Fetal Development: First Trimester 

The first trimester is filled with awe-inspiring changes as your baby begins the incredible journey of growth and development. Your baby (a tiny embryo) grows quickly in these early weeks. 

One of the most reliable ways to track growth in this time of your pregnancy is by measuring from the top of the head to the bottom of the torso, called crown-rump length (CRL), using an ultrasound. Providers use this measurement to estimate how far along your pregnancy is and how big your baby is. 

The First Trimester: Week by Week

During the first 14 weeks of pregnancy, while your embryo is small, it goes through many important changes as it develops into a fetus. Here’s what’s happening week by week: 

Weeks 1–4: Early beginnings 

• After fertilization, the embryo implants in the uterus
• The early “layers” form that will later become every organ, bone and muscle in the body

Weeks 5–8: Organs start to form 

• The heart begins to beat and form chambers
• The brain and spinal cord grow as the neural tube closes
• Tiny arm and leg buds appear
• Eyes, ears, kidneys and the digestive system begin to take shape
• By week 8, fingers and toes are forming, and the face is becoming more recognizable

Weeks 9–14: Rapid growth as a fetus 

• The embryo is now called a fetus
• The face, mouth and nose become more defined
• Fingers and toes lengthen, and bones begin to harden
• Major organs like the heart, brain and digestive system continue developing and maturing 

These early milestones are the foundation of your baby’s development, bringing you closer to the amazing moment when you’ll finally meet face to face. Every week brings new changes — your baby is growing quickly, and there’s so much to look forward to in the weeks ahead!