Honestly, your best options depend on what you mean by 'popular pregnancy manga' as that theme can go from fluffy romance to heavy drama. I read most of mine through the Shonen Jump or Manga Plus apps, but those skew towards action. For series specifically focused on pregnancy, like maybe 'Kono Koi, Hoshi ni Naretara', you'd have to check Comikey or Azuki. Sometimes the official digital release lags way behind the Japanese serialization, which is frustrating when you're hooked on a weekly cliffhanger.
Viz Media's site is a solid legal hub, but you need to search by title—they don't have a 'pregnancy' genre tag. I remember wanting to read 'With the Sheikh in His Harem' (not exactly pregnancy-centric, but close) and found it on Kindle Unlimited through Amazon's ComiXology. That's a sneaky-good resource; you can borrow a ton with a subscription instead of buying each volume outright.
Don't forget about publisher-specific sites like Kodansha's K Manga app. Their library is huge, and they often have titles that fit the 'life changes' niche. The interface can be clunky, but it's legit and supports the creators directly, which matters a lot for niche genres that might not get physical prints over here.
For pure slice-of-life pregnancy stories, I'd start a free trial on Azuki, browse their 'Josei' or 'Slice of Life' categories, and see what pops up. Their curation isn't perfect, but it beats sketchy aggregator sites full of pop-ups.