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A Peek Inside: 5 Amazing Fetal Development Photos

The First Few Days

The First Few Days

Millions of sperm cells embark on a 10-hour race vying to be the first to enter the egg — the moment when conception occurs. The fertilized egg travels down the fallopian tube and implants in the lining of the uterus by the end of the first week following fertilization. At this time, the baby, which is just a cluster of rapidly-growing cells, is smaller than the tip of a pen.


(Pictured: A 6-day-old human embryo implanting in the lining of the uterus.)

Attribution: Yorgos Nikas/Wellcome Images
By Katherine Solem. Published on July 12, 2010. Updated on November 23, 2015. Katherine Solem is a San Francisco–based health writer and editor.
Reviewed by Elisabeth Aron, MD, FACOG on October 5, 2015.