You have to fill this form with your employer: https://www.ahv-iv.ch/de/Merkbl%C3%A...ch%C3%A4digung
If you have an artzeugnis, you have to send it also. My contract finished 1 month before my baby was born, and in the last 3 I worked at 40%, and I had to send them this artzeugnis.
It was only what I had to do!
Good luck, and enjoy you baby...they grow so fast