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Baby Development By Trimester

Baby Development By Trimester

What makes each trimester unique when it comes to fetal development?


First trimester: "The first trimester is the most amazing and spectacular three-month period in human biology," say Dr. Doubilet. Starting from a single cell (the fertilized egg) all of the baby's internal organs will be in place by the end of the first trimester. "It is a time of incredibly rapid growth, with the baby increasing in length from a tenth of an inch at six weeks to three inches at 13 weeks (the end of the first trimester)."

Click here for a detailed look at your baby's first trimester, including 3-D ultrasound pictures.

Second trimester: During the second trimester, your baby begins to look like a newborn as she develops recognizable human features and her body becomes more proportionate. She may be able to make facial expressions, start to swallow and suck her thumb. "By looking closely at the baby's external features and internal organs, ultrasound allows the doctor or other caregiver to make sure that he/she is developing normally," says Dr. Benson.

Click here for a detailed look at your baby's second trimester, including 3-D ultrasound pictures.

Third trimester: The third trimester is when the baby develops into a being capable of surviving on her own as her lungs, brain and other internal organs mature. Her bones are fully developed and she is now able to open and close her eyes and can detect light. (She is considered full-term at 37 weeks). During this trimester, your baby will also gain a lot of weight, going from an average of 1½ pounds at 25 weeks to about 7½ pounds at birth.

Click here for a detailed look at your baby's third trimester, including 3-D ultrasound pictures.

Meet our experts:

Dr. Peter Doubilet and Dr. Carol B. Benson are Harvard Medical School professors of Radiology. They teach and see patients at the Brigham and Women's Hospital in Boston where Peter is Senior Vice Chair of Radiology and Carol is Director of Ultrasound and Co-Director of High Risk Obstetrical Ultrasound. They are co-authors of Your Developing Baby (McGraw-Hill, 2008), a pregnancy guidebook that takes readers through every stage of a baby's development, using 2D and 3D ultrasound images, original explanatory diagrams and reader-friendly prose.