Dear Mr./Ms. Principal,
Thank you again for your congratulations in regard to my pregnancy. As I mentioned, my due date is around the middle of February.
With regard to your comments, while my partner and I had indeed hoped to time our family planning to the summer break, nature had other plans. We are thrilled regardless.
Given the circumstances and your reaction, my decision has been made easier. I am going to resign, effective at the end of the fall semester. Of course, my lesson plans are in order, and I will do all I can to work with my colleagues and ease into the transition. I hope to speak to the students about my absence closer to the end of the semester.