Fetal development all starts with the development of organized gastrula cells, which is called differentiation. Differentiation goes on for thirty eight weeks that are separated into three trimesters. In each trimester essential development takes place. Complications can occur during development, leading to birth defects and miscarriages.
In the first trimester, after four weeks the brain and spinal cord start to form. Eight weeks into the first trimester, fingers and toes become visible along with the development of the kidneys, ears, lungs, liver, and muscles. Sexual distinction is almost complete by twelve weeks.
Sixteen to eighteen weeks into the second trimester, the mother will be able to feel the fetus move. At twenty four weeks the fetus will be able to survive prematurely outside of the mother with the help of specialized care.The fetus' eyelids also open at this stage.
In the third and final trimester, the fetus continues to grow and accumulates a lot of fat causing a significant weight gain. This last trimester also gives the lungs and other important parts of the fetus to finish up their development.
During fetal development, birth defects can occur causing the organ systems to have problems developing, or growth restrictions. Birth defects are caused by teratogens which are substances or factors in the mother that are harmful such as diseases, infections, or chemicals. Most commonly, birth defects happen when the major organ systems are developing. Common birth defects include: spina bifida,brain abnormalities, heart defects, clubfoot, Down syndrome, and fetal alcohol syndrome.
Scientists use comparative embryology to see what similarities and differences there are between different species during fetal development. Through this research, we can see that at the first stage of development all embryos look very similar and that throughout development, more differences start to appear between the compared species. In the last stage of embryonic development you are able to clearly tell what species has been developing. When comparing the embryos, you can see that some have more similarities than others. For example, a human embryo doesn't look like a fish as much as an embryo of an ape would. When embryos look quite similar even in later stages compared to others, it means they are more closely related.
