Introduction

Ultrasonographic foetal biometry is applied both in veterinary and in human medicine. In women accurate measurements are critically important to various aspects of obstetric management, including the determination of gestational age, the prediction of parturition term, the estimation of foetal weight and the detection of growth retardation.

In bitches and queens, predicting the day of parturition is particularly valuable in cases of uncertain mating times or when other information for determining gestation age (that is, day of luteinizing hormone surge, day of initial rise in progesterone or day of ovulation in the bitch, day of coitus-induced ovulation in the queen) are unavailable. Information on gestational age is crucial in clinical practice for planning clinical assistance of whelping or elective caesarean section when the complete foetal maturation has occurred, or for avoiding unnecessary caesarean section in bitches that are erroneously judged to be over term.

Yet, it is useful in case of mismating, when timing of pharmacological treatments for pregnancy termination is requested.

Ultrasonographic Extrafoetal and Foetal Parameters

Several extrafoetal and foetal measurements taken through transabdominal ultrasound examination have been proposed (for review Luvoni and Beccaglia 2006; Zambelli and Prati 2006). In the early pregnancy the ultrasonographic measurement of the inner chorionic cavity (ICC), the outer uterine diameter at the implantation sites, the placental thickness and the length of zonary placenta are significantly and linearly related to the gestational age (England et al. 1990; Yeager et al. 1992; Luvoni, Grioni 2000).

Compared to ICC, the measurements of the aforementioned extrafoetal structures give a less accurate prediction, because the uterine wall and foetal membranes are characterized by less defined margins than those of the chorionic cavity (Luvoni, Grioni 2000).

In mid and late pregnancy different foetal structures can be measured (Luvoni, Beccaglia 2006; Beccaglia et al. 2008b):

 Crown-rump length (CRL) - distance between the most rostral point of the crown to the caudal edge to the perineum;

 Diameter of the foetal heart (HDT ) - major and minor axes of the maximum cross section of the heart;

 Body diameter (BD) - two diameters made at 90° angles in the transverse plane at the level of foetal liver and stomach;

 Foetal head diameter (HD) - the largest cross sectional diameter of the head in an horizontal section;

 Deep portion of diencephalo-telencephalic vesicle (DPTV) - transversal diameter of thalamus and basal nuclei primordia;

 Biparietal diameter (BP) - distance between parietal bones.

In the author's experience, ICC during early pregnancy and BP during late pregnancy are highly correlated with the gestational age and provide a very accurate estimation of the day of parturition.

Technical Approach to Ultrasonographic Measurements of ICC and BP

The measurement of the spherical ICC is determined by taking the mean of two ICC diameters made at 90° angles from one side of the trophoblastic decidual reaction to the other. Accurate measurements can be performed until embryonic vesicles appear regularly spherical with clearly defined margins. Then, the vesicles become elongated through embryo development, and the measurement might be less precise.

The BP is visualized on a longitudinal scan of a foetal head. The distance is recorded when the parietal bones are parallel in order to standardize the measurement. During the examination, the ICC or the BP should be measured on a minimum of two different fetuses.

The mean value of the data obtained from the measurement of each structure on different fetuses is calculated and the predicted day of parturition is obtained by the application of equations. Due to extreme variation in dog breeds, specific formulae for small, medium and large size are needed (Luvoni, Grioni 2000), whereas a correction factor has been suggested for giant size bitches by Kutzler et al. (2003).

The equations we previously derived from specific growth curves (Luvoni, Grioni 2000; Beccaglia et al. 2008a) and that are routinely used in our Department for the ICC and BP measurements in small, medium size bitches and queens are the followings:

 ICC small size bitches: days before part. = (mm - 68.68)/1.53;

 ICC medium size bitches: days before part. = (mm - 82.13)/1.8;

 ICC queens: days before part. = (mm - 62.03)/1.1;

 BP small size bitches: days before part. = (mm - 25.11)/0.61;

 BP medium size bitches: days before part. = (mm - 29.18)/0.7;

 BP queens: days before part. = (mm - 23.39)/0.47.

