Fetal development begins at conception and ends 37 to 41 weeks later with the birth of a full-term baby, according to MedlinePlus. This time period, called gestation, is measured in weeks. Gestational age is measured from the first day of the mother's last menstrual cycle.
A tiny ball of cells form an inner group and outer group of cells. The inner group becomes the embryo implanted in the fetus, while the outer group forms membranes to nourish and protect the baby. MedlinePlus explains this occurs three to four weeks into a pregnancy before differentiation starts.
During week five, the baby's outer features begin to develop and the gastrointestinal tract starts to form. Arms, legs, eyes and ears begin to grow six to seven weeks into the pregnancy. At eight weeks, hands, feet and lungs start to develop, according to MedlinePlus.
By week nine, toes are more distinct and arms grow longer. MedlinePlus reveals that by the end of the 14th week of pregnancy, nails appear on fingers and toes, the liver produces red blood cells and babies can make a fist. After the 21st week, an embryo can swallow, hear and move on its own. Four weeks later, the baby starts to store fat in its cells. Footprints and fingerprints form by week 26. Rhythmic breathing starts by the 34th week, and the embryo's bones are fully developed. By week 37, the baby weighs at least 5.5 pounds and is close to birth.