It was considered a magical incident in the past. But it is completely biological and systematic. Here’s a descriptive procedure of development of a fetus since fertilization.
Fertilization usually takes place in the ampullary region of the fallopian tube. After the fertilization of an egg with a sperm it becomes a zygote.
A zygote divides through mitosis and becomes a morula which is the 16 cell stage. After 3 days the morula becomes the blastocyst and implants in the uterus endometrium. Then the mitosis takes place and the blastocyst divides into an inner cell mass(embryoblast) and an outer cell mass (trophoblast).
The outer cell mass further divides into the cytotrophoblast and syncytiotrophoblast. Maternal blood vessels penetrate the cytotrophoblast to enter the intervillous space. The syncytiotrophoblast goes through the lacunae stage and so the maternal blood enters the lacunar space. Both these structures get together to form the placenta.
The inner cell mass divides into the hypoblast layer and epiblast layer. Hypoblast layer becomes endoderm. All the structures of gastrointestinal system, epithelial lining of respiratory tract, parenchyma of thyroid, parathyroid, liver, gallbladder, pancreas, tonsils and thymus, epithelial lining of urinary bladder and urethra, epithelial lining of tympanic cavity are developed by the endoderm.
The epiblast layer becomes ectoderm. All the structures of the central nervous system, peripheral nervous system, sensory epithelium of ears, nose,eyes, skin epidermis, mammary glands, pituitary gland, enamel of teeth come up by the ectoderm.
Some cells between epiblast and newly formed endoderm form the mesoderm. The mesoderm divides into 3 parts; paraxial mesoderm, intermediate mesoderm, and lateral mesoderm. Structures such as bones, cartilages, muscle, skin and subcutaneous tissue of the skin, come up by the paraxial mesoderm. Structures such as kidneys, gonads and their ducts, spleen and cortex of suprarenal gland come up by the intermediate mesoderm. The lining of body cavities come up by the lateral mesoderm.
The fetal development is measured by trimesters. After the 3rd trimester a fetus is ready to deliver. And this is how you and me is created.