Fetal Development: Week 4
4 weeks after conception (28 days or 1 month)
(6 weeks after the last menstrual period)
The embryo is about 1/6 inch (0.16 inch) long or about the size of 4-5 poppy seeds. The embryo changes from a flat disc to a curved, C-shape as a result of the head and tail folds. Limb buds begin to appear on the side of the trunk. These will later develop into arms and legs. The neural tube forms, which will later become the spinal cord and brain. A primitive gut, trachea, liver, gallbladder and pancreas begin to form. The cardiovascular system begins to develop toward the end of the 3rd week. The heart is forming and begins to beat. Lung buds begin to appear.