One question we 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 age of a foetus through a veterinary ultrasound scan.

While this is something we have become acustomed to in human healthcare, when it comes to scanning dogs, things aren't so straight forward. Breed sizes play 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 (e.g. Chihuahuas) that a single foetus will grow significantly larger than if there are more than one foetus present inside the womb.

Veterinary ultrasound machines come with a number of canine gestational age (GA) calculation packages, which can be somewhat confusing and misleading to the operator who is trying to give the owner an answer.

According to Nyland & Mattoon (2002), 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 can also provide fairly accurate results (but it is difficult to measure after day 48 because the foetus can exceed the screen size). However, they still note that results operate within a tolerance of 3 days and that the highest accuracy is obtained early to mid pregnancy where rapid foetus growth is taking place.

There are a few other things that can help to establish which calculation method may be most appropriate. Remember not to simply 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 as an additional guide:

- 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.

- 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.

- Limb buds and foetal movement can be seen from day 35.

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.