Ultrasound as a Diagnostic Tool in Dating Pregnancies in Dogs and Cats, and Sex Determination of Canine Fetuses
In 2007 I started collecting data from pregnant bitches and queens. Initially the purpose was to determine if valid and precise determination of parturition could be estimated, from measuring canine and feline fetuses. After having collected data from approximately 30 pregnant bitches, 3 pregnant bitches from two different owners, without any information regarding heat or time of mating entered my programme.
Case 1: A Labrador retriever. The owner had detected no sign of heat, and had no idea of when mating had occurred. They had noticed weight gain in the bitch, and came for an ultrasound examination for that reason.
Case 2: Two Yorkshire terrier/Shih Tzu mix living in a shelter after being removed from owner for animal welfare reasons. The person taking care of them noticed changed behavior and weight gain.
At this point I had no idea if and how precise I could predict the time of expected parturition, but any guess could be helpful in both cases. In case 1, I compared biparietal diameter measurements from the fetuses, with similar measurements from Labradors in my data material. In case 2, I compared biparietal diameter measurements from fetuses in the two bitches with similar measurements from Chihuahua, and Danish-Swedish farmdog.
Since I didn’t have any data collected from Yorkshire terriers or Shih Tzus, I chose data from the breeds in my database closest, regarding size and shape. One bitch landed parturition on the exact day I estimated, one the day before and the third the day after. This of course gave me an idea, that information of the biparietal diameter alone, maybe could provide enough information to estimate parturition.
At this point it was too early to determine the nature of the correlation and the degree of breed differences. Since biparietal diameter, by far, is the easiest structure to find and measure in the fetuses, I found that it would be a very useful tool to date pregnancies and/or to detect deviations in fetal growth at all stages of the pregnancy for all veterinarian practitioners who use ultrasound on a regular basis. Later the ultrasound examination was also used as a way of getting information regarding malformations, sex determination, and to estimate number of follicles ready for ovulation, as well as predicting time of ovulation (together with serum progesterone and vaginal smears).
In order to get as much data as possible, all pregnant bitches or queens entering my programme were and still are offered weekly ultrasound examinations from around 19–21 days post-ovulation, or mating if time of ovulation is not known. In every examination, I measure as many as possible of the fetuses and I estimate the expected date of parturition.
In 2016 I had collected data from 145 bitches of all kinds of breeds, types and sizes. A total of more than 850 fetuses have been measured. I have collected data from around 42 different breeds.
At that time my collected data were analyzed. When charts of days to parturition as a function of biparietal diameter (sorted by breed) were made, it revealed that a linear correlation could be made with good approximation. In practical use I can estimate the date of parturition +/- 1–2 days.
Charts of this correlation were made for 34 different dog breeds, and a single for cats in general. The charts were published in September 2016 as the compendium: "Dating Pregnancy in Dogs and Cats - by measuring the biparietal diameter of the fetus." Second edition of the compendium is scheduled for release in September 2017.