| Abstract: | The method of estimating gestational age (GA) was studied in each trimester. CRL, BPD and FFL were selected as the parameters of ultrasonic B-mode measurement, and each standard curve was recorded. The less erroneous range on each parameter was decided. The range of CRL was from 20 to 80mm, that of BPD from 20 to 70 mm and that of FFL from 20 to 55mm. Within these ranges, GA estimated by CRL showed 2.9 days difference on average compared to the GA determined by LMP, compared with BPD 4.6 days and with FFL 4.9 days. CRL was the most accurate, but that with BPD or FFL could be useful in the 2nd trimester. Since the difference in the growth of each fetus would become apparent in late stage, the ranges of GA for each measured value of BPD or FFL were determined in the 3rd trimester by the mean +/- 3/2 SD for GA determined by CRL. Since 67 to 100% of the studied cases were included in the ranges of GA, the estimation will be useful in the 3rd trimester. FFL is more useful than BPD in the 3rd trimester, and it is also utilized in cases where BPD measurement is not possible due to fetal head descent. |
