Is it possible to calculate gestational age in canine ultrasound?
One question I get asked a lot - and I am sure veterinarians and those running their own dog scanning business get asked a lot too - is if it is possible to tell the gestational age of a foetus through an ultrasound scan.
While this is common place in human healthcare, when it comes to canine ultrasound, things aren't so simple. Breed size plays a huge part, with Beagles and Golden Retrievers the only breeds for which accurate figures have been collected so far. It is also common knowledge among breeders of smaller breeds (Chihuahuas, for example) that a single foetus will grow significantly larger than if it has company inside the uterus.
Veterinary ultrasound machines come with a number of canine gestational age calculation packages, which can be somewhat confusing if not misleading to people trying to give an owner an answer as to the gestational age of the foetus.
Nyland & Mattoon (2002) say that the gestational sac diameter method is the most accurate between 20-37 days. From 38-60 days, head diameter (on the transverse
plane) is the most accurate, and crown-rump length is also fairly accurate (but difficult to measure after day 48 because the foetus is so big it can exceed the imaging size). However, they still note that results are always plus or minus 3 days and that the highest accuracy is obtained early-mid pregnancy where rapid foetus growth is taking place.
There are a few other things you can look out for to help you establish which calculation method may be most appropriate. Remember not to just take the owner's word for it: canine sperm lives for at least 4-6 days, hence it is possible for conception to have occurred up to a week after the owner says the last mating happened. Use the following indicators to help decide for yourself:
- The gestational sac changes from spherical to oblong around day 28, so if you're doing your scans at 30 days from mating, this could be a good indicator to at least tell you whether conception occurred on the final day of mating or if it was several days later than the owner thought.
- Limb buds and foetal movement can be seen from day 35.
- Kidneys and eyes are not visible on your scanner until day 40-47. The individual four chambers of the heart aren't visible until the 40th day.
By combining your scanner’s calculation packages with your own observations, it is possible to give a cient an estimation of foetal age.
The days of being able to pinpoint the point of conception within a tolerance of only 24-48 hours for every breed, however, is unfortunately is a very long way off.
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