Unfortunately, Leos will not re-absorb a calcified ova. It however may decompose and break down leading to an internal infection or may perhaps be passed as an indistinguishable mass, appearing as a abnormal stool. Even for them to pass decomposed ova/eggs is a rarity as they usually do not survive long enough for an egg to reach a resonable level of decomposition.
A first time, first year female I would expect some delay and that is a given, but anything after 1.5-2 months after witnessed copulation, subsequent mid-body swelling, and either visual or palpated confirmation of eggs would require chemical
intervention to assist in the release of the eggs. It is the build up of pressure and internal positioning of the eggs causing obstructed blood flow, obstructed stomach passages, and general internal bruising/damages that ultimately will cause death to a Leo. Being that you are witnessing "signs" of deteriorating health, I would recommend, at minimum, an x-ray to confirm your suspicions.
I respectfully disagree with your vet regarding Oxcytocin. Most grass roots retention relief practices will not work on Leos due to their internal makeup, but I would still suggest warm soaks and manual intraventral lubrication until you can be counseled by a vet. If bound, Oxcytocin is a must as it is highly unlikely she will pass the eggs on her own. The administration of Oxcytocin is a common and successful practice. Although it should be considered a "last ditch" effort, we usually cannot determine exactly how much time Leos can retain eggs and Oxcytocin should be used early in diagnosis to releive what is there before more fertilizations take place, from retained sperm, causing more obstructions.
If you search the forums for "egg binding" you will find a long post regarding egg binding that I put out in May.
In the end, she may not be ready to lay, but it is a judgement that should carry a second professional opinion.
I hope your Leo recovers well and keep us informed.
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Galen Clark
www.captivescience.com
