The first trimester of pregnancy ends at the end of the 13th week of pregnancy (often described as 13 weeks and 6 days).

Quick Scoop: When does the first trimester end?

Most medical sources define the first trimester as lasting from the first day of your last menstrual period up to the end of week 12 or 13, depending slightly on who you ask.

  • Many hospitals and organizations (like KidsHealth and Tommy’s) say it lasts until the end of week 12.
  • Others, following common obstetric practice (including ACOG as cited by pregnancy resources), describe it as lasting through the end of week 13 (13 weeks + 6 days).

In day‑to‑day use, most doctors consider you in the second trimester once you hit 14 weeks pregnant.

Why the answers differ a bit

People (and apps, and websites) count slightly differently:

  1. By “weeks completed”
    • Some say, “First trimester is weeks 1–12,” meaning once you finish 12 weeks, you are in your second trimester.
  1. By “up to 13 weeks, 6 days”
    • Other definitions say the first trimester runs from week 1 to 13 weeks, 6 days , and the second trimester starts at 14 weeks, 0 days.
  1. Real‑life example
    • If you’re 12+3 (12 weeks, 3 days), you are still clearly in the first trimester in every system.
 * If you’re **13+4** , some older leaflets might say you’re already in second trimester, but most modern medical definitions still count this as first trimester until you reach 14+0.

What most doctors go by

When you look at recent medical references:

  • A full‑term pregnancy is about 40 weeks , counted from the first day of your last period.
  • Each trimester is roughly 13 weeks.
  • So practically, many clinicians talk about:
    • First trimester: 0–13 weeks
    • Second trimester: 14–26 weeks
    • Third trimester: 27 weeks to birth

If you’re filling out a form, planning travel, or worrying about risk (like miscarriage risk), most guidance and research talk about that risk dropping after 12 completed weeks , even though the trimester label might not switch until week 14.

Mini FAQ

  1. So what should I personally use?
    • Safest, most widely accepted:
      • “My first trimester ends at 13 weeks, 6 days and I’m in my second trimester from 14 weeks.”
  1. Why do some apps say ‘second trimester’ at 12 weeks?
    • They’re using a simple “3 calendar months” or “12‑week blocks” model rather than the week‑by‑week obstetric standard.
  1. When should I say I’m “in my second trimester” to my doctor?
    • Your doctor or midwife will usually consider you entering second trimester at your 14‑week mark.

One‑line TL;DR

In modern medical practice, the first trimester ends at the end of week 13 (13 weeks + 6 days) , and the second trimester starts at 14 weeks.

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