This study aimed to determine phenotypic changes during embryo development in the mouse, and the influence of genetic background, by non-invasive real-time ultrasonography. Serial scannings were performed from Day 4.5 after the appearance of the vaginal plug in a total of 34 adult mice of different strains (9BALB/c, 10C57BL/6 and 15 CD1). Embryonic vesicle diameter was measurable from Day 4.5 of pregnancy. Images of embryos were obtainable from Day 5.5 of gestation onwards, and crown-rump length and trunk parameters (diameter and area) were measured. At more advanced stages, the positions of fetuses prevented accurate measurement of crown-rump length; therefore, head diameters (occipito-snout length and biparietal diameter) were used as an alternative index of fetal size. All measurements correlated highly with gestational age (P<0.0005). No significant differences were observed between genotypes in early pregnancy, but during the last week of gestation trunk measurements were larger in CD1 embryos (P<0.05) while head diameters were larger in C57BL/6 conceptuses (P<0.05). There was a significant effect of genetic background on heart rate throughout pregnancy; although heart rate was similar in CD1 and C57BL/6 embryos (154.5+/-2.8 and 147.8+/-4.5 beats/min, respectively), it was significantly lower in BALB/c mice (127.0+/-2.1; P<0.005 vs. CD1 and C57BL/6).