The 3rd trimester of pregnancy typically starts at the beginning of week 28 and lasts until birth, usually around weeks 39–40.

When does the 3rd trimester start?

Most major medical sources define the third trimester as:

  • Starting: At 28 weeks of pregnancy (the start of your 28th week).
  • Ending: When you give birth, often around 39–40 weeks.
  • Rough duration: About 13 weeks long.

Some organizations or countries may consider week 27 as the beginning of the third trimester, and forums often debate 27 vs 28 weeks, but medical sites and clinics most commonly use 28 weeks as the official start.

Why there’s confusion (27, 28, or 29 weeks?)

People split 40 weeks into three “equal” parts and get about 13.3 weeks per trimester, which lands the third trimester just before 27 weeks mathematically.

However, professional bodies and hospitals usually prioritize fetal development and clinical milestones over pure math, so they standardize on week 28 as the start.

On pregnancy forums, you’ll see all of these views:

  • “It’s 27 weeks” (based on dividing 40 by 3).
  • “It’s 28 weeks” (matching most medical references and calculators).
  • “Some even say 29 weeks” (more informal or app-based definitions).

A popular Reddit pregnancy thread concludes that while there’s gray area, many users and at least one medically knowledgeable commenter “mostly operate on 28 weeks” and note there’s no magical change between 27w6d and 28w0d.

Quick practical takeaway

If you’re wondering what to tell your provider, note on a form, or use for planning:

  • Using 28 weeks as the start of the 3rd trimester will line up with most doctors, clinics, and major pregnancy sites.

Information gathered from public forums or data available on the internet and portrayed here.