After a female egg is fertilized, the resulting one-celled organism becomes known as a zygote. Once the egg is fertilized, the zygote begins a two-week period of rapid cell division and will eventually become an embryo. The zygote divides through a process known as mitosis, in which each cell doubles by dividing into two cells. This two-week stage is known as the germinal period of development and covers the time of conception to the implantation of the embryo in the uterus.
In most cases, each male and female sex cell contain 23 chromosomes. When these two haploid cells join, they form a single diploid cell that contains a total of 46 chromosomes. The zygote begins a journey down the fallopian tube to the uterus where it must implant in the lining in order to obtain the nourishment it needs to grow and survive.
The period of the zygote lasts for about four days. Around the fifth day, the mass of cells becomes known as a blastocyst. The germinal period will last for fourteen days, after which the embryonic period will begin. The second period of development lasts from two weeks after conception through the eighth week, during which time the organism is known as an embryo. At the ninth week post-conception, the fetal period begins. From this point until birth, the organism will be known as a fetus.
Researchers estimate that nearly 60 percent of all naturally occurring conceptions fail because the zygote never becomes properly implanted in the uterus.
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