Pregnancy Week by Week: Baby's Development in the Womb
Your baby's development from a two-cell embryo to a full-term newborn is one of biology's most remarkable processes. All major organs form in the first trimester. The rest of pregnancy is about growth, maturation, and getting ready for life outside the womb.
Key Takeaways
- All major organs are formed by the end of week 12 — the fetal period (weeks 9–40) is primarily about growth and maturation
- The heart begins beating at around week 5–6 and can be detected by ultrasound
- Most mothers feel the first fetal movements (quickening) between weeks 16–22 (earlier with subsequent pregnancies)
- Week 22 is the viability threshold — with intensive NICU care, survival is possible; by week 28, the survival rate reaches approximately 90%
- Lungs are the last organ to mature — they are not ready for breathing air until approximately 34–36 weeks
First Trimester: Weeks 1–12 (The Embryonic Period)
Weeks 1–4: Conception and Implantation
- Fertilization occurs within 24 hours of ovulation
- Zygote divides rapidly as it travels to the uterus
- Week 3: Blastocyst implants in uterine wall
- Week 4: Positive pregnancy test possible — size of poppy seed (0.1cm)
Weeks 5–8: Major Organ Formation Begins
| Week | Development |
|---|---|
| 5 | Heart begins beating (ultrasound can detect ~6 weeks) |
| 6 | Brain, spinal cord forming; small arm/leg buds appear |
| 7 | Facial features taking shape; heartbeat ~100–160 bpm |
| 8 | All essential organs forming; embryo = raspberry (1.6cm) |
Weeks 9–12: Embryo to Fetus
- Week 9: Called a fetus from this point; fingers and toes forming
- Week 10: All organs present, fingers and toes separated
- Week 11: External genitalia forming (sex determined soon)
- Week 12: Reflexes develop; size of lime (5.4cm); risk of miscarriage drops significantly
Second Trimester: Weeks 13–27 (Rapid Growth)
| Week | Size | Key Development |
|---|---|---|
| 13 | Peach | Bones hardening, fingerprints forming |
| 16 | Avocado (11.6cm) | Can suck thumb, may hear sounds |
| 18–22 | Banana | Mother feels movement (quickening) |
| 20 | Banana (25.6cm) | Anatomy scan — major organs visible |
| 22 | Papaya | Viability threshold, survival possible with intensive care |
| 24 | Ear of corn (30cm) | Lung development milestone; brain growing rapidly |
| 27 | Cauliflower | Eyes can open and close; REM sleep cycles begin |
Third Trimester: Weeks 28–40 (Preparation for Birth)
| Week | Size | Key Development |
|---|---|---|
| 28 | Eggplant (37.6cm) | Brain development accelerating; 90% survival if born |
| 30 | Cabbage | Fat deposits increasing; immune system developing |
| 32 | Squash (42.4cm) | Practice breathing movements; ~95% survival if born |
| 34 | Butternut squash | Lungs approaching maturity |
| 36 | Romaine lettuce (47.4cm) | "Early term", most systems ready |
| 37 | Term: Baby is "full term" (37–40 weeks) | |
| 38–40 | Watermelon | Final fat and brain development |
| 40 | Average: 51.2cm, 3.4kg (7.5 lbs) | Due date |
Fetal Movement: When to Feel It
| Pregnancy | When to Feel Movement |
|---|---|
| First pregnancy | 18–22 weeks |
| Subsequent pregnancies | 16–18 weeks |
| Posterior placenta | Earlier |
| Anterior placenta | May feel later, normal |
From 28 weeks: Count kicks daily. Contact your midwife if you notice significantly reduced movement, don't wait until the next appointment.
Viability by Week
| Gestational Age | Survival Rate | Notes |
|---|---|---|
| Under 22 weeks | Not viable | |
| 22–23 weeks | ~10–30% | Intensive NICU care required |
| 24 weeks | ~50–60% | Significant disability risk |
| 28 weeks | ~90% | Better outcomes |
| 32 weeks | ~95%+ | Usually good outcomes |
| 37+ weeks | >99% | Full term |
🔗 Track your pregnancy week by week with our Pregnancy Tracker, updated with your baby's size and development each week.
Sources: NHS UK. Fetal Development; American College of Obstetricians and Gynecologists (ACOG). Fetal Development; WHO. Standard Gestational Age Definitions; Moore KL, "The Developing Human: Clinically Oriented Embryology"
Frequently Asked Questions
How big is my baby at each week of pregnancy?
Key size milestones: Week 8 = raspberry (1.6 cm), Week 12 = lime (5.4 cm), Week 16 = avocado (11.6 cm), Week 20 = banana (25.6 cm), Week 24 = ear of corn (30 cm), Week 28 = eggplant (37.6 cm), Week 32 = squash (42.4 cm), Week 36 = romaine lettuce (47.4 cm), Week 40 = watermelon (average 51.2 cm, 3.4 kg / 7.5 lbs).
When does the baby start moving in the womb?
Most first-time mothers feel fetal movement (quickening) between weeks 18 and 22 — often described as fluttering, bubbles, or light tapping. Those who have been pregnant before may feel movement as early as week 16. From 28 weeks, movement should be felt daily, and doctors recommend kick counting from this point.
When is the baby fully formed?
All major organs are formed by the end of the first trimester (12 weeks). The fetal period from week 9 onward is primarily about growth, weight gain, and organ maturation. The lungs are the last organ to fully mature — they are not ready for breathing until approximately 34–36 weeks.
What is the viability milestone in pregnancy?
Week 22 is the current threshold of viability — with intensive NICU care, survival is possible but rare (approximately 10–30%). Each additional week significantly improves outcomes: at 24 weeks survival is 50–60%, at 28 weeks approximately 90%, and at 32 weeks 95%+. A baby is considered full term from 37 weeks.
When is the anatomy scan done and what does it check?
The anatomy scan (mid-pregnancy ultrasound) is typically done between weeks 18 and 22. The sonographer checks the baby's brain, heart, spine, limbs, organs, placenta position, and amniotic fluid levels. It is the most comprehensive scan of the pregnancy and usually takes 30–45 minutes.
Sources
- ACOG — Clinical guidance on pregnancy and women's health
- WHO — Global recommendations for maternal health
- Mayo Clinic — Evidence-based pregnancy health information
🔧 Helpful Tools
- Due Date Calculator — Calculate your estimated due date
- Contraction Timer — Track your contractions during labor
- All Baby Tools — Browse all free tools for pregnancy and baby care
