From the moment of conception, rapid cellular division begins and initiates the start of a child's development in the womb. The developing embryo will go through many changes. From the development of organs and bones to eyes, ears, and tiny feet and hands, what once was an egg and sperm will become a child.
A sperm must locate the mature egg in the fallopian tube. Once the sperm fertilizes the egg, the egg, which is now a blastocyst, will continue to move down the fallopian tube into the uterus. On or about the seventh day, the blastocyst develops chorionic villi, hairlike structures used to absorb nutrients for the growth of the embryo, which enable it to implant into the lining of the uterine wall. According to Biology of Reproduction, the chorionic villi in the placenta function as necessary structures in exchanges between the fetus and mother.
The blastocyst will develop into two separate parts. The outer cells develop into the placenta. According to What to Expect, the placenta carries nutrients and oxygen to and waste materials away from the fetus. The inner cells of the blastocyst will form into the embryo. The amniotic sac will form an inner layer of membranes that will fill with amniotic fluids, which will protect the developing embryo for the remainder of the pregnancy.
The embryonic stage of development is characterized by the formation of the external body structures and internal organs. At this stage of development, the cell is made up of three layers: the endoderm, the mesoderm and the ectoderm. The endoderm is the structure that will form into the baby’s digestive system. According to National Institute of Health, the mesoderm is what will form the baby’s kidneys, muscles and bones. The outer layer, called the ectoderm, will form the baby’s eyes, skin, hair and nervous system. Approximately three weeks after fertilization, the embryo begins taking on a human shape. A line of cells form; eventually these cells will become become the spinal cord and brain. Major blood vessels form around day 17 and the heart begins to pump blood through the vessels at around day 20. The embryo will enter the fetal stage of development at week eight of pregnancy.
Around week eight, the embryo is now called a fetus. The lungs, brain, kidneys and liver are beginning to function and the fetus has distinct fingerprints. According to Discovery Health, the fetus will measure about three inches long and weigh approximately an ounce at 12 weeks gestation. During this stage of development, the external sex organs become visible on an ultrasound. Tiny hairs called lunugo begin to cover the baby’s entire body. At approximately 40 weeks gestation, the fetus is considered full-term and ready to make the journey through the birth canal.
- "Biology of Reproduction"; The Cytotrophoblast Layer of Human Chorionic Villi Becomes Thinner; Miki Mori et al.; October 2006
- National Institute of Health: Stem Cell Information: Glossary
- Discovery Health: How Pregnancy Works
- What to Expect: Week 4 of Pregnancy: Development of the Embryo and Placenta