Most colubrid snakes are oviparous (egg-laying). Two exceptions are garter snakes (Thamnophis spp.) and water snakes (Nerodia spp.) which are viviparous (live-bearing). Dystocia has been reported to occur in approximately 9% of captive reptiles. Dystocia can be classified as obstructive or nonobstructive based upon etiology. Obstructive dystocia is relatively uncommon in snakes. Examples of obstructive dystocia are fetal morphologic abnormalities such as excessively large or fused eggs, retained mummified products of conception from a previous pregnancy, pelvic compromise, oviductal strictures, and extraluminal compression due to space occupying lesions. In snakes most cases of dystocia are nonobstructive in nature; furthermore, the exact cause is often idiopathic. It is suspected that improper husbandry practices are largely responsible for this phenomenon. In the author's experience, the most commonly suspected causes of nonobstructive dystocia in order of decreasing frequency are breeding of physically immature subadult specimens, unsuitable substrate availability and poor muscle tone due to inactivity. Physically immature, first-time gravid snakes appear to be more prone to dystocia. Another group predisposed to dystocia are females that are double-clutched each year. The second clutch of eggs is prone to retention possibly due to uterine inertia.
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