Fetal Development Timeline
It's confusing, but doctors typically measure the start of pregnancy as the first day of your last menstrual period. This is called the 'menstrual age' and is about two weeks ahead of when conception actually occurs, so throughout the timeline, fetal development lags 2 weeks behind your week of pregnancy.
4 Weeks
Cells are already specializing, with the outer layer of the fertilized egg becoming the nervous system, skin, and hair. The inner layer will be the breathing and digestive organs. The middle layer will be the skeleton, muscles, circulatory system, kidneys, and sex organs. Home pregnancy tests are often positive now.
Reviewed by Matthew Hoffman, MD on October 28, 2008
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