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Your plecostomus, or pleco, is not pregnant - they do not bear live young like guppies do. Plecos lay eggs. If you have a female pleco, she could be carrying eggs, but generally, breeding is difficult in the aquarium. Do you know what kind of pleco it is, or have a picture? There are many, many species of plecos. To determine the sex of the pleco, you first should determine what species it is, and look at sexing guidelines for that species. The male and female of some species will have differences in fins, body size, head size or characteristics, etc.
Some plecos are nocturnal (active at night) and some are active and visible during the day. It is normal for plecos to stay at the bottom of the tank on the gravel, or rummage through the gravel looking for food. They also stick to the side of the tank as you've seen. They are bottom dwellers and usually don't swim through the water a lot like other fish do.
Here is an article to help you understand the nitrogen cycle your new tank will go through:
Here is an article about feeding vegetables to fish:
When I had a pleco, he liked to eat broccoli, spinach leaves, green beans, zucchini, and algae wafers. Most plecos also like some driftwood to hide behind or rasp on.