Let’s go step by step to fill in the information for each stage of pregnancy. We’ll focus on two things for each stage:
1.
Average size of the baby (in inches or centimeters, and sometimes compared to everyday objects).
2.
Important growth changes — what new body parts form, what movements start, etc.
We’ll use standard medical knowledge about fetal development that is commonly taught in schools.
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By 4 weeks:
- The embryo is very small — about the size of a poppy seed.
- Size: Around 0.04 inches (1 mm) long.
- Growth changes:
- Heart begins to beat.
- Brain and spinal cord start forming.
- Tiny arms and legs begin as buds.
- Eyes and ears start developing.
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By 12 weeks:
- Baby is now called a fetus.
- Size: About 2.5 inches (6 cm) long — like a lime or plum.
- Growth changes:
- All major organs are formed.
- Fingers and toes have nails.
- Can make fists and move limbs (but mom can’t feel it yet).
- Sex organs are developed enough to tell boy or girl on ultrasound.
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By 20 weeks:
- Halfway through pregnancy!
- Size: About 6.5 inches (16 cm) long — like a banana.
- Weight: Around 10 ounces (280 grams).
- Growth changes:
- Mom can feel baby moving (“quickening”).
- Baby has hair on head, eyebrows, eyelashes.
- Skin is covered with soft hair called lanugo.
- Can hear sounds from outside the womb.
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By 26 weeks:
- Baby is getting bigger and stronger.
- Size: About 9 inches (23 cm) long — like an ear of corn.
- Weight: Around 1.7 pounds (760 grams).
- Growth changes:
- Lungs are starting to mature — may survive if born early with help.
- Eyes can open and close.
- Responds to sound and light.
- Has regular sleep and wake cycles.
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From 32–40 weeks:
- Final stretch before birth!
- At 32 weeks: ~17 inches (43 cm), ~3.7 lbs (1.7 kg)
- At 40 weeks: ~20 inches (51 cm), ~7.5 lbs (3.4 kg) — average newborn size.
- Growth changes:
- Gains fat under skin to stay warm after birth.
- Turns head-down position for delivery.
- Lungs fully mature near end.
- Practices breathing motions.
- May suck thumb and grip tightly.
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Final Answer:
By 4 weeks:
Size: Poppy seed (0.04 in / 1 mm)
Changes: Heart beats, brain/spine form, arm/leg buds appear
By 12 weeks:
Size: Lime (2.5 in / 6 cm)
Changes: Organs formed, fingers/toes with nails, can move, sex visible
By 20 weeks:
Size: Banana (6.5 in / 16 cm), 10 oz
Changes: Mom feels movement, hair grows, hears sounds
By 26 weeks:
Size: Ear of corn (9 in / 23 cm), 1.7 lbs
Changes: Eyes open, lungs maturing, responds to light/sound
From 32–40 weeks:
Size: 17–20 in (43–51 cm), 3.7–7.5 lbs (1.7–3.4 kg)
Changes: Gains fat, turns head down, lungs ready, practices breathing
Parent Tip: Review the logic above to help your child master the concept of stages of prenatal development worksheet.
