Identify the trimester when each fetal development finding occurs. Fetus has a heartbeat: Brain develops most rapidly during this time: Gender can be determined: Fetus can hear and respond to external stimuli: Formation of fingers, toes, and facial features: Increase in body fat reserves: 1st Trimester (Conception-13 weeks) 2nd Trimester (14-26 weeks) 3rd Trimester (27-40 weeks)