Definition

• "Embryo" is the term for an organism in the beginning stages of development. For the human embryo, this period is defined as the time from fertilization to the third month. According to the Mayo Clinic, the embryonic period starts around the fifth week of gestation and ends in week 13, at which point the fetal stage of development begins.
  
  

Fetal Age vs. Gestational Age

• There are two age measurements during this early stage of development: fetal age and gestational age. According to Parentsconnect.com, fetal age is the age of the baby counted from the moment of conception. Conception, however, does not occur until approximately two weeks after the beginning of the last menstrual cycle. Gestational age counts from the last menstrual cycle, and is therefore usually about two weeks more than the fetal age.
  
  

Gestational Age: Weeks 3-6

• According to Babygurgles, during this stage, the embryo divides into many different cells, and by the end of the second week, the embryo is made of 150 cells. According to the Mayo Clinic, these cells are divided into three layers. The top layer, called the ectoderm, will become the outside layer of the skin. The middle layer, the mesoderm, will become the baby's heart and circulatory system, and will later serve as the basis for the development of bones, muscles, reproductive organs and kidneys. The bottom inner layer, called the endoderm, is the place that the baby's lungs, intestines and bladder will develop.
  
  

Gestational Age: Weeks 7-10

• At the beginning of this stage, the average embryo measures 1/4-inch long, according to the American Pregnancy Association. During this stage the embryo will quadruple in size, and at the end of the eighth week, the average length of a human embryo is 1 inch. At this point, beginning in the sixth week after fertilization, the baby's heart is beating and moving blood throughout the organism.
  The brain is developing very quickly--around 250,000 new cells per minute, according to Babygurgles--and facial features are beginning to emerge. Hair follicles, eyelids and tongue have begun to develop, and genital organs begin to develop, although the sex of the baby is still unidentifiable. The baby also begins to move.
  
  

Gestational Age: Weeks 11-13

• The embryo begins to kick its feet, open its mouth, and swallow amniotic fluid, which is the fluid present in the uterus that makes up the embryo's surroundings. At the end of this period, the average embryo will be around 3 inches in length, according to the American Pregnancy Association. The head takes up most of the embryo, measuring around half of its total size. According to the Mayo Clinic, fingernails begin to develop, and a human profile becomes more prominent. From week 13 of gestation (fetal week 11), the baby will be medically described as a "fetus," and the embryonic stage of development comes to a close.