Pregnancy Week by Week Foetal Development: Complete Milestone Guide

Ever wondered what's really happening inside during those 40 weeks? I remember staring at my first positive test thinking, "Okay, what now?" That curiosity led me down a rabbit hole of research. Let's cut through the fluff and talk real foetal development week by week. No sugarcoating, just practical insights you can actually use.

The First Trimester: Tiny Beginnings (Weeks 1-12)

Honestly, this phase can feel brutal. Morning sickness hit me like a truck at week 6. But knowing what was happening inside helped me power through.

Weeks 1-4: The Silent Start

Funny thing: Your "week 1" starts before conception! Doctors count from your last period. By week 4, that tiny cluster of cells (now called a blastocyst) implants in your uterus lining. I didn't feel a thing yet, but:

My tip: Start prenatal vitamins NOW if you haven't. I used Nature Made Prenatal Multi + DHA ($25/100ct). The fishy aftertaste isn't awesome, but it covers all bases.

Weeks 5-8: Major Organ Boot-Up

This is when things get real. The embryo grows from 2mm to 16mm! I remember my first ultrasound at 7 weeks - seeing that flickering heartbeat was surreal.

• Week 5: Heart starts beating (110bpm!)
• Week 6: Arm/leg buds appear. Facial features begin forming
• Week 7: Hands/feet look like paddles. Brain hemispheres develop
• Week 8: Official transition from embryo to foetus. All major organs present

Weeks 9-12: Looking Human

By week 10, your baby's no longer a tailed creature! Fingers and toes separate, bones harden, and sex organs form (though too early to see on scan).

Second Trimester: Growth Spurt (Weeks 13-27)

The "golden period"! For me, energy returned around week 14. Finally felt human again.

Weeks 13-16: Motion Activated

Your foetus can now suck its thumb! Mine loved midnight acrobatics. Key developments:

• Skin transparent but covered in lanugo (fine hair)
• Bone marrow starts making blood cells
• First movements happen (though you won't feel them yet)

Product I loved: BabyDoppler Fetal Monitor ($69). Heard my baby's heartbeat at home from week 14. Warning: It's addictive and can cause unnecessary panic if you can't find it. Use sparingly.

Weeks 17-20: Hello, Kicks!

This is when most moms feel first flutters ("quickening"). Mine felt like gas bubbles at week 18. Gender reveal time if you want!

Weeks 21-27: Viability Threshold

By week 24, survival outside womb becomes possible with medical help. Lung development accelerates:

• Week 21: Taste buds work! Swallows amniotic fluid
• Week 23: Rapid eye movement (REM) sleep begins
• Week 25: Hand dominance shows (left/right preference)
• Week 27: Opens/closes eyes. Recognizes light/dark

Kick Count Warning: Start tracking movements around week 26. Less than 10 kicks in 2 hours? Call your doctor. I used the Count the Kicks app - simple and free.

Third Trimester: Final Countdown (Weeks 28-40)

The home stretch! Sleep becomes mythical as baby hogs your ribcage.

Weeks 28-32: Brain Building Boom

Babies gain half their birth weight now! My appetite was insane - ate pickles with peanut butter daily.

Must-have: Pregnancy pillow. I tried three - the Pharmedoc C-Shape ($65) won. Lifesaver for hip pain.

Weeks 33-36: Practice Mode

Lungs mature by producing surfactant. If born now, baby might need NICU time but usually does well. Final growth spurts happen:

• Skin turns pink and smooths out
• Fingernails reach fingertips (hello, scratches!)
• Immune system borrows antibodies from you

Weeks 37-40: Full-Term Readiness

You made it! Baby's just perfecting skills and gaining weight:

Final Note: Only 5% deliver on their due date! My water broke at 41+3 while grocery shopping. Embarrassing but memorable.

Your Week-by-Week Foetal Development Questions Answered

When will I feel my baby move?

First-time moms usually feel flutters between 18-22 weeks. Earlier with subsequent pregnancies. If you have an anterior placenta (like I did with my second), it muffles movement - might not feel consistent kicks until 24+ weeks.

What if my baby measures small/large?

Ultrasounds have a 15% margin of error! My niece was predicted to be 10lbs - came out 7lbs 2oz. Unless there's drastic deviation, don't stress. Your doctor monitors growth curves.

Are prenatal ultrasounds safe?

Yes, when done by trained professionals. But avoid "keepsake" 3D/4D ultrasounds at mall boutiques. They often use higher intensity than medical scans. My hospital's 3D scans were safer and included measurements.

Can I influence my baby's development?

Partially. Good nutrition matters most - especially folic acid early on. But don't beat yourself up over occasional junk food. Stress? Worse than sushi slip-ups. I cried over spilled ice cream at 3am once. Baby turned out fine.

When is the anatomy scan done?

Typically 18-22 weeks. Checks all major organs and limbs. Ask for printed images - they make great baby book additions! Pro tip: Bring orange juice to make baby move if they're sleepy.

What Actually Affects Foetal Development

Beyond genetics, these factors matter:

• Nutrition: 300 extra calories/day in 2nd/3rd trimesters. Focus on protein, iron, calcium
• Harmful Exposures: Alcohol, tobacco, certain medications (check with OB)
• Chronic Conditions: Diabetes/hypertension need tight control
• Stress Levels: High cortisol impacts development. Try prenatal yoga

My biggest regret? Obsessing over every little thing. The fact you're reading this already makes you an awesome mom-to-be. Trust your body - it's done this for millennia.

Tracking Tools Worth Trying

Skip the gimmicks. These actually helped me:

Final thought: This week-by-week foetal development journey is wild. One day you're googling "poppy seed size," next thing you're holding a human. Document it - take bump photos even when you feel huge. I hated mine then, treasure them now. You've got this!