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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.

Image of fetus in utero month 1 (weeks 1-4). 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.

Image of fetus in utero month 2 (weeks 5 - 8). 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.

Image of fetus in utero month 2 (weeks 9 - 12). 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.


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