How did my mouse get pregnant?

Question

Bleu   Hi Natasha,
It has been a while. I previously contacted you about my two mice Fern and Bleu because they had upper respiratory infections. I took them to the vet and Fern is doing lovely. Bleu however is still sneezy. This however is not why I am contacting you. About 20 days ago I came home from a lovely vacation. We were having friends take care of our fish, guinea pigs, and mice. The mice which usually reside upstairs in my room were moved on the floor of the first floor for the convenience of their new temporary care takers. Bleu has always been thinner than Fern and when I came home she looked as though she gained a little bit of weight and looked a healthy weight for the first time. I joked that she was pregnant. Not a funny joke anymore. I have noticed that Bleu has been getting bigger and bigger. Fern is 100% a female mouse so she could not have impregnated Bleu. Bleu and Fern have been very busy building a nest, that I have unfortunately disturbed a couple of times thinking it was just very cute mouse behavior. Yesterday morning as I went to take them out and play with them Bleu tried to nip me. I took her out and her sides were so much larger than usual. All of this makes me believe she is pregnant. The weight gain, the nest building, and the change in her behavior (the nip).  So now I am left believing that may have been impregnated by a wild mouse who would have had a better chance of reaching her when she was on the first floor. We have in the past had mice in our basement, though she was not down there. Is it possible she could be pregnant and that a wild mouse could be the culprit? I don't know how a mouse could have gotten in but there are bars so it might be possible they mated between the bars. Does this sound at all feasible? I'll attach a picture below. I moved them into a glass aquarium with a screen on top so that if she is pregnant the babies can't get out. I put in plenty of cozy nesting material and a new nest has already been made. I'm really quite confused as to what is happening. If you have any clue it would be great to hear from you!

Thanks so much,
Susan

Answer
Hi,

I bet if you searched under How did my mouse get pregnant, you would find I have out that not only is it common, but it has happened to me!  Those little wild boys can slip through such tiny cracks:I know of no barred wire mouse cage that they can't get through. I now have a bird cage - a 4 story converted multi bird cage-which must be made to keep little tiny parakeet bills from fingers.

It is all quite cute but there is a big problem that comes along with these apparent virgin births- these half wild babies are notoriously crazy. Much worse than a plain wild mouse.

Get them into a clean cage if she hasn't already given birth, and once they are born write back. I would explain more now but I am juggling a suicidal dwarf rat. Lol!

Squeaks n giggles,  
Natasha