Serial ultrasonographic examinations were daily performed on 8 bitches (5 Maltese and 3 Yorkshire terrier) from day 15 until parturition to determine the size of gestational structures. Gestational age was timed from the day of ovulation (day 0), which was estimated to occur when plasma progesterone concentration was first increased above 4.0 ng/ml. When the size of fetal and extra-fetal structures according to gestational age was measured, there was a difference between Maltese and Yorkshire terrier in extra-fetal structures (P<0.001), and no difference in fetal structrures (P<0.001). The correlations between the gestational age and the growth in fetal and extra-fetal structures were examined by regression analysis. The extra-fetal structure which showed high correlation from day 20 to 35 was inner chorionic cavity diameter in two breeds. The fetal structures which showed high correlation from day 36 until parturition was fetal head diameter in two breeds. In conclusions inner chorionic cavity diameter was the most accurate for estimation of gestational age and prediction of whelping day from day 20 to 35, and fetal head diameter from day 36 to parturitions respectively. Ultrasonography is very useful tool to evaluate the gestational age and to predict the whelping day in pet dogs.