[BACK STORY: This sow has been underweight, and she was getting a little bit thin, and I've been trying to help her gain weight by changing her diet and weighing her. Recently, she did gain weight, which I attributed to the new diet regime. I didn't even know she was pregnant, nor did I think she could be. I have several female piggies, and only one boar whom I adopted but *I was told that he was neutered.* He has never shown a lot of interest in the females, and he is a small runt, and he is housed separately. But I have children, and they must have allowed them together at some point, because last night the sow in question gave birth to just one baby! It's been years since any of my guinea pigs have had babies, as I do not breed them, and only keep neutered boars (or so I thought.)]
Side Question: Just one baby is unusual, in my experience. Do you think she is waiting to give birth to another baby? How can I tell?
I had to make a change that helped her. I noticed that Squeakerella kept going to one corner of her cage, and attempting to "dig" obsessively. (This is not a normal behavior for her.) This led me to believe that she felt the need to create some sort of nest or protected area there for some reason. (She already had several areas like this in her cage, but she was not going to them.) I put a spare plastic "igloo" type hidey over the corner where she kept trying to dig. She immediately purred, and went inside the igloo. The baby followed her in, and at first Squeakerella didn't want to be in the igloo with the baby. But within about 2 minutes, she started grooming and cleaning the baby inside the igloo (which she hadn't done before.) They have not left the igloo for 15 minutes, and it appears that the baby is also nursing.
Squeakerella had a complete turnaround after getting into the igloo in the area of the cage where she had the nesting instinct. Apparently, her need to create the protected nest was even more overwhelming than her instinct to care for the baby. So, one tip I'm learning from this is to observe the sow, and see if she is exhibiting signs of having to meet other instinctual needs. Try to help her get those needs met, and then she may have "room" in her mind to care for the baby!
I will write with an update later.
If the male you have is older than several months, it should be obvious whether he is neutered or not. We neutered ours as soon as we realized he was male and that our sows were pregnant. If he is neutered, perhaps it was a recent thing before you got him. Guinea pig sperm must be some of the hardiest stuff on the planet because it can stay viable within the "ductwork" of male up to 3 or 4 weeks after having been neutered. He could still impregnate a sow several weeks after having been neutered. Of course, if it had been that recent, you should have been able to see some evidence of surgery.
The difference between a neutered and intact adult male is pretty obvious. Intact males have sacs that often drag the ground. Neutered males do not. A male that has been neutered young will also not grow as large as an intact one. Ours was neutered at around 5 or 6 months and now he is the same size as all the intact females he lives happily with. (I have six all together.) His two sons, which we re-homed with a friend so we could stay in contact with them, stayed intact and they grew to a huge size compared to dad.
A neutered male (assuming it's been a month or more after the procedure) can still "do it" but there won't be any babies.