Calculating gestational age in chihuahuas

biparietal diameter

Most veterinary ultrasound machines come with gestational age calculation packages inbuilt. Within any one species, however, results are not universally accurate across all breeds. There can be huge breed variation, with some species (e.g. goats) and some breeds (e.g. chihuahuas) suffering more than others.

Vieira et al.’s 2020 study sought to improve the accuracy of gestational age calculations in chihuahuas – a small dog which, nevertheless, has a large head in comparison to its body size. As chihuahuas are at increased risk of complications during delivery, it is important to be able to predict the estimated due date. Even more precise timing of delivery may be possible with a Doppler study of the umbilical artery.

The authors found that biparietal diameter (BPD) was the most useful measurement. The standard gestational age calculation using the BPD method in dogs is:

Gestational Age = biparietal diameter × 15 + 20 (Nyland and Mattoon, 2002)

The corrected calculation for chihuahuas, expressed in days before parturition, is:

Gestational Age = biparietal diameter × 15 + 25


Days before delivery

The authors devised the following formula for calculating the number of days to delivery in chihuahuas:

Days before parturition = biparietal diameter × -15.46 + 38.72  (with a standard deviation of ± 1.21 days)

In other words, if your BPD diameter measurement came to 2cm, your ultrasound machine would give you a gestational age of (2 * 15) + 20 = 50 days, i.e. 13 days before parturition.

Using the new formula, however: (2 * -15.46) + 38.72 = 8 days before parturition (or, expressing as gestational age, 55 days).



As this is a linear equation, however, one thing that is immediately apparent is that the adjusted values for the gestational age in chihuahuas are simply 5 days older than the generic canine BPD results. Rather than remember a formula, therefore, all we need to remember is that whenever we are performing the BPD measurement in chihuahuas to calculate gestational age, simply add 5 days onto the answer.

These manual adjustments have been used for years by experienced scanners who have noticed consistent under- or over-estimations in their breeds: for chihuahuas, at least, science has finally caught up!

Interested in establishing the standard for other breeds? Click here to learn more about the Animal AI Project.



Vieira CA, Bittencourt RF, Biscarde CEA, Fernandes MP, Nascimento AB, Romão EA, Carneiro IMB, Silva MAA, Barreto RO, Loiola MVG. Estimated date of delivery in Chihuahua breed bitches, based on embryofetal biometry, assessed by ultrasonography. Anim Reprod. 2020;17(3):e20200037


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