Serial ultrasonographic examination was daily performed on 12 pregnant Shih-tzu bitches 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. Extra-fetal structures were measurable from days 17 to 46. Outer uterine diameter increased from $9.9{pm}0.4$ mm (Mean${pm}$SD) at day 20 to $44.3{pm}0.8$ mm at day 46 and inner chorionic cavity diameter increased from $7.2{pm}0.2$ mm at day 17 to $36.9{pm}1.5$mm at day 41. Length of chorionic cavity or zonary placenta increased from $6.6{pm}0.4$ mm at day 21 to $46.4{pm}0.9$ mm at day 44. Of the extra-fetal structures, inner chorionic cavity diameter was the most accurate for estimation of gestational age until day 38. Fetal structures were measurable from days 23 to 60. Crown-rump length increased from $3.9{pm}0.3$ mm at day 23 to $55.2{pm}3.3$ mm at day 41, fetal body diameter increased from $7.9{pm}0.6$ mm at day 32 to $47.8{pm}2$ mm at day 60, and fetal head diameter increased from $6.3{pm}1.1$ mm at day 29 to $25.6{pm}0.2$ mm at day 60. Of the fetal structures, fetal head diameter was the most accurate for estimation of gestational age from day 38 until day 60.