Fetal Development: Your Baby's Growth Week by Week

From a tiny cluster of cells to a fully formed baby ready to meet the world—fetal development is one of nature's most remarkable journeys. Understanding what's happening inside the womb week by week helps you connect with your growing baby and appreciate the incredible transformations taking place.

Every pregnancy is unique, but the general pattern of development follows a predictable sequence. Here's what's happening as your baby grows. ACOG

First Trimester (Weeks 1-12)

The first trimester is a period of rapid development. By the end of week 12, all major organs have begun forming. Mayo

Weeks 1-4: Conception and Implantation

• Fertilization occurs when sperm meets egg
• The fertilized egg (zygote) divides rapidly
• Implantation in the uterine wall around day 6-10
• The placenta begins forming
• Neural tube (future brain and spinal cord) starts developing

Weeks 5-8: Embryonic Period

• Heart begins beating (around week 5-6)
• Brain develops rapidly
• Arm and leg buds appear
• Facial features begin forming
• Major organs start developing
• By week 8, called a "fetus" instead of "embryo"
• About 1/2 inch long

Weeks 9-12: Systems Develop

• Fingers and toes become distinct
• Bones begin hardening
• External genitalia forming
• Fetus can make small movements
• Facial features more defined
• By week 12: about 2 inches long, weighs about 1/2 ounce

Second Trimester (Weeks 13-27)

Often called the "honeymoon trimester"—you may feel better and your baby is growing rapidly. ACOG

Weeks 13-16: Movement Begins

• Skeleton continues to develop
• Baby can make sucking motions
• Fingerprints forming
• May begin feeling movements ("quickening")
• Can hear your heartbeat
• About 4-5 inches long

Weeks 17-20: Halfway Point

• Vernix (protective coating) covers skin
• Hair (lanugo) grows on body
• Baby is more active
• You'll likely feel definite movements
• Anatomy ultrasound typically done
• May learn sex of baby
• About 6-7 inches long

Weeks 21-24: Senses Develop

• Hearing more developed—responds to sounds
• Eyes can open
• Taste buds forming
• Fingerprints and footprints complete
• Sleep-wake cycles emerge
• About 10-12 inches long

Weeks 25-27: Rapid Growth

• Lungs developing but not yet mature
• Brain growing rapidly
• Baby responds to light and sound
• More defined sleep patterns
• About 13-14 inches long, weighs about 2 pounds

Third Trimester (Weeks 28-40)

Your baby is gaining weight, practicing breathing, and preparing for life outside the womb. Mayo

Weeks 28-31: Gaining Weight

• Fat deposits forming under skin
• Brain development accelerates
• Bones fully developed (but still soft)
• Can see light through the womb
• Practice breathing movements
• About 15-16 inches long, weighs 2.5-3.5 pounds

Weeks 32-35: Nearly Ready

• Lungs nearly mature
• Baby "drops" lower in pelvis
• Less room to move (movements feel different)
• Fingernails reach fingertips
• May sleep 90-95% of the day
• About 17-18 inches long, weighs 4-5 pounds

Weeks 36-40: Full Term

• Lungs fully mature
• Baby gains about 1/2 pound per week
• Head may engage in pelvis
• Vernix and lanugo shed
• Ready for birth
• Full term: about 19-21 inches, 6-9+ pounds

What Your Baby Can Do in the Womb

It's amazing what babies can do before they're born: ACOG

Hear:

• Recognize your voice by third trimester
• Respond to music
• Startled by loud noises

Taste:

• Swallow amniotic fluid
• May prefer flavors you eat often

See:

• Detect light through the womb
• Eyes open and close

Move:

• Kick, stretch, roll
• Hiccup
• Suck thumb
• Grasp umbilical cord

Dream:

• REM sleep observed
• May be dreaming or processing sensory information

Milestones to Watch For

Heartbeat detected: Around 6 weeks (via ultrasound)

First movements felt: 16-25 weeks (earlier in subsequent pregnancies)

Viability: Around 24 weeks (with intensive medical care)

Full term: 39-40 weeks (early term 37-38 weeks)

Supporting Healthy Development

You can support your baby's development by: ACOG

• Taking prenatal vitamins (especially folic acid)
• Eating a nutritious diet
• Staying hydrated
• Getting regular prenatal care
• Avoiding alcohol, tobacco, and drugs
• Managing stress
• Getting adequate sleep
• Staying active (with provider approval)

What Other Parents Ask

Q: Can my baby hear me talking?
A: Yes! By the third trimester, your baby can hear and may recognize your voice. Talking, singing, and reading to your belly is a wonderful way to bond. Mayo

Q: Are kicks and movements a good sign?
A: Yes. Fetal movement is a sign of health. You'll start counting kicks in the third trimester—generally 10 movements in 2 hours is reassuring. Report any significant decrease to your provider.

Q: Why does my baby seem most active when I'm trying to sleep?
A: When you're moving during the day, the motion rocks baby to sleep. When you stop and rest, baby often wakes up. This pattern may continue after birth!

Q: Does stress affect my baby?
A: Chronic, severe stress may impact fetal development. Normal everyday stress is generally not harmful. Managing stress through relaxation techniques is helpful for both you and baby. ACOG

The Bottom Line

Fetal development is an incredible journey from a single cell to a fully formed baby. Each week brings new developments as your baby grows, develops organs and senses, and prepares for life outside the womb. Understanding this process helps you appreciate the amazing changes happening and connect with your baby before birth.

Key milestones:

• Week 6: Heart begins beating
• Week 12: All major organs forming
• Week 16-20: First movements felt
• Week 24: Viability milestone
• Week 28: Eyes open, brain rapidly developing
• Week 37+: Full term, ready for birth

Clara is here to help you understand your baby's development throughout pregnancy.