Fetal development is one of the most intricate, awe-inspiring processes in biology. From a microscopic cluster of cells to a fully-formed baby, the journey from conception to birth involves rapid and precise transformations. Each week brings new milestones — the heart beats, limbs bud, brain structures form, and organs begin to function.
This detailed guide explains fetal development week by week during the first trimester, using visual cues and developmental benchmarks. Whether you're a parent, biology student, or medical learner, this breakdown reveals how life begins — and evolves — inside the womb.
Week 3: The Heart and Central Nervous System Begin
At just 3 weeks of pregnancy, the embryo is no larger than a poppy seed, yet it marks the beginning of organogenesis — the development of organs.
- Heart: Begins forming as a simple tube and will soon start to beat.
- CNS (Central Nervous System): The neural tube forms, which later develops into the brain and spinal cord.
Fun Fact: Even before most women realize they’re pregnant, the groundwork for the brain and heart is already in motion.
Week 4: Neural Tube and Early Shape
In week 4, the embryo now appears like a curved tadpole.
- Neural Tube Closes: This will later become the brain and spinal cord.
- Somites Form: These are precursor structures that give rise to the spine, ribs, and skeletal muscles.
This week is critical — folic acid intake is vital here to prevent neural tube defects like spina bifida.
Week 5: Eyes and Limb Buds Appear
As the fifth week begins, the embryo enters the organogenesis peak.
- Eyes: Begin as dark spots on the side of the head.
- Limb Buds: Small buds appear, which will grow into arms and legs.
- Brain: Grows rapidly and begins to develop 3 primary sections — forebrain, midbrain, and hindbrain.
At this stage, the embryo is only about 1.5 mm long — but its development is exponentially fast.
Week 6: Teeth and Ears Begin Forming
By week 6, the embryo begins to resemble a tiny human.
- Teeth: Tooth buds start forming beneath the gums.
- Ears: Begin as small pits on the sides of the head.
- Heartbeat: Becomes more regular and detectable via transvaginal ultrasound.
Key Tip: Most early ultrasounds are done around 6 weeks to confirm pregnancy and fetal heartbeat.
Week 7: Genitalia and Palate Begin
During the 7th week, the focus is on facial and genital development.
- Genitalia: Start to differentiate, although sex is not visible yet via ultrasound.
- Palate: The roof of the mouth (hard and soft palate) begins to form.
- Brain: Continues rapid growth with neuron production in the millions per minute.
Week 8: Ears and Facial Features Sharpen
In week 8, the embryo is now officially referred to as a fetus.
- Ears: Move from the neck region to the sides of the head.
- Facial Features: Nose tip appears; eyelids begin covering the eyes.
- Muscles and Nerves: Begin functioning together, allowing the fetus to move slightly.
Interesting Note: Movements are not felt by the mother yet, but they can be observed on ultrasound.
Week 9: Brain Development Peaks
Week 9 marks an explosion in brain development.
- Cerebral hemispheres grow rapidly.
- Neural Connections: Start forming between muscles and nerves.
- Fingers and Toes: Begin to separate and form distinct digits.
This is a week where the fetus truly begins to look human — with visible facial expressions, limb movement, and structured organ systems.
Fetal Development Summary Table (Week 3 to Week 9)
| Week | Major Developments | System Involved |
|---|---|---|
| 3 | Heart tube forms, neural tube develops | Cardiovascular, Nervous |
| 4 | Neural tube closes, somites develop | Nervous, Musculoskeletal |
| 5 | Eye spots and limb buds appear | Sensory, Musculoskeletal |
| 6 | Ear pits, tooth buds, regular heartbeat | Sensory, Cardiovascular |
| 7 | Palate and genitalia begin forming | Digestive, Reproductive |
| 8 | Ears shift, facial features develop | Sensory, Facial |
| 9 | Brain growth surges, digits visible | Nervous, Musculoskeletal |
Real-Life Perspective: Why These Early Weeks Matter
While the fetus is still small and fragile, these early weeks lay down the blueprint for the entire human body. Miscarriages, birth defects, and developmental issues often stem from disruptions in this critical period — whether due to nutritional deficiencies, infections, or toxic exposures.
Prenatal vitamins, early screenings, and lifestyle choices can have a lifelong impact on the baby’s health. That’s why understanding fetal development isn't just academic — it’s life-saving.
Common Questions
When does the heart start beating in a fetus?
The fetal heart begins forming in week 3 and usually starts beating by week 5 or 6.
What is developing in the fetus at 5 weeks?
At 5 weeks, the eyes begin forming as dark spots, and limb buds appear. The brain and heart continue rapid development.
How developed is a fetus at 9 weeks?
By week 9, the fetus has distinct fingers, toes, a developing brain, and begins spontaneous movement. Facial features are more defined.
When do teeth begin to form in the fetus?
Tooth buds start forming under the gums during week 6 of fetal development.
What is the difference between embryo and fetus?
The developing baby is called an embryo from fertilization until the end of week 8. From week 9 onward, it is referred to as a fetus.
