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Pregnancy: Your First Trimester Changes

The first trimester is a time of rapid development for your baby. Because your baby is growing so quickly, it is important that you start a healthy lifestyle right away. By the end of the first trimester, your baby has formed all of its major body organs and weighs just over an ounce.

Month 1 (weeks 1 to 4)

The placenta (the organ that nourishes your baby) begins to form. The brain, spinal cord, heart, gastrointestinal tract, and lungs begin to develop. Your baby is about ¼ inch long by the end of the first month.

Side view of female body showing reproductive system and inset of 1 month embryo.
Actual size of baby is 1/4".

Month 2 (weeks 5 to 8)

All of your baby’s major body organs form. The face, fingers, toes, ears, and eyes appear. By the end of the month, your baby is about 1 inch long.

Side view of female body showing reproductive system and inset of 2 month embryo.
Actual size of baby is 1".

Month 3 (weeks 9 to 12)

Your baby can open and close its fists and mouth. The sexual organs begin to form. As the first trimester ends, your baby is about 3 inches long.

Side view of female body showing reproductive system and inset of 3 month embryo.
Actual size of baby is 3".

Online Medical Reviewer: Daniel N Sacks MD
Online Medical Reviewer: Donna Freeborn PhD CNM FNP
Online Medical Reviewer: Heather Trevino
Date Last Reviewed: 8/1/2020
© 2000-2021 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.