Fetal development at any stage is very exciting, but I think there is something really special about fetal development weeks 2 â 4. This is where all human life begins and starts to take shape.
Fertilization occurs around week 2 (the way most doctors calculate it from the last menstrual period). Once the egg has become fertilized it becomes a zygote.
From that point the zygote begins to divide approximately every 20 hours. It is known as a morula once it is approximately 16 cells.
About four days later, the morula starts it's journey into the uterus. It continues splitting and starts to form two types of cells for the inner and outer areas. It then becomes a blastocyst.
As it attaches to the uterus, it starts to produce HCG (human chorionic gonadotropin) which is what produces a positive pregnancy test later.
HCG also tells the corpus luteum to keep producing progesterone. Many changes are occurring in the uterus at the point of attachment to start forming the placenta.
The inner cells continue to split into different types. There are now cells to become the yolk sac and also cells to become the embryo and amniotic sac.
There is a tiny stalk which attaches the developing embryo to the placenta and it will later become the umbilical cord. The primitive streak starts to become visible which will differentiate the three different layers of the forming embryo.
The top layer builds skin, hair, ears, mouth nose, and nervous system. The second layer forms bones, muscles, heart lungs and spleen.
The third layer creates the digestive tract, bladder, surface of the lungs, tongue and tonsils. Finally, at four weeks, the neural tube starts to form.
This is a crucial period of spinal cord formation where birth defects such as spina bifida can occur. It is critical to take folic acid to help prevent birth defects such as this â even before you know you are pregnant.
The most ironic thing about the period of fetal development weeks 2 â 4 is that these are the things that occur before a woman has even learned that she is pregnant.
So much has taken place and at week 4 when her period is missed will she learn of all the changes that have happened in her body.