Tips, Tricks and Techniques – Fetal Aging: Plan Ahead for Success!

By Dr. Andrew Bronson, DVM | Created on Fri Aug 4, 2017

One of the greatest benefits of ultrasounding is the ability to age the fetus and determine the stage of gestation of the pregnant cow.  When determining fetal age, it is important that the animals are presented at an estimated stage of gestation that gives you the results that you and your client desire.  For example, if accurate fetal aging is required to confirm that pregnancies are the result of artificial insemination versus cleanup bulls, then the timing of the pregnancy testing is critical to one’s success.  With some practice, one can categorize fetal age into 5 day increments using the grid lines on the ultrasound’s display for measuring.  It is best to do this in the 28 to 85 day window.  As you get closer to 100 days of gestation, there is too much variability due to genetics, fetal sex, nutrition, position of fetus in the uterus and other factors to ensure the same accuracy in fetal measurements and fetal aging. 

There are several different ways to measure fetuses:  Crownà Rump Length (CRL), head diameter, head length, and trunk diameter. The most common way to age smaller embryos and fetuses is by using the crown rump length with the ultrasound’s grid. For example: on average, a 40-day fetus will have a CRL of about 2 cm, a 50-day fetus will be 4 cm and a 60-day fetus will be about 7 cm. A simple rule for aging under 50 days is: CRL(mm) + 18 = age in days. After 50 days, it is recommended that you refer to the fetal aging table. I like using head diameter from 65 to 90 days.  Beyond 90 days, there is more difficulty in getting an accurate head diameter, so you will have to measure any fetal part that you can get the probe on with or without the extension arm. 

There are a couple developmental stages that are helpful when determining the age of the fetus.  Around 40 days the amniotic membrane becomes more visible.  Between days 50-60 the fetal shape is complete.  After 60 days, the rib cage is evident.  And, from this point on, one can use the head diameter measurement to determine fetal age. As the fetus continues to grow, it is often easier to position the probe over the head to take a measurement, as it can be challenging to get the entire fetus into the ultrasound view window for fetal length measurement.  This is especially true if you are using a linear probe.

At 120 days, the fetus is now the size of a small cat. At this stage, it is easiest to determine age when zoomed out to 22 cm on a convex probe machine.  Zooming out will allow for a larger viewing area, thus decreasing the time it takes to find the fetus and other structures.  At 120 days and beyond, it becomes more difficult to accurately age the fetus. Often the placentomes are all that one sees when scanning; this is especially true in deep bodied cattle. Due to the large variation of placentome size, it is not recommended that you age by placentome size alone. It is best to explain to your clients that advance pregnancies are more difficult to age accurately.  This difficulty is due to a multitude of factors including the limited ability to accurately measure fetal and uterine structures when the fetus and uterus are large and deep in the abdomen.  Also, there are large variations in the size of these structures due to the factors such as genetics of the cow and bull, fetal sex, nutrition and other environmental factors. 

Pregnancies over 120 days are best estimated by the month.  Some states, such as Missouri, use the trimester system.  In Missouri sale barns, cows are pregnancy tested and marked with a 1, 2 or 3 to indicate stage of pregnancy by trimester.  The trimester 1/2 split at 90 days can be done accurately with measurements.  The trimester 2/3 split can be estimated using size of placentomes, size of fetal structure including eye socket diameter greater than 2 cm.  Veterinarians that use ReproScan XTC or BoviScan Curve have developed systems that work for their sale barn customers.  Please contact ReproScan if you wish to discuss these unique sale barn systems in more detail. 

 ReproScan ultrasounds have two different grid settings available: a one-centimeter and a two-centimeter grid. If you have questions on how to set your grid, please refer to your manual or call the office at 877.890.2411.