Dystocia, a common murine reproductive condition, is classified as either obstructive, a result of fetal factors such as an oversized fetus, or functional, a result of dam factors such as advanced age. Treatment is based on the dam’s clinical condition and the underlying etiology, but usually requires euthanasia. A prospective study was conducted to characterize the etiology of murine dystocia to determine if treatment is warranted. The signalment and experimental, clinical, and breeding histories were obtained, and a targeted serum chemistry panel, radiographs, and a gross necropsy were conducted on mice presenting with clinical signs consistent with dystocia. Obstructive dystocia was diagnosed if the pelvic canal width was less than the diameter of the fetal head closest to the cervix or a fetus was lodged in the pelvic canal. Functional dystocia was diagnosed based on clinicopathologic abnormalities. A total of 54 mice were evaluated over 7 mo with 45/54 (83%) confirmed to have dystocia with the remaining 9 (17%) having other reproductive abnormalities. Of the confirmed cases, 27/45 (60%) were C57BL/6 or on a C57BL/6 background, and the average age at presentation was 181 ± 85 d. The number of mice categorized as having an obstructive (n = 16) compared with a functional (n = 11) dystocia was not significantly different than those in which the definitive category could not be ascertained (n = 18). Neither clinical signs nor clinical pathology were significantly different between mice categorized as having an obstructive compared with a functional dystocia. Hunched posture, lethargy, and vaginal discharge were the most common presentation. Azotemia (BUN: 66.6 ± 10.2 mg/dL, mean ± SE), hypoglycemia (96.11 ± 8.5 mg/dL), and hyperglobulinemia (3.13 ± 0.14 mg/dL) were common. Differentiating obstructive from functional dystocia could not be determined cageside with strong confidence.
Number of litters prior to presentation with dystocia.
Correlation between number of pups compared with the average weight per pup in the litter of all dystocia cases (R2 = 0.0959; P = 0.04).
Dystocia classification based on likelihood and confidence level.
Radiographs of mice with obstructive dystocia. (A) Lateral (left) and ventrodorsal (right) views of a dystocic mouse with a fetus lodged in the pelvic canal. (B) Lateral (left) and ventrodorsal (right) views of a dystocic mouse with a macerated fetus lodged in the pelvic canal. A second macerated fetus can be found in the right uterine horn.
Gross images of dystocic mice classified as obstructive or functional. Note that all mice have perineal hemorrhagic staining. (A) Obstructive dystocia. Uterine horns are markedly distended with pups and a pup is lodged in the birth canal. The cranial half of the lodged pup extends into the left uterine horn. (B) Obstructive dystocia. Pup’s rump is observed protruding from the vulva. (C) Obstructive dystocia. The caudal half of a pup is seen extending from the vulva. (D) Obstructive dystocia. Dam with marked vaginal bleeding. Pup hindlimb protruding from the vulva. The other limb was cannibalized. The retained pelvic region of the pup was discolored tan to yellow. (E) Functional dystocia. The proximal aspect of the left uterine horn is markedly distended, soft, and filled with abundant yellow to tan, pasty, friable material (left image, intact left uterine horn; right image, left uterine horn dissected open). Mixed bacterial species were isolated. (F) Functional dystocia. The right uterine horn is distended with multifocal to coalescing dark red areas. The placental implantation sites in the right uterine horn are hemorrhagic. Presumptive placental defects and/or a circulatory disturbance.
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