Don't want to write this by hand? GodCraft sets minecraft:is_pregnant for you with a switch.Get GodCraftOverview
minecraft:is_pregnant is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Marks the entity as pregnant, used by breeding with "causes_pregnancy".
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:is_pregnant": {}
}