Pregnancy has been traditionally defined as the period of time eggs are incubated in the body after the egg-sperm union.[1] Although the term often refers to placental mammals, it has also been used in the titles of many international, peer-reviewed, scientific articles on fish, e.g. Consistent with this definition, there are several modes of reproduction in fish, providing different amounts of parental care

en.wikipedia.org/wiki/Pregnancy_in_fish

Going off of this, it’s just a matter of the term “pregnancy” being co-opted to describe something completely different from what it means in its original context. As does happen, even in science.