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Implantation bleeding occurs between 7 and 14 days after fertilization, [57] and is a small amount of light vaginal bleeding or spotting that can occur in early pregnancy due to the blastocyst penetrating the lining of the uterus during implantation.
The blastocyst stage occurs between 5 and 9 days after conception. During embryonic development, after fertilization (approximately 5–6 days in the human), the cells of the morula begin to undergo cell differentiation, and the morula changes into the blastocyst by pumping fluid to grow a lumen.
In many organisms including Xenopus and Drosophila, the midblastula transition usually occurs after a particular number of cell divisions for a given species, and is defined by the ending of the synchronous cell division cycles of the early blastula development, and the lengthening of the cell cycles by the addition of the G1 and G2 phases.
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After four divisions, the conceptus consists of 16 blastomeres, and it is known as the morula. [8] Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the blastocyst by the fifth day of development, just as it approaches the site of implantation. [9]
Human embryology is the study of this development during the first eight weeks after fertilization. The normal period of gestation (pregnancy) is about nine months or 36 weeks. The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus.
An embryo is the initial stage of development for a multicellular organism.In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm cell.
During these cellular divisions, the zygote remains the same size, but the number of cells increase. The morula enters the uterus after three or four days—during which a cavity, called the blastocoel, is formed to produce the blastocyst. [4] Once the blastocyst is formed, it undergoes implantation into the endometrium. [4]