The idea behind Wonder Weeks is that babies go through certain developmental periods called "leaps".
The app gives you an estimate of when to expect these periods based on your baby's due date. This will let you know when to expect extra fussiness as babies are often extra cranky and in need of snuggles when going through these changes.
The app will also give you information on what new skills are being developed and activities you can do to help them with their new skill set.
There are debates as to whether so called "leaps" really exist or not, but I've seen a lot of parents who see behavior that coincides with "leaps". I use the app myself with my two kids
The app was developed from information in the "Wonder Weeks" book if you want more in depth information.
Flo is my go to app for tracking periods and ovulation. It can predict when you're period will start, when you will ovulate, and your fertile window based on your past cycles. (You'll have to enter data for a couple months for it to start getting more accurate)
The app is great if you're using natural family planning or just like to have an idea as to when Aunt Flo will visit again.
Remember: The app can only track your fertile window once your period returns and natural planning isn't 100% accurate. You can get pregnant while breastfeeding & you can get pregnant before your period returns!