Prenatal Development: Embryonic and Fetal Stages

Prenatal development is divided into two main periods: the embryonic period, which occurs during the first 2 months, and the fetal period, which follows. The embryonic period begins in the oviduct with the segmentation of the zygote. This involves successive mitotic divisions that produce 2 cells, called blastomeres, approximately 36 hours after fertilization. These cells then divide into 4, then 8, then 16, and so on, forming a morula.

About 5 days after fertilization, the morula transforms into a hollow, spherical structure called the blastocyst. The blastocyst consists of a cavity and two sets of cells: trophoblast cells (or the outer cell mass) and the embryoblast (the inner cell mass). After 7 days, the trophoblast attaches to the endometrium and contributes to the future placenta, while the embryoblast gives rise to the structures that form the embryo and a fluid-filled membranous sac called the amnion, which surrounds and protects the fetus during pregnancy.

Gastrulation and Tissue Layer Formation

In utero, the embryoblast undergoes gastrulation, a process that differentiates into three tissue layers: endoderm, mesoderm, and ectoderm. A cylindrical cartilaginous structure called the notochord also forms. Subsequently, differentiation occurs within each of these three layers, creating various tissues. These tissues then organize to form organs and systems.

Key Milestones in Fetal Development

• 1st Month: The heart begins to beat, and the head and limb buds start to appear.
• 2nd Month: Facial features develop, and limbs take shape. The embryo is now considered a fetus.
• 3rd Month: All major organs are formed, and the fetus begins to resemble a human.
• 4th Month: The circulatory system completes its formation.
• 5th Month: The mother begins to feel fetal movements, and nervous system maturation occurs.
• 6th Month: The fetus exhibits early reflexes, such as thumb sucking.
• Beginning of 7th Month: The fetus is considered viable. It reaches a height of approximately 36 cm and a weight of about 1200 grams.
• 7th Month: The baby opens its eyes, can hear, and moves more actively.
• End of 9th Month: Delivery occurs. The fetus typically has a height of around 50 cm and a weight.