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Baby's Development in the First Trimester

Weeks 11 to 13

By the end of the first trimester, the baby's body proportions have changed to become much more similar to those of a newborn. By 13 weeks, the baby is three inches long, measured from the top of the head to the bottom of the rump. (About the size of a small peach.) This represents a 30-fold increase in length in the seven-week interval from six to 13 weeks gestation.

13-week 3-D Ultrasound

Meet our experts:

Peter Doubilet, MD, PhD and Carol B. Benson, MD are professors of radiology at Harvard Medical School and obstetrical ultrasound specialists at the Brigham & Women's Hospital in Boston. You can learn much more about pregnancy through the "eyes" of ultrasound in Peter and Carol's new book Your Developing Baby (McGraw-Hill, 2008). It offers an illuminating "tour" of life in-utero using remarkable 2D and 3D ultrasound images, original explanatory diagrams and reader-friendly prose. For more information, visit www.YourDevelopingBaby.com.