Let's say they developed a genetic mutation after getting bitten by a radioactive spider when they were a teenager, and this mutation gave them the abilities of a spider while still keeping their general human shape. What are the chances of the mutation progressing again later on in life and turning them into a man-spider?
And then let's say there was a supervillain who injected them with another mutagen that turned them into an actual spider and then somehow they got pregnant and erupted from themselves with all new powers?