While the initial fertilized cells divide into the fetus and placenta, tiny arms form on the baby. The brain, spinal cord, lungs and heart begin to develop and, by the end of the first month, a heartbeat can be detected. Through the second month, eyelids and ear canals form as well as fingers and toes. Bones and genitals appear. By the third month, all major organs are in place. At just three months, a fetus should weigh about an ounce and measure nearly 3.5 inches. Appendages, internal organs and muscles continue to develop.