When my Mother in law and Brother in law took Miya to be spayed, when
they opened her up, they found that she was pregnant with 4 pups, 4 to
5 weeks along.
Since she was already under anaesthesia, and opened up, it was too
late to save the pups, but according to the vet the sire must have
been pretty big, and there almost certainly would have been
complications had she gone full term.
Miya recovered from her surgery quite well, and is going through a bit
of nesting and carrying around toys like pups, but doesn't seem any
worse for the wear.
The vet techs were all amazed at what a sweet and beautiful girl she is.
The vet didn't even charge my MIL extra for the pregnant spay, or the extra overnight stay to keep an eye on her.
I noticed that the first post in this thread was from 2/23. Since Miya's spay was on 3/31, she must have been bred just before being turned in!
It was fortunate that my MIL and BIL adopted her when they did, because she would have started to show within about the next week, and then she would have been very hard to adopt out.