3 wk Hr, 4 Brain: Start of your last period is used as part of the 40 weeks (so for the first two weeks, sperm and ovum haven't even been introduced). Then conception occurs. By the time you realize you missed a period, heart (3rd week) and brain/spinal cord will have formed (4th week post conception, or 6th week of pregn. Of 40 weeks). Amazing! http://www.Mayoclinic.Com/health/prenatal-care/pr00112.
Answered 4/18/2013
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