Bovine Fetal Aging With Ultrasound: How Accurate Are We After 100 Days?
By Elle Terhaar, with guest contributor Jacob Geis, DVM
Ask a group of bovine veterinarians how accurately they can fetal age a 60-day pregnancy with ultrasound, and you'll probably get a fairly confident answer.
Now ask the same question about a 130-, 150- or 180-day pregnancy.
That's where the conversation gets more interesting.
In a recent conversation with Dr. Jacob Geis, a bovine veterinarian with extensive experience working with cattle and evaluating pregnancies, he shared a fairly strong opinion on the subject: Once we get much beyond 100 days of gestation, we need to be careful about the amount of precision we attach to fetal aging.
That doesn't mean ultrasound suddenly stops being useful on Day 101. It certainly doesn't mean we can't estimate whether a cow is four, five or six months pregnant.
It means something different:
Estimating the stage of pregnancy and predicting an exact calving date are not the same thing.
And understanding that distinction can make better veterinarians, better ultrasound operators and better communicators with clients.
The Problem: Fetal Size Isn't the Same as Fetal Age
Most fetal aging techniques ultimately depend on a relationship between what we can observe or measure and the known development of the bovine fetus.
During earlier gestation, that relationship can provide remarkably useful information.
Classic research by White and colleagues found that crown-rump length was the most precise of several ultrasound measurements evaluated for estimating bovine gestational age, with a residual standard deviation of approximately 4.5 days. Other measurements had greater variation, ranging from approximately 6.9 to 12.6 days.
But there is an important concept hiding in those numbers:
There has always been a range.
Ultrasound isn't reading a date stamped on the side of the calf. We are observing biological structures and using their relationship with gestational age to make an estimate.
As the fetus gets older, that distinction becomes increasingly important.
Think about the calves that are delived every spring. One mature calf may weigh 60 pounds at birth. Another may weigh 100 pounds or more. It makes sense that we shouldn't expect every fetus to follow precisely the same growth curve throughout gestation.
That's why we have to be careful about equating fetal size with fetal age , particularly as pregnancy advances.
What Does the Research Say About Accuracy?
There is good evidence that bovine fetal aging with ultrasound can provide useful management information. There is also good evidence that we shouldn't mistake that usefulness for perfect precision.
One of the largest real-world studies examined fetal-age estimates in 83,104 cows from 116 seasonally calving dairy herds . Experienced veterinarians performed transrectal ultrasound examinations, and the researchers compared predicted with actual calving dates.
Overall, the results were impressive: 90.3% of cows calved within 10 days of their predicted calving date.
But the details are important.
The probability of cows calving more than 10 days later than predicted increased when fetuses were 13 weeks of age or older at pregnancy diagnosis.
Thirteen weeks is approximately 91 days.
That doesn't establish a scientific cliff at 100 days. But it does support something we see clinically: As pregnancies advance, we should become increasingly cautious about how precisely we communicate fetal age.
Another study illustrates the problem from a different direction.
Lawrence and colleagues examined cows from 60 to 160 days of gestation and evaluated whether placentome measurements could accurately predict gestational age. Their work demonstrated substantial uncertainty around an individual prediction and highlighted an important statistical problem: a strong relationship between a measurement and gestational age does not necessarily mean that measurement can accurately predict the age of an individual pregnancy.
That distinction deserves more attention in everyday practice.
Correlation Isn't Necessarily Prediction
This may be one of the most important concepts in fetal aging.
We can demonstrate that a fetal measurement gets larger as pregnancy advances. That gives us a relationship between the two variables.
But the question we're being asked at the chute isn't:
"Does this structure generally get larger as gestation advances?"
The producer wants to know:
"How old is this calf?"
Or, more often:
"When is this cow going to calve?"
Those are prediction questions.
A veterinary commentary reviewing the fetal-aging literature pointed out that much of the research historically focused on correlation and regression rather than measuring the accuracy of predictions in individual cows. The authors noted that the evidence base for the predictive accuracy of bovine fetometry was more limited than might be expected for such a commonly used technique.
That doesn't invalidate fetal aging.
It tells us to respect its limitations.
So, Is 100 Days the Cutoff?
Not exactly.
There is not enough evidence to declare Day 100 a universal biological cutoff beyond which fetal aging becomes impossible.
In fact, researchers have successfully used ultrasound measurements to estimate gestational age beyond 100 days. Wright and colleagues examined 300 beef cows between 35 and 125 days of gestation. The mean difference between predicted and actual calving dates was only 0.9 day, but the standard deviation was 9 days .
That's an important combination of numbers.
Across the population, the predictions centered quite closely on the correct date. But for the individual cow standing in the chute, there was meaningful uncertainty around that prediction.
That’s why Dr. Geis uses approximately 100 days as a practical threshold, not an absolute biological rule.
If a pregnancy is substantially beyond 100 days, Dr. Geis is comfortable estimating a general stage of gestation. He may be comfortable putting her into a month or even a half-month calving group depending on what he can visualize. In the New Zealand based study, when estimated fetal age increased to 15 weeks or more, correct predictions dropped markedly to 72%.
What Dr. Geis is much less comfortable doing is turning that observation into a highly specific due date.
There is a difference between saying:
"This cow is approximately four-and-a-half months bred."
and:
"This cow is 137 days pregnant."
The second statement implies considerably more precision.
Set Yourself Up for Success
At ReproScan ultrasound courses, we talk a lot about setting yourself up for success .
Usually that phrase comes up when we're discussing technique and managing expectations: cattle flow, restraint, probe positioning, facilities, image quality, operator experience and ultrasound settings.
But setting yourself up for success also means deciding before you start scanning what information you realistically expect to collect.
