Definitions
- Zygote: the cell that is formed when a female reproductive cell [egg] and a male reproductive cell [sperm] join.1
- Blastocyst: a ball of cells that forms early in a pregnancy, about five to six days after a sperm fertilizes an egg. It implants in the uterine wall, eventually becoming the embryo and then the fetus.2
- Embryo: “The collection of cells that has developed from the fertilized egg of a vertebrate animal, before all the major organs have developed. A collection of such cells of a human, especially from implantation in the uterine wall through the eighth week of development.” 3
- Fetus: “A fetus is a developing offspring after the embryonic stage prior to birth. In humans, the young is at nine weeks after fertilization. This fetal period is characterized by the presence of distinct organs, tissues, and systems (e.g., circulatory, nervous, and digestive systems). The fetal stage is characterized by rapid growth, refinement of functional abilities, and maturation of various tissues and structures.” 4
As a human being grows and develops in the womb, it passes through three stages of fetal development. Unless otherwise specified all information was taken from the Cleveland Clinic website.
The Germinal Stage
The first is the germinal stage. This runs from conception, when the mother’s egg is fertilised by the father’s sperm, and continues until the third week of pregnancy. Once the egg is fertilised it becomes a zygote and begins to divide. At the same time it moves down the Fallopian tube, divides into two sections one of which becomes the placenta and the other develops into the baby. After a week it has changed from a zygote into a blastocyst, has made its way out of the Fallopian tube and attaches to the wall of the uterus.
The Embryonic Stage
The second is the embryonic stage. During this stage, which starts around 3 weeks and lasts for between 5 and 6 weeks, up until the end of the 8th week of pregnancy, The developing baby begins to take on human characteristics. Organs and limbs begin to form. Most of the organs and systems are present in some form at the end of this period. The heartbeat starts during this time: “Cardiac tissue starts to pulse at around 5–6 weeks of pregnancy, registering as a heartbeat on the ultrasound, though the heart has not developed yet.” 5 Morning sickness may also begin.
The Fetal Stage
After 8 weeks the fetal stage begins and continues throughout the rest of pregnancy. The fetus’ sexual development begins at around 9 weeks and all of the major organs and systems continue to develop. Fingernails, eyelashes and hair begins to grow and the fetus can move his/her limbs. However the expectant mother may not be able to notice the movement of her baby until around 20 weeks or about nearly 5 months. As well most of the baby’s growth in-utero happens during this period.
References
1. Cambridge Dictionary
2. Cleveland Clinic
3. From The American Heritage® Dictionary of the English Language, 5th Edition. Wordnik site.
4. Biology Online
5. Medical News Today