According to Animal Wised, the mating habits of dolphins share some characteristics with other mammals. Ovulating female bottlenose dolphins can be impregnated by males, whereupon the baby (or calf) develops in the female's womb.
Whale & Dolphin Conservation USA adds that the duration of the pregnancy varies, depending on the species. For an orca — sometimes referred to as a Killer Whale, but actually a type of dolphin — it's a mammoth average of around 18 months, while for bottlenose dolphins it tends to take about a year.
Fish and other aquatic species have a variety of intriguing ways of bringing their young into the world. As Thierry Lodé writes in the paper "Oviparity or viviparity? That is the question..." (via Science Direct), "embryonic development ... occur[s] inside ... the ventral pouch of the male seahorse." A seahorse can birth up to 2,000 babies at one time, according to National Geographic, but the dolphin will give birth to only one calf.
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