Pregnancy and Embryonic Development — Core Principles

Core Principles

Pregnancy is the period of gestation from conception to birth, typically lasting 40 weeks. It begins with fertilization, forming a zygote, which undergoes cleavage to become a morula, then a blastocyst.

The blastocyst implants into the uterine wall, marking the start of pregnancy. The inner cell mass of the blastocyst develops into the embryo, while the trophoblast contributes to the placenta. The embryonic period (weeks 3-8) is crucial for gastrulation, forming three germ layers (ectoderm, mesoderm, endoderm), and organogenesis, where major organs begin to form.

After week 8, the developing organism is called a fetus, and the fetal period (week 9 to birth) focuses on growth and maturation of organs. The placenta is a vital temporary organ for nutrient/gas exchange and hormone production (hCG, progesterone, estrogen).

Amniotic fluid protects the fetus. Hormones regulate the entire process, maintaining pregnancy and preparing the mother's body for childbirth and lactation.

Important Differences

vs Embryo vs. Fetus

Aspect	This Topic	Embryo vs. Fetus
Time Period	Embryo (Weeks 3-8 post-fertilization)	Fetus (Weeks 9-40 post-fertilization)
Key Developmental Process	Organogenesis (formation of major organs from germ layers)	Growth and maturation of existing organs, functional development
Vulnerability to Teratogens	Highest vulnerability due to rapid cell differentiation and organ formation	Reduced vulnerability, but still susceptible to growth retardation or functional defects
Appearance	Rapidly changing, developing distinct human features	Increasingly resembles a miniature human, growing in size and weight
Size at end of stage	Approximately 3 cm long	Approximately 50 cm long (at term)

The distinction between an embryo and a fetus is a critical concept in developmental biology, marking different phases of prenatal development. The embryonic stage, spanning from week 3 to week 8 post-fertilization, is characterized by intense organogenesis, where all major body systems begin to form from the three germ layers. This period is highly sensitive to external factors. Following this, from week 9 until birth, the developing human is termed a fetus. The fetal stage is primarily dedicated to the growth, maturation, and functional refinement of the organs already established during the embryonic period. Understanding this temporal and developmental difference is essential for NEET aspirants.