Important milestones in fetal development occur in the fifth, 12th and 24th weeks. The fifth week is the start of the embryonic period. In the 12th week, the fetus starts to move, and in the 24th week, the baby responds to external sounds, explains WebMD.
The embryonic period begins in the fifth week of pregnancy, explains Mayo Clinic. During this period, crucial organs and systems begin to form. The fetus's heart starts to develop along with an early version of the circulatory system. The brain and spine begin to develop as well, while specialized cells on the fetus's outer layer form the precursor to skin and connective tissue.
During the 12th week, the fetus begins to move on its own, but some women are not able to detect the fetus's movements until 20 weeks, notes WebMD. During this time, the heart is much more developed, and doctors can detect a heartbeat with specialized instruments. The fetus's facial features solidify and start to become recognizably human, with an obvious forehead, nose, mouth and chin, according to Mayo Clinic.
The fetus begins to respond to external movements during the 24th week of pregnancy, according to WebMD. The fetus may respond with voluntary movements, such as moving around, or involuntary procedures, such as demonstrating a quicker heart rate.