I've quite a few pregnant piggies and births since keeping rescue guinea pigs (obviously all coming into my care pregnant!). I remember my first experience of it was when I was a teenager - before I knew about rescues and the thousands of small animals in need of homes I did what most people do and went to a pet shop.
I chose 2 jet black, female pigs. I trusted they were female and granted they were. What I didn't expect was 1 month later waking up to 6 guinea pigs. Both had given birth over night. I just remember lots of little eyes staring at me. Because males can reproduce at 3 weeks, females 4 weeks (possible but obviously varies) if you don't correctly sex and separate pronto you have problems.
Missexing is a common problem (from pet shops to breeders) and I would advise always getting this double checked by a vet or piggy savvy person. Obviously I would also say go to a reputable rescue instead. I've learnt from my mistakes and never 'bought' an animal again only adopted.
But anyway, I woke up this morning to my partner cooing over the babies. He barged into the room telling me they were trying to eat hay like 'big pigs'. Pig babies suckle from mum from birth being weaned at around 3 weeks but from the start they try and copy mum and nibble at food. Amazing huh?