Dear …
Please accept this letter as my formal notice of resignation effective __(date)__.
This wasn`t an easy decision for me, but after long consideration, I have decided to assume the role of full-time mother. My experience with __(name of company)__ has been a very rewarding one and I do regret leaving behind the friendships I made here, but I feel this is the right decision for me.
Thank you for allowing me to have been part of your team over the past __(#)__ years. It was a memorable experience and I wish you and __(name of company)__ continued success.
Sincerely,