Animal time

I have been thinking of animals alot lately, mainly because of something that happened....... first I will share these pics with you. My brown dog is Sasha, other dog is Jenny and the little piggie is Peanut when he was a baby. The piggies are the reason for all this..... peanut has a sister, Lulu, a mum Rosie and a dad called Mum (Adel chose this - I take it as a compliment!). So we have 2 girls and 2 boys. These piggies have been seperated for almost a year, and we have had no problems......until Wednesday. When cleaning their cages we came across 3 dead baby piggies :( I am in shock. It is very sad that they died, but..... can someone PLEASE EXPLAIN how my piggy (Rosie) got pregnant in the first place?! I just dont know, I mean I KNOW but how can it happen when there cages are apart from each other? Well they were until 2/3 weeks ago, but the dead ones looked full term. How how how ??!!