My doctor said week 12, not because of the timeline, but because of fetal development.
If it broke down by time, then 2st would start at 13 weeks 3 days, and 3rd would start at 26 weeks 6 days, but most doctors(and books) say 3rd starts at 28 weeks.
It has more to do with how the baby is growing, rather than dividing 40 weeks into 3.
Aww, I'm just stalking your Kerr and you have a baby lime!!! it!