In the next two weeks, the embryo becomes much more recognizable, complete with fingers, eyelids, ears and even some important parts of the brain (including the cerebral cortex and pituitary gland). Even though the physical details are developing rapidly, the embryo is still only about 1.25 centimeters long.
In the second trimester of pregnancy, the basic systems and organs established during the first 12 weeks continue to develop and differentiate. By week 15, bones have begun to develop. Around 18 weeks, a fetus can begin to hear the maternal heartbeat and even sounds coming from the outside world. And around week 26, the fingernails are apparent.