Fetus at 3.5 weeks To use the sharing features on this page, please enable JavaScript. Overview 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. Review Date 8/23/2023 Updated by: LaQuita Martinez, MD, Department of Obstetrics and Gynecology, Emory Johns Creek Hospital, Alpharetta, GA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team. Learn how to cite this page