (Fig. 4).
a. Intrauterine growth retardation (IUGR).
b. Foetal weight prediction (with abdominal circumference).
c. As indication for appropriate time for amniocentesis.
d. Composite mean BPD also shows degree of pulmonary maturity known to be complete by 36 weeks. Head and abdominal circumference (AC) are also useful in conjunction with BPD. Conclusion:
Most reliable methods at different stages: In first trimeter of pregnancy - CRL. In second trimester - Femur length and BPD (Better if both are taken and corn pared). In third trimester - composite BPD with abdominal circumference and centres of ossification.