The application of the equations mentioned above resulted in negative figures representing the number of days before parturition.

Accuracy of Parturition Day Prediction

Overall Accuracy in Bitches and Queens

The prediction is considered accurate when the difference between actual and predicted parturition dates is within ± 1 day or ± 2 days.

We previously reported an overall accuracy at ± 1 day of ICC and BP of 70% and 77%, respectively. The accuracy within ± 2 days was 86% and 85%, respectively (Beccaglia, Luvoni 2006). As mentioned before, specific formulae for small and medium size dogs were applied. The evaluation of the accuracy of ICC and BP based on bitch size showed that it was satisfying in both size bitches either at ± 1 day or ± 2 days (Beccaglia, Luvoni 2006).

In queens overall accuracy of the prediction of parturition day based on ICC measurements was 65% at ± 1 day and 85% and at ± 2 days; while with BP was 73.9% and 91.3%, respectively (Beccaglia et al. 2008a).

Accuracy on Foetal Sex Ratio and Litter Size Basis in Bitches

We previously demonstrated that the accuracy of ICC and BP measurements is not affected by the prevalence of one gender in the litter. Thus, individual variability due to the male or female growth is negligible.

Litter size does not affect the overall accuracy of parturition date prediction in bitches. Accuracy at ± 1 day ranges between 56% and 67% and at ± 2 days between 76% and 83% irrespective of the measurement taken (Kutzler et al. 2003). However, considering single ultrasonographic parameters we found that, whereas the parturition date prediction based on ICC is not affected by litter size, a higher accuracy is obtained with BP in normal litter size when compared with small and large litters (Beccaglia, Luvoni 2006). Biparietal measurement, which is taken during late gestation, is likely affected by individual variability of growth when few fetuses are present or it may be less accurate when the overlapping of multiple fetuses in the same ultrasonographic image field occurs.

Accuracy on Gestational Period Basis in Bitches and Queens

Canine and feline pregnancy lasts about 9 weeks from the ovulation day. The pregnancy diagnosis and the ultrasonographic identification and measurement of extrafoetal structures can be performed starting from week 2 to 3 of pregnancy in queens and bitches, respectively.

In the remaining 6–7 weeks, a progressive appearance and development of foetal organs allow the measurements of different parameters and their definition changes following their growth through pregnancy.

In clinical practice it is crucial to know at what stage the ultrasonographic measurements of ICC and BP provide the highest accuracy of the prediction of parturition term. We investigated whether the reliability of the determination of gestational age might be influenced by the stage of embryo/foetal development. Recent results demonstrated that the accuracy of the prediction in bitches and queens is influenced by the gestational period when ultrasonographic measurements are performed, particularly for BP (Beccaglia, Luvoni 2012).

Measurement of BP is highly accurate (± 1 day) within the first 6 weeks of pregnancy; afterwards a gradual decrease of the accuracy is observed, thus maintaining a good accuracy at ± 2 days until week 8. On the other hand, ICC measurement leads to a similar accuracy of the prediction at week 4 and 5.

With the measurements of multiple parameters as ICC, crown-rump length, body diameter and BP, other authors found that the highest accuracy of the parturition date prediction is when the bitch is examined in early (20–30 days) or mid (31–43 days) gestation (Kutzler et al. 2003), which correspond approximately to 3–4 and 4.5–6 weeks.

Approaching to the complete foetal development, the foetal fluid reduction and the overlapping of multiple fetuses increase the risk of inconsistent measurements. This affects the accuracy of the parturition term prediction.

Conclusions

Ultrasonographic foetal biometry provides useful information for monitoring the foetal growth and for assessing the gestational age and it has important implications in managing clinical assistance and planning caesarean section.

Among foetal parameters, ICC and BP are easily identifiable and allow an accurate prediction of parturition term. Particularly for BP, care should be taken with small or large litters and when the examination is performed in the last week of pregnancy. Unless it is carried out by an experienced sonographer with a high-resolution ultrasound equipment, the accuracy can be lower than expected.

References

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