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Preimplantation period - Week 1:
At the beginning of the first week, conception takes place where a woman's ovum is penetrated and united with a man's sperm during fertilization.
- This union forms a fertilized egg, known as the zygote. During fertilization, the final stages of meiosis occur in the ovum, resulting in the joining of chromosomes from the ovum and sperm. After fertilization, the zygote undergoes mitosis and splits into more cells due to cleavage, which creates a blastocyst. -
Embryonic period - Week 2:
The blastocyst grows due to proliferation and differentiation of embryonic cells, which in turn forms embryonic layers within the blastocyst. A bilaminar embryonic disc is formed. -
Embryonic period - Week 2:
During week two, the bilaminar embryonic disc is developed from the blastocyst, creating a superior epiblast layer, and inferior hypoblast layer. -
Embryonic period - Week 3:
The primitive streak is formed within the bilaminar embryonic disc. This results from an increased proliferation of cells in the midline area. The primitive streak causes the disc to have bilateral symmetry. -
Embryonic period - Week 3:
Some cells from the epiblast layer migrate toward the hypoblast layer only in the area of the primitive streak. These cells locate in the middle between the epiblast and hypoblast layers, and become the mesoderm and endoderm. -
Embryonic period - Week 3:
With three layers present, the bilaminar embryonic disc has thickened into trilaminar embryonic disc. With the creation of the new embryonic cell layers of the mesoderm and endoderm, the epiblast layer is now considered the ectoderm. -
Embryonic period - Week 4:
The trilaminar embryonic disc undergoes embryonic folding. The face and neck begin to develop, with the primitive eyes, ears, nose, oral cavity, and jaw areas. -
Embryonic period - Weeks 5-8:
The brain, spinal cord, heart, and other organs begin to form. Facial features begin to become more defined. -
Fetal period - Weeks 9-12:
Development of external genitalia and sexual differentiation marks the period of weeks 9-12. -
Fetal period - Weeks 20-30:
Maturation of organ systems and rapid brain development -
Fetal period - Weeks 35-40:
In this final month, you could go into labor at any time. At this point, the fetus's position may have changed to prepare for birth. The fetus also reaches its full term size.