A 12.5-MHz ultrasound probe was used to monitor 11 pregnancies in 6 queens from a breeding colony, and results were compared to previous findings using 5-, 7.5-, and 10-MHz probes. Cats were monitored daily from day 7 to day 28; the mean duration of the pregnancies was 65.2 days ± 0.7 days. The first embryonic vesicle was seen on different days depending on which probe was used; the higher MHz probes provided the earliest detection (day 11) compared with the 5-MHz probe (day 16-17). There was also delayed visualization of embryos with the 5-MHz probe (day 20) vs the higher MHz probes (days 14-16). Measuring the length of the embryonic vesicle after day 11 or the crown-rump length after day 17 could be used to evaluate stage of gestation; however, this became less reliable after day 25. Visualization of the limbs, neural tube, and stomach when using the 12.5-MHz probe indicated a pregnancy of at least 19, 20, and 26 days, respectively. The use of the 12.5-MHz probe did not improve the diagnosis of pregnancy in comparison with the 7.5- or 10-MHz probes but allowed significantly earlier diagnosis than the 5-MHz probe.