Factors affecting gestation duration in the bitch
Introduction
Accurate prediction of a bitch's gestation duration is useful when managing whelping or scheduling an elective cesarean section. Gestation duration in the bitch is most commonly cited as being 65.1 ± 0.1 d (S.E.M.) from the LH peak in Beagles [1]. Gestation duration from the progesterone rise, which is concurrent with the LH peak, is reported as 65 d, with 67% whelping within ± 1 d, 90% within ± 2 d, and 100% within ± 3 d [2]. When counted from a single mating on the ‘the optimal day for mating’ based on plasma progesterone, with matings not necessarily a uniform time after the progesterone rise, gestation duration was estimated as 61.4 ± 1.5 d (S.E.M.) among six different breeds [3] and 62.1 ± 0.2 (S.E.M.) among five large breeds [4]. When counted from the first day of cytologic diestrus, gestation duration has been stated as 56.9 ± 0.18 d (SEM) [5]. Factors such as breed [2], [3], [4], [6], litter size [2], [3], [4], [5], [6] and parity [4] have been examined for their influence on gestation duration.
Okkens et al. reported that breed influenced gestation duration [3], [4]. The most recent study by Okkens et al. had at least 12 bitches representing each breed; they reported that West Highland White Terriers had longer gestation durations than German Shepherd Dogs, Labrador Retrievers, and Dobermans [3]. The same group, analyzing at least five bitches from each breed, previously found German Shepherd Dogs had shorter gestation durations when compared to Boxers, Bernese Mountain dogs, Old English Sheep Dogs, and Bouvier des Flandres [4]. However, other investigators have shown no breed effect on gestation duration when 274 bitches representing 75 breeds (average of 3.6 females/breed) were retrospectively examined [6] or when 63 bitches representing 19 breeds (only four breeds represented by more than three bitches) were grouped by body size [2].
Litter size is another factor that may influence gestation duration. Litters of a single Beagle pup had longer gestation durations [5] in one study and litter size was negatively correlated with gestation duration by one group [3], [4]. Litter size did not affect gestation duration in three reports [2], [6], [7]. It is commonly accepted in sheep that the fetus initiates parturition [8]. It is not known what initiates parturition in the bitch, however clinical impressions and data [3], [4], [5] suggest that bitches with smaller litters have longer gestation durations.
The effect of age, or parity on gestation duration has not been studied extensively in the dog. A single study found primiparous bitches to have similar gestation durations to multiparous bitches [4]. Within breed, age had no effect on litter size [3].
This study used data collected retrospectively to test the hypothesis that age, breed, parity, and litter size affect duration of gestation in the bitch. The objective was to have substantial numbers of each breed and to have consistent, accurate data for examination of gestation duration. We hypothesized that decreasing litter size would increase gestation duration, but age, breed or parity would not affect gestation duration.
Section snippets
Animals
Bitches from the canine breeding colony at Louisiana State University in Baton Rouge, LA, USA (LSU) and Guide Dogs for the Blind, Inc., in San Rafael, CA, USA (GDB) were monitored from breeding to whelping.
Bitches at LSU were bred when the vaginal cytology was 100% cornified. Bitches had vaginal cytologies performed and were bred on a Monday, Wednesday, Friday schedule. Bitches were bred anywhere from one to six times during cytologic estrus. The first day of diestrus was identified as the day
Results
At LSU, 36 large hound bitches whelped 152 litters from 16 December 1992 to 21 April 2003. The age at whelping was from 12 to 155 mo. There were 36 first litters, 29 second litters, 23 third litters, 19 fourth litters, 14 fifth litters, 11 sixth litters, 7 seventh litters, 6 eighth litters, 5 ninth litters, and 2 tenth litters. Whelping averaged 56 ± 2.8 d from the first day of cytologic diestrus, with a range of 5 d before to 12 d after the predicted whelping date. The litter size ranged from one
Discussion
This study supported the hypothesis that litter size affects gestation duration. However, the hypothesis that breed or parity does not influence gestation duration was not supported. Although the duration of gestation was measured at two different facilities by two different methods, it was the difference between actual and predicted gestation duration that was analyzed by the model. By using this difference, data could be compared, regardless of the method used to predict parturition.
When
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