Stages of Development> Prenatal Developmental Period

• The prenatal period begins at conception (when the sperm fertilizes the egg) and continues until about 280 days, or 9 months.
• During this time, the human baby grows inside the mother's womb.
• Almost all body organs form and develop in this rapid phase before birth.

Prenatal development is divided into three main stages:

1. Germinal Stage (Weeks 1–2)

• Sperm and egg join, forming a single cell called a zygote.
• This cell multiplies quickly and travels to the uterus.​
• Around a week later, it implants itself in the uterine wall.

2. Embryonic Stage (Weeks 3–8)

• Specialized cells form three layers:
• Ectoderm (outer): skin and nervous system.
• Mesoderm (middle): heart, muscles, bones.
• Endoderm (inner): digestive system and lungs.​
• The embryo’s heart starts to beat, and organ systems begin to form.

3. Fetal Stage (Weeks 9–40) 

• The embryo is called a fetus.
• All major organs are present; now they grow bigger and stronger.
• Movement starts, hair grows, and features become clearer. The baby gets ready for birth.​

Human gestation (pregnancy period) lasts about 9 months. In contrast, cats or dogs are pregnant for about 2 months, and elephants for 22 months.

• The prenatal period is the fastest growth phase before birth.
• All organs begin forming in the mother’s womb.
• Germinal Stage (Weeks 1–2): Fertilization happens, and the cell divides and implants in the uterus.
• Embryonic Stage (Weeks 3–8): Major organs begin forming; the heart starts beating.
• Fetal Stage (Weeks 9–40): The baby grows, organs mature, and the body prepares for birth.