If a producer needs cattle separated into early-, middle- and late-calving groups, ultrasound may provide very useful information across a relatively broad gestational range.
If the producer needs to distinguish an AI pregnancy from a bull-bred pregnancy, the timing of the examination and the required degree of precision become much more important.
If the expectation is that every cow will leave the chute with an exact predicted calving date, we need to ask whether the biology can actually support that level of confidence and more importantly, when are the cattle being presented for ultrasounding .
In other words:
Start with the management question. Then determine whether ultrasound can reasonably provide the answer.
That's setting yourself up for success.
Your Ultrasound Settings Matter, Too
There is another part of setting yourself up for success that is easy to overlook: know your ultrasound machine.
Probe type, depth, frequency, and focus affect the image you're interpreting.
For example, increasing frequency generally improves detail while reducing penetration. Increasing imaging depth provides a larger field of view but changes how much of the anatomy occupies the screen. Gain and dynamic range influence the appearance and contrast of the structures you're trying to identify.
Those aren't minor details when you're fetal aging.
ReproScan's own training materials recommend different starting depths and frequencies depending on whether an operator is examining short-bred cattle, older pregnancies or a group with an unknown breeding range. For shorter-bred cattle, a shallower examination with a higher frequency can provide greater detail. When examining cattle with an unknown or broader gestational range, a deeper general-purpose examination can provide a larger window in which to locate the pregnancy.
Knowing how to optimize those settings won't eliminate biological variation.
But it can help ensure that image quality isn't adding unnecessary uncertainty on top of biological uncertainty.
That's an important distinction.
We can't control how quickly an individual fetus grows.
We can control whether we've given ourselves the best reasonable opportunity to see and interpret it.
Manage the Producer's Expectations — and Your Own
Good ultrasound work isn't just about what appears on the screen.
It's also about what you tell the person standing next to the chute.
If we communicate an estimated fetal age as an exact date, it is reasonable for a producer to expect the cow to calve on or very close to that date.
If she doesn't, ultrasound gets blamed.
Instead, communicate fetal-aging results in a way that reflects the confidence you actually have in the estimate.
For an early pregnancy, that window may be relatively narrow.
For an older pregnancy, it may be more appropriate to communicate a broader range or calving group.
That's not being less skilled.
It's being more accurate about what we know.
The Equipment Is a Tool. The Operator Makes the Decision.
One of the things we've learned through teaching bovine reproductive ultrasound is that good equipment and good training need to work together.
ReproScan's training resources and courses cover ultrasound fundamentals, pregnancy diagnosis, fetal aging and hands-on technique as well as practical hands-on tips about setting up for success and ultrasound technique.
But regardless of the ultrasound brand you use, understanding the fundamentals matters.
Know what you're looking at.
Know your machine.
Know your settings.
Know the limitations of the measurement.
And, perhaps most importantly, know when the image in front of you doesn't justify the precision being requested of you.
If you're using a ReproScan system and aren't sure whether your settings are helping or hurting your image, our team is always available as a resource. Sometimes a small adjustment to depth, frequency or gain can make the examination considerably easier.
But no setting can remove normal biological variation.
Age the Pregnancy. Respect the Range.
Ultrasound has given bovine veterinarians and producers an incredibly valuable tool to age pregnancies and therefore manage herds.
We can identify pregnancies earlier, group cattle more intelligently and gather information that helps producers make better management decisions.
The challenge is resisting the temptation to turn every observation into a more precise number than the evidence supports.
The takeaway is simple:
Set yourself up for success. Know what information you need before you start. Optimize the equipment for the examination in front of you. Understand what you're seeing. And communicate an appropriate level of confidence in your fetal-age estimate.
When pregnancies get beyond approximately 100 days, don't confuse precision with accuracy.
You can still learn a lot about that pregnancy.
Just respect the range.
About the Author and Guest Contributor
Guest contributor Jacob Geis, DVM, is a South Dakota veterinarian whose career has included extensive work with beef cattle and bovine reproductive medicine. Through years of veterinary practice and pregnancy evaluation, Dr. Geis has developed a practical perspective on the capabilities — and limitations — of fetal aging in the field.
Elle Terhaar is the author of this article and CEO of ReproScan Technologies and a RACE-approved instructor for advanced bovine reproductive ultrasound education. Her work focuses on helping bovine and mixed-animal practitioners successfully integrate reproductive ultrasound into their practices. Elle and her family also raise Charolais cattle in Iowa.
References
Brownlie TS, Morton JM, McDougall S. Accuracy of fetal age estimates using transrectal ultrasonography for predicting calving dates in dairy cows in seasonally calving herds in New Zealand. New Zealand Veterinary Journal. 2016;64(6):324-329. doi:10.1080/00480169.2016.1207573.
Lawrence KE, Adeyinka FD, Laven RA, Jones G. Assessment of the accuracy of estimation of gestational age in cattle from placentome size using inverse regression. New Zealand Veterinary Journal. 2016;64(4):248-252. doi:10.1080/00480169.2016.1157050.
Wright IA, White IR, Russel AJ, Whyte TK, McBean AJ. Prediction of calving date in beef cows by real-time ultrasonic scanning. Veterinary Record. 1988;123(9):228-229. doi:10.1136/vr.123.9.228.
White IR, Russel AJ, Wright IA, Whyte TK. Real-time ultrasonic scanning in the diagnosis of pregnancy and the estimation of gestational age in cattle. Veterinary Record. 1985;117(1):5-8. doi:10.1136/vr.117.1.5.
Laven RA. Estimating fetal age: how accurate is it really? New Zealand Veterinary Journal. 2016. Commentary discussing the distinction between correlation and individual predictive accuracy in bovine fetal aging.