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.

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Actual size of baby is 1/4"

Month 1 (Weeks 1–4)

The placenta (the organ that nourishes your baby) begins to form. The heart and lungs begin to develop. Your baby is about 1/4 inch long by the end of the first month.

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Actual size of baby is 1"

Month 2 (Weeks 5–8)

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.

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Actual size of baby is 4"

Month 3 (Weeks 9–12)

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 4 inches long.

Online Medical Reviewer: Dolan, Mary S, MD, MPH
Online Medical Reviewer: Morell, Patrick W, MD
Last Review Date: 12/22/2011
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