One of the most common questions asked of ultrasound technicians performing pregnancy scans – particularly on dogs – is the due date. In breeds where parturition can be difficult, it is important to know when the bitch is due so that the owner can be present, or arrangements for a Caesarean section be made in advance.

A study by Kutzler et al. (2003) found that due dates could be predicted with 24 hours with ultrasound to an accuracy of 75%, and within 48 hours to an accuracy of 87%. This is because the canine embryo grows at a rate of 1mm diameter per day between 17-30 days, after which the growth rate becomes exponential. The accuracy of standard measurement packages can be improved by adjusting for breed size; +1 day for small (under 9kg), and -2 days for giant (over 40kg) breeds. Bearing in mind that the data for this study was collected between 1987 – 2001, veterinarians and breeders purchasing modern-day equipment should have no difficulties achieving similar success rates.

Kutzler et al. (2003) recommend taking measurements from at least two foetuses (unless only one is present), and found that the optimal time for measuring was at day 30. Predictions made after 39 days gestation could only use the biarietal and BD foetal measurements, and were less than 50% accurate within a tolerance of two days.

Very similar accuracy rates were obtained by Lenard et al. (2007), who found that a good substitute for taking measurements in late gestation (which has been proven to be too inaccurate) was to look for organ development markers. This method also has the advantage of not having to adjust for maternal and breed size. The only downside of scanning at the 30 day mark is that resorption may still commonly occur; however, as long as litter size predictions are not being made, this should not be a serious issue.

Useful developmental markers are:

Day 23-24: Foetal heartbeat seen

Day 25-28: Foetus changes from circular to bipolar shape

Day 27-31: Anechoic (dark) area develops in the head

Day 33-35: Limb buds develop

Day 34-36: Foetal movement can now be seen

In summary, when using your scanner's gestational age calculation packages, remember that:

- "From Days 20 to 25, embryonic vesicle diameter (EVD) is the only fetal measurement that can be made."

- "The embryo becomes visible at Day 26 and from that day on, crown-rump length (CRL) and body diameter (BD) can be measured."

- "From Days 26 to 29, CRL was measured as the total length of the embryo and BD was measured in the same plane."

Taken from Kutzler et al. (2003).

By using any of these gestational age calculation methods at the appropriate stage in gestation, and by adjusting for breed size, you can provide accurate predictions of whelping date - but never 100% of the time. Inaccuracies in caliper placement, or the fact that dogs can give birth prematurely or be overdue, mean that getting it right 100% of the time is simply not a realistic target.

Perhaps a good starting point for those enthusiastic about giving gestational age estimates would be to provide a date with a tolerance of 72 hours to the owner, and being sure to follow up with them to find out what the final parturition date is. If it is found to be consistently within 48 hours of the given date, the technician may choose to begin giving 48 hour windows to owners with the clearly stated caveat that their dog is not a machine, and that no parturition date should ever be relied upon as guaranteed.

Further Reading:

For an earlier discussion of landmarks during gestation, click here.

For help on calculating gestational age with the Teknova TH80v ultrasound machine, click here.

To calculate gestational age using your Scan Pad, you must be scanning in obstetric mode. To do this, click "FUNCTION," then "PROBE," and select the "OB" setting. Once you have frozen your image, you then go to "FUNCTION" and "MEASURE" in the normal way, but down the left hand side you will see all of the gestational age calculation packages now available to you.