What is the course of prenatal development, and how do teratogens affect that development?

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Prenatal development refers to the series of stages through which a fertilized egg develops into a fetus before birth. It is a complex and delicate process that spans approximately nine months. During this period, the developing embryo/fetus undergoes significant changes in structure and function. However, it is also vulnerable to the influence of various external factors, such as teratogens, which can harm development.

The course of prenatal development can be divided into three main stages: the germinal stage, the embryonic stage, and the fetal stage.

1. Germinal Stage: The germinal stage is the initial phase of prenatal development and lasts approximately two weeks. It begins with fertilization when the sperm and egg fuse to form a single-celled zygote. The zygote then undergoes rapid cell division through mitosis as it moves down the fallopian tube toward the uterus. By the end of the germinal stage, the zygote develops into a blastocyst and eventually implants into the uterine lining.

2. Embryonic Stage: The embryonic stage occurs from approximately the third week to the eighth week after fertilization. During this stage, the blastocyst differentiates into three distinct layers: the ectoderm, mesoderm, and endoderm. These layers give rise to various organs, tissues, and body systems. Major organ systems, including the nervous system, heart, limbs, and reproductive organs, begin to form. The developing organism is now called an embryo. It is highly susceptible to teratogens, external agents that can disrupt normal development and cause birth defects.

3. Fetal Stage: The fetal stage begins around the ninth week and continues until birth. By this stage, the major organs and body systems have developed, and the embryo is now called a fetus. The fetus grows rapidly during this period and undergoes further refinement and maturation. Organs continue to develop and become functional, and the fetus exhibits movements that the mother can feel. The nervous system develops further, and the senses begin to function. However, the fetus remains vulnerable to the impact of teratogens.