Any website please Ex: Wattpad, Webnovel, Here, etc If even after reading the bullet points you don’t know what I want then here for example https://my.w.tt/Zctaeld49V start from the beginning Either rebirth/transmigration/System/World hopping Either the MC is pregnant when it happens or already has a baby that he’s going to treat right Childcare More than 20 chapters
You can browse this tag: https://www.novelupdates.com/stag/mpreg/ and if there is Pregnancy tag in it, usually it also includes the pregnancy period. I do have a few I wish to recommend, but your (More than 20 chapters) have removed them from being eligible.
The second world of Playing the Victim in wattpad has what you're looking for. The first world is also good but there are no children of his own in the first world
Here's some good ones I've read: Florist Little Boss Entertainment Industry’s Gourmet Food Service I Don't Want You to Be Responsible! 老子不要你负责!