First Trimester The first trimester is one of the busiest as all of the major organs and external structures are fashioned including, but not limited to the brain, the heart, the spine, the eyes, and the lungs. Involuntary movement begins at five to six weeks. A few weeks later, the four chambered heart is complete and beating like an adult's and her tiny brain is becoming highly developed. By just eight weeks, generally around the time a mother realizes she is pregnant, her baby has grown from a single cell to over one billion cells. The baby now has more than 90 percent of the body structures found in an adult.