The milestones that matter
Viability (~23–24 weeks, with NICU support) marks survival possibility; early term (37–38) marks functional maturity with some trade-offs; full term (39–40) is the elective-delivery standard; late term (41) triggers increased monitoring; post-term (42+) usually prompts induction. Each boundary is a clinical decision point with data behind it.
Why 39 became the magic number
The 39-week rule came from large data showing elective deliveries before 39 weeks carrying higher NICU admission and respiratory risk — brains and lungs finish specific business in those final weeks. The exception list (hypertension, growth restriction, diabetes complications) exists, but "done at 37" is not among them by preference.
Counting with the right units
Weeks + days is the only unambiguous unit; months create double-counting (a "9-month pregnancy" is really ~40 weeks ≈ 9.2 calendar months). The week calculator on this site keeps one series consistent from positive test through delivery — the same units your charts use, minus the app-vs-provider disagreements.