How the pregnancy week calculator works
How obstetric counting works
Gestational age counts from the LMP in whole weeks plus days — "23+4" means 23 weeks and 4 days. Weeks are 7-day blocks; months are the messy conversion everyone argues about (lunar months of 28 days ≠ calendar months). The calculator speaks the clinic's language: weeks + days, with the trimester boundary in your chosen convention.
The two trimester conventions
The "classic" split (T2 from week 13, T3 from 27) and the "clinical" split (14 and 28) both appear in books and apps — neither is wrong; they slice the same 40 weeks differently. What matters is consistency with your provider's usage. The calculator supports both and labels which it used.
Milestone anchors worth knowing
- ~6–7 weeks: first ultrasound typically sees a heartbeat
- 11–14 weeks: dating/Nuchal scan window — the scan that refines the due date
- 18–22 weeks: anatomy scan
- 24 weeks: a major viability threshold in clinical terms
- 28 weeks: third-trimester schedule begins (growth scans, GD testing)
- 37–42 weeks: term window
Counting from the due date backwards
Given a due date, everything runs in reverse: LMP = due − 280 days, conception ≈ due − 266. The calculator shows today's position and the milestones ahead — a legible map between appointments.