The 280 days is full term plus 2 weeks prior to ovulation--when you weren't even pregnant. If you go by the 40 weeks (280 days) the prior poster is right. The math would be 13.3 weeks and you would be in your third about 27 weeks. If you go by 266 days, you would be starting your third trimester in the 25th week. So, docs go between 25-28 weeks for that reason. Why any doc would ever say that the third trimester starts in the 30th week is beyond me. That is weeks late by either way.