Safe pregnancy scanning in very small animals: rabbits and guinea pigs

How early can I scan a rabbit?

Positive confirmation of pregnancy has been documented as early as 7 days in rabbits, but only by the presence of fluid inside the uterus, which is a sensitive but not specific sign of pregnancy: in other words, you’ll always see it in a pregnant rabbit, but you could also see it in a rabbit coming in or out of season, or when something is wrong.

It is also worth noting that in such studies, rabbits were completely shaved and mechanically restrained on their backs. While this undoubtedly yields the best scanning conditions, it is clearly not the way we would ever scan a pet. Animals should be scanned in the position that they feel most comfortable – standing, lying, or in their owner’s arms – and no animal should ever be forced to have an ultrasound scan. Shaving should be done selectively, because does like to use their fur to prepare their nests for their young. Some rabbits will only do this from their neck and chests, but many do like to take it from their bellies. It is only necessary to shave a small ‘window,’ just large enough for the footprint of the probe, which should be a small microconvex. A convex probe will simply be too big, with too much of the probe overlapping fur.

Under these more realistic conditions, at 11 days I found that all that could be identified clearly was the presence of fluid in the uterus. It was not possible to identify individual embryos or confirm viability. I would therefore advise scanning rabbits a little later than this if the goal is to confirm a viable pregnancy for the owner.

Day 16 scan:

By day 16, things had become a lot clearer, although the presence of fur of course remained an issue. In the video below, you can see where the probe is losing contact with the skin to the left of the screen.

Fur really is the enemy here. By way of contrast, scanning a similarly sized but much shorter haired animal (guinea pig) on the same machine yields far more spectacular images:

KEEPING TINY ANIMALS SAFE DURING SCANNING

A study by Dom et al. (2013) showed that exposing rabbits to a typical ultrasound scanning protocol as performed on pregnant women resulted in significantly reduced parathyroid hormones in all of the offspring, which would have implications for the kidney, intestines and bones. This was regardless of the stage of pregnancy the does were scanned at.

This protocol involved 60 minutes of ultrasound exposure (the maximum permitted in a human obstetric scan by the British Medical Ultrasound Society), at a thermal index of 0.1 and mechanical index of 0.7, displayed on-screen at all times.

In guinea pigs, temperature rises of 2.5 degrees Celsius have been shown in foetal brain after 2 minutes of scanning time, and up to 5 degrees Celsius over foetal bone, the implications of which (in terms of birth defects) are unconfirmed.

Many poorly trained scanners in the UK use ultrasound machines that do not measure or display MI or TI. They do so unwittingly, because they have not been trained in ultrasound safety. A typical animal pregnancy scan will only take 5-10 minutes and will be perfectly safe when performed with the right machine, by the right person. A scan performed by someone without proper training or with an unsafe machine, however, could theoretically be causing thermally-induced teratogenesis, particularly on smaller animals.

To guard against this risk in this pregnancy scan, I took the following steps:

• Total exposure time was limited to a cumulative total of 5 minutes
• The probe was removed from the skin for a few seconds, every 60 seconds
• MI and TI were both kept at 0.1

This way, MI was seven times lower than in the Dom et al. study, and exposure time was at least twelve times lower – in reality, far lower than this because in this study, rabbits were insonated continuously for 60 minutes, but during this scan no single exposure exceeded 60 seconds.

References

Abramowicz, J. (2007). Prenatal exposure to ultrasound waves: is there a risk? Ultrasound Obstet Gynecol, 29.

Dom, S., Razak, H., Zaiki, F., et al. (2013). Ultrasound exposure during pregnancy affects rabbit foetal parathyroid hormone (PTH) level. Quant Imaging Med Surg, 3(1):49-53.

Ypsilantis, P., Saratsis, P. (2010). Early pregnancy diagnosis in the rabbit by real time ultrasonography. World Rabbit Science, 7(2):92-99.