I've never heard of plain ivermectin causing a problem with pregnant animals in general. I believe it's generally considered safe for them. You can even hatch eggs of hens while treating with it and not expect damage caused by it to the developing embryos. If you want to be safe, I'd consider following the longest withdrawl period for slaughter listed on the box. You may want to get your parasite problem under control before breeding though, if that's what you're using it for.
Anybody with experience otherwise is welcome to contradict me, btw. We've used it several times on different animals with no problems, but several times is still limited experience.
I doubt Ivermectin would be an issue with breeding at all, it is pretty mild and according to the tube of horse ivemectin on my desk here "Ivermectin paste may be used in horses of all ages, including mares at any stage of pregnancy. Stallions may be treated without adversely affecting their fertility" - This is 1.87% Ivermectin paste. It does say not to use it in horses intended for human consumption though.
I doubt you used anything like this percentage, what, 1%? I think there's a 10 or 30 day withdrawal for animals to be consumed on that - I don't think my label says anything about breeding animals or adverse effects.
I used to have a bottle of injectable Ivomec Cattle/Swine wormer. In the ant print circular that came with the bottle, there was a warning about not using it within a few weeks post-breeding (don't remember how many, and can't find it now) because Ivermectin has been shown to be teratogenic (causing birth defects) and cytogenic (causing changes in DNA). One time when I wormed my chickens with Ivermectin, I tried to hatch the eggs rather than just discard them. That was one of the worst hatches I ever had; low hatch rate, and more than the usual number of deformed chicks. Related? Perhaps.
I'm not sure if any formulation of ivermectin has ever been approved for use in rabbits, so any use of it is considered off-label. But when you have a parasite problem, you do what you gotta do. I, too, would go with the longest withdrawal period on the label.