BookmarkBookmarkTick

How Long Is Each Trimester?

When exactly does each trimester begin and end?
ByJayme Klock
Updated
February 28, 2017
Hero Image

If you’ve tried searching the web to find out when each trimester begins and ends, you’ve probably noticed that there are many answers to this question. Different practitioners may use different markers to determine the start and end dates of each trimester. Some just break it up into three chunks of 13 weeks, but most add on an extra week for a grand total of 40 weeks of pregnancy. Here’s the most common breakdown, along with official weeks from the American Congress of Obstetricians and Gynecologists:

First Trimester: 0 weeks–13 and 6/7 weeks (Months 1–3)
Starts with the first day of your last menstrual period (LMP) and ends with the last day of the 13th week thereafter. During your last period (which will be your last for a while), your uterus is preparing for ovulation and your body is gearing up for pregnancy, so technically, you can start counting from this point.

Second Trimester: 14 and 0/7 weeks–27 and 6/7 weeks (Months 4 –7)
Starts at the beginning of the 14th week after your LMP and lasts through the 27th week of pregnancy.

Third Trimester: 28 and 0/7 weeks– 40 and 6/7 weeks (Months 7–9)
Starts at the beginning of the 28th week after your LMP and ends with labor.

Still confused? Use our Due Date Calculator tool help pinpoint which trimester you’re in based on your estimated due date.

Related Video

Congenital Heart Defects

profile picture of Jennifer L.W. Fink
Jennifer L.W. Fink
Registered Nurse
backpack spilling open with supplies

5 Ways to Make Baby Smarter Before Birth

profile picture of Celia Shatzman
Celia Shatzman
Contributing Writer
pregnant woman looking at her sonogram

This Common Pregnancy Condition Could Up Baby’s Risk of Autism or ADHD

profile picture of Ashley Edwards Walker
Ashley Edwards Walker
Contributing Writer
Published
09/23/2019
sonogram machine at doctor's office

Mom-to-Be’s Hilarious ‘Demon’ Baby Sonogram Goes Viral

profile picture of Laurie Ulster
Laurie Ulster
Contributing Writer
Published
09/13/2019
smiling pregnant woman in urban setting

This Is How Mom’s and Baby’s Cells Communicate During Pregnancy

profile picture of Stephanie Grassullo
Stephanie Grassullo
Associate Editor
Published
08/22/2019
pregnant woman holding her sonogram

Study: Industrial Chemicals Pass From Mother to Fetus Throughout Pregnancy

profile picture of Stephanie Grassullo
Stephanie Grassullo
Associate Editor
Published
01/31/2019
pregnant woman in floral dress touching her belly

Why Your Baby Is Really Kicking in the Womb, According to New Research

profile picture of Stephanie Grassullo
Stephanie Grassullo
Associate Editor
Published
11/30/2018
fruits and veggies representing different weeks of pregnancy

How Big Is Baby: Fruit & Veggie Edition

profile picture of Darcy Strobel
Darcy Strobel
Senior Visuals Editor
pregnant woman holding sonogram

Research Shows Why Babies Need to Move in the Womb

profile picture of Anisa Arsenault
Anisa Arsenault
Associate Editor
Published
03/12/2018

This Is How Hard Your Baby Is Kicking, According to Science

profile picture of Anisa Arsenault
Anisa Arsenault
Associate Editor
Published
01/25/2018
Article removed.