Hey, I've noticed that there is already some discussion on the story as to pregnancy mode. That's why I decided that I would also try to offer a vision of how the story itself could begin, so that it hooks into all the stages we are given to play in and explains the issue of fertilization in most beings.
Stage 1
The story begins with a female scientist falling in love with a certain mobian who does not reciprocate her affection. Desperate, the woman decided to create a serum with which she could drug her other half to make him fall in love with her. Unlucky for her, the recipe for creating this serum was old, making some of the ingredients and the proportions for them not very clear. The young woman nevertheless made an attempt and got to work. Not knowing if all the ingredients were right and if she had added everything in the right amount, she sincerely hoped that she would succeed. To her misfortune, at the same moment the city was attacked by Eggman and his robots. A numerous rocket attack caused tremors, and these led to the dropping of several vials of strange substances in the laboratory. The mixture led to a huge explosion with the release of a huge pink cloud. The seemingly ordinary cloud brought with it some side effects, such as an increase in libido in living organisms, and interestingly the creation of a peculiar virus that resembled sperm in structure and showed similar abilities, such as fertilizing a female individual. Due to its gaseous structure, it was only a matter of time when this cloud would get everywhere, even into a woman's vagina and then into an egg cell. And this caused the immediate fertilization of most of the female beings in the area of the cloud's spread. In addition, this virus, when combined with the woman's egg cell, released certain substances that strengthened the body, and accelerated the process of fetal development itself. During this event, our characters are captured as well as taken to the main ship, unsuspecting of what awaits them when they wake up.
Although the story is not somehow sophisticated, I think it would somehow explain why everything looks the way it does.