Pregnancy Week by Week: Complete Trimester Guide and Fetal Development

Every week of pregnancy brings new developments. From a cluster of cells smaller than a poppy seed to a fully-formed baby ready for the world, the transformation is nothing short of miraculous.

Quick orientation: Pregnancy lasts 40 weeks and is divided into three trimesters. The first trimester (weeks 1-13) is when all major organs form. The second trimester (weeks 14-27) is when baby grows rapidly and you start feeling movement. The third trimester (weeks 28-40) is when baby gains weight and prepares for birth.

During my own pregnancy, I obsessively checked what was happening each week -- I first felt flutters at week 19, had my anatomy scan at week 20 (everything looked perfect), and started kick counts religiously at week 28 after my OB recommended it. My daughter arrived at 40 weeks and 3 days, and looking back, the week-by-week tracking helped me feel grounded when symptoms came and went unexpectedly. That experience is what shaped this guide: knowing what's normal at each stage genuinely eases the anxiety.

Use our Pregnancy Calculator to find your current week and see key milestones for your stage.

This comprehensive guide walks you through every week of pregnancy—what's developing, what you might feel, and what to expect.

How Pregnancy Weeks Are Counted

Before diving into the week-by-week breakdown, understand the counting system:

• Gestational age: Counted from the first day of your last menstrual period (LMP)
• Fetal age: Counted from conception (about 2 weeks less)

When we say "week 8," we mean gestational age—your baby has actually been developing for about 6 weeks since conception.

First Trimester: Weeks 1-13

The first trimester is the most critical period for development. By week 13, all major organ systems are in place.

Weeks 1-4: Conception and Implantation

Week 1-2: Preparation

• Your body prepares for ovulation
• Uterine lining builds up
• Not technically pregnant yet

Week 3: Conception

• Ovulation occurs (typically around day 14)
• Fertilization happens if sperm meets egg
• The zygote begins dividing

Week 4: Implantation

• Blastocyst implants in uterine wall (around day 22-28)
• hCG production begins
• You might miss your period by week's end

Learn more about this stage in our Implantation Guide.

Week 5: The Heart Begins

This is often when women discover they're pregnant—a missed period prompts a test.

What's forming:

• Primitive heart begins pulsating
• Neural tube (future brain and spinal cord)
• Early circulatory system

Week 6: Facial Features Start

What's forming:

• Facial features beginning to develop
• Arm and leg buds appearing
• Brain developing rapidly

Week 7: Hands and Feet

What's forming:

• Fingers and toes (webbed at first)
• Mouth and tongue
• Both brain hemispheres

Week 8: Major Organs Present

A milestone week—by now, all major organs are present in primitive form.

What's forming:

• Heart now has 4 chambers
• Intestines developing
• Bones beginning to form
• Embryo now called a "fetus"

According to the American Pregnancy Association, by week 8, the embryo has a distinct human appearance.

Weeks 9-10: Movement Begins

What's forming:

• All essential organs continue developing
• Tiny movements begin (not felt yet)
• External genitalia forming (too early to determine sex)
• Fingers and toes separating

Weeks 11-12: Reflexes Develop

What's forming:

• Reflexes emerging (grasping, sucking)
• Fingernails and toenails
• Tooth buds under gums
• Vocal cords

Week 13: End of First Trimester

Milestone: The risk of miscarriage drops significantly after week 13, which is why many families announce their pregnancy around this time.

Second Trimester: Weeks 14-27

Often called the "honeymoon phase"—morning sickness typically subsides, energy returns, and you'll start to show and feel baby move.

Week 14-15: Gender Forming

What's developing:

• Sex organs are differentiating
• Baby can squint, frown, grimace
• Sucking reflex strengthening
• Body is growing faster than head

Week 16-17: Hearing Develops

What's developing:

• Ears are functional—baby can hear your voice
• Skeletal bones hardening
• Baby can make facial expressions
• Hair pattern forming on head

Week 18-20: Anatomy Scan

Key milestone: The anatomy scan (typically 18-22 weeks)

This detailed ultrasound checks:

• Brain structure
• Heart chambers and function
• Spine alignment
• Kidney function
• Limb development
• Placenta location
• Often: baby's sex (if you want to know)

The Cleveland Clinic notes this is one of the most comprehensive scans of pregnancy.

Week 21-22: Taste Buds Active

What's developing:

• Taste buds can detect flavors in amniotic fluid
• Eyebrows and eyelashes growing
• Hearing more refined—baby recognizes your voice
• Sleep-wake cycles beginning

Week 23-24: Viability Milestone

Milestone at 24 weeks: This is considered the "age of viability"—with intensive medical care, babies born at 24 weeks have about a 40-70% survival rate, though often with significant health challenges.

