Pregnancy describes the period of time from conception to birth as an egg develops into a baby. Because no one knows exactly when an egg becomes fertilized, doctors calculate the length of pregnancy as 40 weeks from the first day of the last menstrual cycle. This means that during the first two weeks of pregnancy, the mother is not actually pregnant yet. The eight main stages of pregnancy encompass the time from fertilization through birth, which normally occurs between 37 and 42 weeks gestation.
Fertilization
Fertilization describes the stage in which the female reproductive cell, known as the egg, and the male reproductive cell, known as the sperm, comes together. Fertilization begins as soon as the sperm penetrates the egg and the process lasts for approximately 24 hours, according to The Visible Embryo.
Zygote
Once the sperm penetrates the zona pellucida--the tough outer membrane surrounding the egg, it fuses with the nucleus of the egg, forming a zygote. As the zygote moves through the fallopian tube toward the uterus, it divides from one cell into two cells, then from two cells info four cells, and so on.
Morula
Approximately four days following fertilization the zygote contains 16 cells, referred to a morula for its shape, and enters the uterus.
Blastocyst
As cell division continues, a blastocele, or cavity, forms in the center of the morula, marking the beginning of the blastocyst stage. The cells inside the cavity flatten creating two types of cells; the cells inside the blastocele known as embryoblasts and the cells outside the blastocele known as trophoblasts.
Implantation
Approximately five to eight days following fertilization, according to the The Merck Manuals Online Medical Library, the blastocyst attaches to the uterine wall. During this stage, known as implantation, the trophoblasts secrete the hormone human chorionic gonadotropin--hCG--to stimulate progesterone production. The increase in progesterone signals the uterus to maintain the blood-rich lining and circulation between the mother and the blastocyst begins.
Placenta Formation
The outer cells, or trophoblasts, continue to burrow into the uterine lining beginning the formation of the placenta--a pancake shaped organ that connects the fetus to the mother. The placenta contains small blood vessels that carry the baby's blood and fills with the mother's blood, allowing the exchange of nutrients, oxygen and waste products.
Embryo
The cells in the blastocyst begin to differentiate, marking the beginning of the embryo. During the embryo stage the group of cells elongates into a human-like shape and the internal organs begin to develop. Eight weeks following fertilization, or by the tenth week of pregnancy, most organs with the exception of the brain and spinal cord are fully formed transforming the embryo into a fetus.
Fetus
During the remaining weeks of pregnancy the fetus continues to develop. By week 12, the fetus fills the uterus making it extend beyond the pubic bone, according to the Merck Manual. Between weeks 16 and 20 the mother can feel the fetus moving. The brain continues to accumulate new cells and the lungs continue to mature until the time of delivery when the fetus becomes a newborn baby.
