The Mayo Clinic reports that the fetus develops a heart approximately 21 days following conception which begins to pump blood approximately 28 days after conception. The third and fourth weeks following conception. During this time, various facial features develop and the body begins to curve.
The American Pregnancy Association explains that during the fourth week following conception, the lungs, jawbone and nasal cavity begin to develop. During this period, the hands and feet develop small buds that eventually become fingers and toes. During the fifth week following fertilization, every bodily organ has developed. The fetal size is 1/2 inch in length and can be compared to the size of an aspirin tablet.