By the end of 12 weeks of pregnancy (the end of the first trimester), most of the major organ systems are formed. This marks a crucial milestone in fetal development.

Key Highlights of Development by 12 Weeks:

• Organogenesis: Most major organs are structurally present, though not fully functional.
• Central Nervous System: The brain and spinal cord continue to develop and grow in complexity.
• Circulatory System: The heart is beating and actively pumping blood throughout the body.
• Digestive & Excretory Systems: Intestines begin settling into place; kidneys start producing urine.
• Limb Development: Arms, legs, fingers, and toes are formed; slight fetal movement may begin.
• Facial Features: Eyes, ears, and nose take shape; external genitalia begin to differentiate.

Importance of the First Trimester:

The first trimester is a critical period for development. All major organ systems are established, making the fetus particularly vulnerable to environmental influences such as drugs, infections, and radiation.