Go to 婦科檢孕 today, and then ask for 懷孕證明.
Keep the record yourself, and then go to public hospital and for 'waiting' check list.
However, you don't need to give this letter to your company now.
If 公司"材" you suddenly, you just give them the letter. The effective day will be count as the date writen in the 懷孕證明.