Week 25-27: Rapid Brain Growth

What's developing:

• Brain is growing rapidly
• Eyes can open and close
• Baby responds to light
• Lungs continuing to mature

Glucose screening: Around weeks 24-28, you'll have a glucose challenge test to screen for gestational diabetes.

Third Trimester: Weeks 28-40+

The home stretch! Baby is gaining weight rapidly, and your body is preparing for labor.

Week 28-30: Practice Breathing

What's developing:

• Lungs producing surfactant
• Practice breathing movements
• Brain developing wrinkles (gyri)
• Adding body fat

Week 31-33: Immune System Building

What's developing:

• Immune system building
• Bones fully developed (but still soft)
• Baby is urinating about a pint per day
• Most babies are head-down by now

Week 34-36: Preparing for Descent

Important milestones:

• At 34 weeks: Most babies would do well if born (may need some support)
• At 35-37 weeks: Group B Strep (GBS) test
• At 36 weeks: Considered "late preterm" if born

According to the March of Dimes, babies born at 34-36 weeks generally do well but may need some extra support initially.

Week 37-38: Early Term

"Early term" classification:

• Baby is considered viable
• All systems functional
• Still some brain and lung development occurring
• Delivery is safe if medically necessary

Week 39-40: Full Term

Full term means:

• Optimal development complete
• Best outcomes for baby
• Lower respiratory issues
• Better temperature regulation

Week 41+: Late Term and Post-Term

What happens:

• Increased monitoring (NSTs, fluid checks)
• Discussion of induction
• Most providers recommend delivery by 42 weeks

Symptoms by Trimester: What to Expect

First Trimester Symptoms

Second Trimester Symptoms

Third Trimester Symptoms

Prenatal Appointment Schedule

Your visit frequency changes throughout pregnancy:

Key Milestones Summary

Frequently Asked Questions

How do I know what week of pregnancy I'm in?

Count from the first day of your last menstrual period (LMP). If your LMP was 8 weeks ago, you're 8 weeks pregnant. Our Pregnancy Calculator can determine your exact week and display relevant milestones.

When will I start showing?

Most first-time mothers begin showing noticeably around 12-16 weeks, though body type affects this significantly. Second-time mothers often show earlier. By 20 weeks, most women have an obvious bump.

When should I feel baby move?

First-time mothers typically feel movement between 18-22 weeks. Second-time mothers may feel it as early as 16 weeks. Early movements feel like flutters or bubbles. Strong kicks become apparent by 24-28 weeks.

What if I don't feel movement every day?

In early pregnancy, intermittent movement is normal. By week 28, you should feel regular movement and begin kick counts. If you notice a significant decrease in movement, drink cold water, lie on your side, and count movements. Contact your provider if you don't feel 10 movements in 2 hours.

Can my baby hear me?

Yes! Hearing develops around weeks 16-18, and by week 24, babies can clearly hear and recognize familiar voices. Talk, read, or sing to your baby—they're listening.

What determines baby's size at birth?

Multiple factors affect birth weight: genetics, placental function, gestational age, maternal health, and nutrition. Birth weight typically ranges from 5.5-10 lbs, with 7.5 lbs being average for full-term babies.

Is it safe to exercise during pregnancy?

For most healthy pregnancies, exercise is beneficial and encouraged. Walking, swimming, prenatal yoga, and modified strength training are generally safe. Consult your provider for personalized guidance, especially with high-risk pregnancies.

What are the most important nutrients during pregnancy?

Key nutrients include:

• Folic acid: Neural tube development (especially first trimester)
• Iron: Blood production
• Calcium: Bone development
• DHA: Brain development
• Protein: Overall growth

Prenatal vitamins help ensure adequate intake.

Related Articles

• Complete Pregnancy Journey: From Conception to Due Date — Overview of the entire pregnancy experience
• How to Track Ovulation — For those planning pregnancy
• When Did I Conceive? — Understanding conception timing
• Implantation Guide — The earliest pregnancy days
• Due Date Accuracy Guide — Understanding when baby will actually arrive

Related Calculators

• Pregnancy Calculator — Track your current week and milestones
• Due Date Calculator — Calculate your estimated delivery date
• Conception Calculator — Determine when conception occurred
• Ovulation Calculator — For planning future pregnancies
• Implantation Calculator — Estimate early pregnancy timing

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This article provides general information for educational purposes. Every pregnancy progresses differently. For personalized guidance about your pregnancy, consult with your healthcare provider.