Well, this time of year, it’s definitely possible. Spring is when koi generally spawn. I’ve written before about how to determine the sex of your koi as well as how to get your koi to spawn, but how do you decide if your koi have decided to get spawny without your intervention?
Well, first off, it’s worth noting that a koi that appears pregnant is not, one way or the other, pregnant in the same way that a human would be. They don’t give live birth, and the eggs are fertilized outside of the body, so there really aren’t baby koi in your fish either way. However, your koi will appear bloated and pregnant for up to several days before she’s ready to release her eggs. At this point, you could still stop the spawning from happening if you don’t want eggs or baby fish (fry) in your pond by simply removing the females from the pond, keeping them separate from the males. This shouldn’t hurt the females (assuming that you keep them in good conditions). Without males to agitate her into releasing the eggs, she’ll just reabsorb them as protein. She’ll also do this if you simply have no male koi in your pond with whom to mate.
If your fish is full of eggs, she’ll catch the eyes of the males around her and they’ll start to chase her around the pond to get her to release the eggs. This can get quite rough, though your females are likely to be fine, as they’re generally larger than males. The males will push against her sides, and eventually she’ll release her eggs into a protected area, usually among vegetation. The males will then release their milt over the eggs. Often several females in the same pond will have eggs at the same time, but usually only one female per day will actually release her eggs.
At this point serious koi breeders will remove the eggs from the pond to protect them (koi tend to eat their own eggs as well as fry). This isn’t entirely necessary, though, as some of the eggs are likely to survive either way. Be prepared, though, as generally very few fish will survive to adulthood.