Pregnancy and the development of a child is an amazing process. During the 40 weeks of pregnancy one single cell transforms into a human being. Throughout this transformation there are many identifiable stages. The early stages of pregnancy take place during the first few weeks following conception and when changes are rapidly taking place.
Fertilization
Fertilization is the joining of an egg, the woman's reproductive cell, and a sperm, the male reproductive cell. Though it sounds simple, the journey a sperm must take in order to find and penetrate an egg is not easy. Of approximately 300 million sperm that enter the vagina, only 3 million will make it all the way to the uterus, according to Visible Embryo. Those sperm must then find their way to the fallopian tube, where fertilization normally takes place. The process of fertilization takes approximately 24 hours.
Zygote
Once a sperm finds the egg, it must then penetrate the membrane surrounding the egg, known as the zona pellucida. After the membrane is pierced, the sperm enters the egg. The fusion of the nuclei (center of the cell that contains the genetic material) of the egg and sperm creates the stage known as a zygote.
Morula
The zygote begins to travel through the fallopian tube towards the uterus. On this journey the single celled zygote divides into two cells, known as blastomeres. Those two cells then divide into four cells and so on. When there are 16 cells present, this is known as the morula stage, according to Visible Embryo.
Blastocyst
Once in the uterus, the group of cells continues to divide into a hollow ball known as a blastocyst. At this stage, which is usually between 3 and 5 days after fertilization according to the Merck Manual, the blastocyst attaches to the wall of the uterus. This process, called implantation, is essential to the continued success of the pregnancy.
Embryo
A line of cells known as the primitive streak appears approximately 13 to 14 days after fertilization. At this point spontaneous division ceases and the blastocyst is officially known as an embryo. During the embryo stage the mass of cells begin to differentiate into organs and elongates taking on a human shape. The neural tube, the area that will become the brain and spinal cord, forms and the heart and blood vessels start to form during day 16 or 17 according to the Merck Manual. Around day 20 the heart begins to pump which can be seen on an ultrasound.
By 8 weeks following fertilization, which is the 10th week of pregnancy, most of the organs are formed. This embryo stage, which lasts until the 10th week of pregnancy at which time the embryo becomes known as a fetus, is the most vulnerable time as most birth defects occurring during these early stages of pregnancy.
