Goldfish do not get pregnant. They Lay eggs. in the summer they can get spawn-bound - this means full of eggs with no place to lay them. This produces swelling of the abdomen (belly) and can be fatal. the male has a roll to play in her releasing eggs (if she has them) usually by tickling her with water plants (water weeds) which promotes her releasing eggs. However there are more common reasons for swelling (of the belly) abscesses, over feeding, tumors. Since this is a swollen head this is most likely infection or a tumor and hits being on its own means it is sick and most likely ready to leave the pond (die). Your best bet is to isolate the fish in its own container. If its small enough you might be able to take the fish to a vet or a fishery/aquarium shop and they might be able to tell you the cause and might have a treatment. There are various fish diseases out there which have their own set of symptoms, and treatments - but at times those symptoms do overlap so its hard to pick the right disease to treat. http://www.kokosgoldfish.com/disease.html Is just the tip of the iceberg on what can happen to fish. these are the more common ailments to strike a pond/tank. When in doubt take the fish or a Few good photographs of the fish to an expert who can ask other questions and who has experience dealing with lots of fish.