It is important to know the precise gestational age in the early stage of gestation for deciding expected delivery date, and for estimating fetal development and well-being in the last trimester. For these purposes crown-rump length (CRL), diameter of gestational sack (GS) and the total volume of GS were measured by ultrasonic scanning techniques between 7 to 14 weeks of gestation. Coefficient of correlation (R) between CRL and gestational age estimated by the ovulation date according to basal body temperature (BBT) record (n=46) and that between CRL and gestational age calculated by the last menstrual period (LMP) (n=141) were 0.977 and 0.796, respectively. The relationship between CRL (y cm) and gestational age (x week) was: y=9.2722×10-1-4.5085×10-2x+1.0126×10-3x2. R between the length of GS and the gestational age estimated by BBT, CRL (n=141) and by LMP were 0.864, 0.762 and 0.593, respectively. R between total volume of GS and individual factors described above showed the same tendency in the same orders as mentioned above. These results indicated that the measurement of fetal CRL is very useful and reliable parameter for estimating the gestational age in the early stage of gestation.