Picture of First Trimester (4 Weeks) At 4 weeks gestational age, a baby in the first trimester starts to grow a brain, spinal cord, and heart. Arm and leg buds begin to form. The baby is considered an embryo at this point and is 1/25 of an inch long. Image Source: Copyright © Medical RF / Phototake -- All rights reserved. Text Reference: American Pregnancy Association: "Fetal Development"