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Baby's Development in the First Trimester

December 9th, 2014 | By: Dr. Peter Doubilet and Dr. Carol Benson
From single cell to human fetus: a detailed look at your baby’s first three months

In This Article

Baby's Development in the First Trimester

After first discovering they’re pregnant, most women can’t wait to find out more about the changes that are about to take place in their bodies. The first trimester is the most amazing three-month period in biology; what begins as a single cell (the fertilized egg) will become a tiny fetus with all of the baby’s internal organs in place and recognizable human features in just 13 short weeks. Here’s a look at your baby’s development in the first trimester:

Weeks 1 to 4

Your pregnancy countdown starts from the first day of your last menstrual period. About two weeks before you notice a missed period, an egg is released from your ovary and a sperm fertilizes it to create a zygote. This zygote divides repeatedly as it travels along the fallopian tube. By the time it reaches the uterus, it's a bundle of 12 to 15 cells.

While the uterus waits for its arrival, it transforms itself into a welcoming home, ready to accept and nurture the zygote when it appears. The uterus creates extra blood vessels and special glands in its inner lining to nourish the developing baby. Four to five days after fertilization, the zygote has implanted itself in the uterus, where cell division and growth continue throughout your pregnancy.