Multiple Extrauterine Pregnancy with Early and Near Full-Term Mummified Fetuses in a New Zealand White Rabbit (Oryctolagus cuniculus)

Extrauterine pregnancy (EP) is infrequent in mammalian species and occurs when fertilized ova implant and develop outside the uterus. A common outcome is abdominal pregnancy resulting in mummified fetuses (lithopedia). Here we describe an unusual case of abdominal pregnancy with early and near full-term lithopedia. Macroscopic findings supported the diagnosis of lithopedia with distinct age differences and facilitated further characterization of primary ectopia and risk factors leading to this occurrence.Abbreviation: EP, extrauterine pregnancyExtrauterine pregnancy (EP) occurs infrequently in most mammalian species.¹² The term derives from the Latin prefix meaning ‘outside’ or ‘beyond’ and refers to the implantation of a fertilized ovum outside the uterine cavity. Extrauterine pregnancy was first recognized more than 900 y ago² among other discoveries with a hereditary nature.¹³ Early reports compared EP in women, cats, dogs, and rabbits⁷ and described the presence of mummified fetuses in laboratory rabbits.^16,35EP is a serious obstetric complication that occurs asymptomatically in most cases.¹⁷ There are 4 classifications of EP: tubal, ovarian, abdominal–peritoneal, and cervical. The fallopian tube is the most common location and leads to tubal pregnancy. When gestation occurs in the abdominal–peritoneal cavity, abdominal pregnancy results and is subdivided as primary, when fertilization occurs outside the uterus after an oocyte is accidentally released from the fimbria, and secondary, when an oocyte is released due to direct tubal trauma.⁴⁵ A rare form of EP associated with high maternal morbidity and fetal mortality is called heterotopic (or combined) pregnancy, which occurs when 2 fertilized eggs coexist, one outside the uterus and the other inside.^18,33,44Undetected EP is frequently associated with fatal outcomes to the dam and offspring, including the formation of mummified fetuses, which may eventually become calcified and are called lithopedia (from the Greek: lithos, stone; paidion, child).^11,51,53 The condition is infrequent, and the factors that influence the unexpected outcomes of this pathology are not well understood.^26,34,35 Epizootiologic investigations are few,¹² although a recent report outlined the prevalence of EP in large NZW rabbit breeding colonies.⁵¹Examples of EP have been documented in dogs,¹⁷ cats,^14,39,42,49 rabbits,^20,29,45,51 hamsters,^9,46 rats,²⁶ mice,^8,12 guinea pigs,^3,30 lambs,⁴⁰ nonhuman primates,^10,34,38,50 and other species including humans.^11,12 However, despite the number of documented species, the majority of reports failed to note detailed clinical symptoms that interfered with reproduction even in instances that led to the formation of lithopedions.⁴²Experimentally, mouse embryos have successfully been transferred to a variety of sites including the peritoneal cavity, kidney, spleen, muscles, testis, and the anterior chamber of the eye.^1,6,21,31 The aim of the current report is to describe a rare case of abdominal pregnancy in which lithopedia developed clinically silently and coexisted with multiple pregnancies in a healthy doe rabbit.