Fetal aging is a great service for clients. There are many uses for this information: calving groups, sale barn sorts, a measure of bull performance, etc. Fetal aging heifers in the 35 to 85 days post-breeding window can be very accurate. As pregnancy progresses and cow size increases, there are greater challenges that affect accuracy.
In mid-gestation, it is often difficult to visualize the fetus within the ultrasound image. Large frame and older cows add to this challenge. The fetus is deep in the abdomen and cannot be reached with the extension arm and 22cm depth setting. The only “information” available for fetal aging may be placentome size. Use this information with caution. Yes, placentomes get larger as the pregnancy advances but placentome size varies with position on the uterus and other factors. The closer the placentomes are to the fetus, the larger the placentomes. See the image below. This dissection of an 80-day pregnancy is from The Visual Guide to Bovine Reproduction (formerly called the Drost Project) on the University of Florida website. The placentomes are shown here range in size from less than 1 cm to over 2 cm. If one cannot reach the fetus with the extension arm and 22cm depth setting on the ultrasound unit, it would stand to reason that the placentomes being visualized are smaller than other placentomes that will be found near the uterus (if we could see them).
Gerben Blankenvoorde concluded in his research shown above that placentome size beyond 190 days of gestation was unlikely to be useful for diagnosing the stage of gestation. Blankenvoorde was working with smaller-frame dairy cattle in New Zealand primarily.
Where does this leave North American cattle scanners? If there are multiple placentomes greater than 6 cm in length, we can usually assume that the cow is at least 6 months pregnant. Then again, we have some large framed well-nourished cows in the Northern States and Canada that have large placentomes. Proceed with caution is the bottom line.