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Pregnancy Week-by-Week: A Complete Guide

Forty weeks, walked through one at a time — what's happening with your baby and your body, with the tools you'll need at each stage.

Pregnancy isn't a single experience — it's three trimesters with very different feelings, milestones, and concerns. This guide walks through all 40 weeks and tells you what's happening week by week.

How weeks are counted: by convention, pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. So you're 'two weeks pregnant' before fertilization actually happens. Most people don't know they're pregnant until around week 4-5, when the home test turns positive.

Each week below has a baby-size comparison and one notable thing happening. As individual deep-dive pages ship, they'll link from this table — meanwhile this is a quick scannable reference.

Quick reference

First Trimester (Weeks 1–13)

Weeks 1-13. Implantation, organ formation, and the most rapid development period. Morning sickness peaks around weeks 7-10 and eases by the end of this trimester.

StageWhat's happening
Week 1Pre-conception — your last period starts the clock.
Week 2Ovulation around now. Fertilization may happen this week.
Week 3Conception complete — embryo is a tiny cluster of cells.
Week 4Implantation. Most home pregnancy tests turn positive now.
Week 5Size of a sesame seed. Heart begins to form.
Week 6Size of a lentil. First heartbeat detectable on ultrasound.
Week 7Size of a blueberry. Brain dividing into regions.
Week 8Size of a raspberry. Tiny fingers and toes forming.
Week 9Size of a cherry. Basic body shape established.
Week 10Size of a strawberry. Vital organs are functioning.
Week 11Size of a fig. Baby can hiccup, even if you can't feel it.
Week 12Size of a lime. NT scan often happens this week.
Week 13Size of a peach. End of first trimester — morning sickness often eases.

Second Trimester (Weeks 14–27)

Weeks 14-27. Often called the 'honeymoon' trimester. Energy returns, morning sickness fades, and around week 18-20 you'll feel the first kicks.

StageWhat's happening
Week 14Size of a lemon. Baby can squint, frown, and grimace.
Week 15Size of an apple. Baby can hear your voice.
Week 16Size of an avocado. Baby's eyes start moving.
Week 17Size of a turnip. Skeleton hardening from cartilage to bone.
Week 18Size of a bell pepper. You may feel first kicks ('quickening').
Week 19Size of a mango. Vernix forms to protect baby's skin.
Week 20Size of a banana. Halfway! Anatomy scan typically this week.
Week 21Size of a carrot. Baby's movements become more coordinated.
Week 22Size of a papaya. Tiny eyebrows forming.
Week 23Size of a grapefruit. Baby's hearing is well-developed.
Week 24Size of a cantaloupe. Viability threshold — major NICU milestone.
Week 25Size of a head of cauliflower. Baby responds to your voice.
Week 26Size of a head of lettuce. Eyes start to open.
Week 27End of second trimester. Baby has regular sleep cycles.

Third Trimester (Weeks 28–40)

Weeks 28-40. The home stretch. Kick counts become daily practice from week 28. Hospital bag packed by week 36.

StageWhat's happening
Week 28Size of an eggplant. Kick counts are now recommended daily.
Week 29Size of a butternut squash. Brain billions of neurons developed.
Week 30Size of a large cabbage. Baby practices breathing.
Week 31Size of a coconut. All major organs nearly fully developed.
Week 32Size of a jicama. Time to start packing your hospital bag.
Week 33Size of a pineapple. Bones harden except the skull.
Week 34Size of a cantaloupe. Lungs nearly ready for the outside world.
Week 35Size of a honeydew melon. Group B Strep test typically this week.
Week 36Size of a romaine lettuce. Hospital bag must be ready.
Week 37Size of a Swiss chard bunch. Early term — baby is fully developed.
Week 38Size of a leek. Most growth complete; gaining fat.
Week 39Size of a watermelon. Full term — labor can start any day.
Week 40Estimated due date. About 5% of babies arrive today.

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