At 3.5 weeks, the fetus will have formed the heart, begins development of the brain and spinal cord, and starts forming the gastrointestinal tract.
Update Date 9/26/2015
Updated by: Daniel N. Sacks MD, FACOG, obstetrics & gynecology in private practice, West Palm Beach, FL. Review Provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.