Warning, I used to study/teach/work in fish reproductive biology:

Fish, reptiles, amphibians, and birds don't actually get "pregnant" in the sense that mammals do. In some species they have internal fertilization, but the baby is nourished entirely by the egg, not by either parent or a placenta, until it hatches. Most hatch externally, some hatch internally, although it is less common. That said, among all animals sex is determined by which parent produces the large gametes (F) and which produces the small gametes (M) and some animals (not humans) sex can change over time due to environmental conditions. In most animal species, whichever parent is the last to deposit genetic material is usually burdened with childcare. Interestingly, in fish and other species that have primarily external fertilization (some fish are internal, but it's uncommon) the female often lays eggs, then "fucks off" while the male deposits sperm and cares for the eggs until they hatch. There is of course variation where one or both or neither perform childcare.

In seahorses they are usually a mated pair where the female deposits eggs into the male's pouch where he fertilizes them and carries them until they hatch. The male does not produce a placenta or produce nourishment for the babies and is therefore not pregnant, in the same way if I'm carrying around a hamster in my closed hands I am not pregnant with said hamster, and if I am carrying gum in my mouth I am not pregnant with the gum. The seahorse pouch is not the same as a uterus in either structure or function. After hatching seahorse babies often return to the pouch if there is danger. Other male fish will look after fertilized eggs in nests they've made or in their mouth. The nest is not pregnant and neither is the mouth.