Week by week, from the inside

Forty weeks, one growing story.

What baby is building, what your body is doing, and what the placenta is quietly managing -- every week from before conception to the due date. Scroll, and the page changes season with you.

40weeks
5chapters
3perspectives
Chapter One

Preconception

Before the two lines
🌻
1–2 Week 1-2
BabyPregnancy is dated from your last period, so conception hasn't happened yet -- but the clock starts now.
YouOvulation happens roughly two weeks after your period starts. This is the best window for conception.
🌱
3 Week 3
about the size of a poppy seed
BabyFertilization! A single cell begins dividing rapidly on its journey to the uterus.
YouToo early for a test to detect anything yet -- hang tight.
Chapter Two

Trimester 1

Weeks 4-13 · the foundation
🌱
4 Week 4
about the size of a poppy seed
BabyThe blastocyst implants in the uterine lining -- the moment pregnancy officially begins.
YouhCG starts rising. A positive test is often possible around now.
PlacentaPlacental tissue begins forming from these same early cells.
🌽
5 Week 5
about the size of a sesame seed
BabyThe neural tube -- the earliest brain and spinal cord -- starts closing. This is why folic acid matters before conception.
YouMissed period. Many first notice symptoms starting around now.
PlacentaPlacental tissue develops rapidly to take over hormone production.
🥦
6 Week 6
about the size of a lentil
BabyThe heart begins beating, and tiny arm and leg buds appear.
YouFatigue and nausea often begin. Iron and vitamin B6 can help both.
🫓
7 Week 7
about the size of a blueberry
BabyArm and leg buds lengthen, and the brain is generating new neurons at a remarkable pace -- roughly 100 every minute.
YouSmell sensitivity often spikes -- a very common early quirk.
🍇
8 Week 8
about the size of a raspberry
BabyFingers and toes begin forming (webbed at first). Facial features take shape.
YouBreast changes begin: tenderness, darkening areola, visible veins.
🍇
9 Week 9
about the size of a grape
BabyBaby is now officially called a fetus, and tiny muscles are starting to function -- baby may be stretching and twisting, even though you can't feel it yet.
YouRound ligament pain may start as the uterus stretches.
🍒
10 Week 10
about the size of a kumquat
BabyTooth buds are forming beneath the gums, and the webbing between fingers disappears.
YouNausea often peaks around now. It's also worth being extra careful with raw or unpasteurized foods -- bacteria like listeria carry a higher risk in pregnancy.
🔵
11 Week 11
about the size of a fig
BabyExternal genitals begin forming (not yet visible on ultrasound), and baby may start hiccupping or sucking a thumb -- tiny, coordinated movements you can't feel yet.
YouThe uterus is now roughly the size of a grapefruit.
🍐
12 Week 12
about the size of a lime
BabyReflexes are developing -- baby can curl its toes and make a fist.
YouEnd of the highest-risk window; many feel a wave of relief here.
PlacentaThe placenta has largely taken over hormone production.
🍋
13 Week 13
about the size of a lemon
BabyFingerprints are forming, and vocal cords are developing.
YouMany start feeling noticeably better as trimester-one symptoms ease.
Chapter Three

Trimester 2

Weeks 14-27 · the golden window
🌷
14 Week 14
about the size of a peach
BabyExternal genitals are now distinguishable -- sex may be visible on ultrasound.
YouEnergy often returns. Welcome to the golden trimester.
🍎
15 Week 15
about the size of an apple
BabyBaby can sense light through closed eyelids. Bones are hardening.
YouRound ligament twinges may continue as the belly grows.
🥑
16 Week 16
about the size of an avocado
BabyFine hair (lanugo) covers the body. Legs are now longer than arms.
YouSome feel the first flutters of movement around now.
🍒
17 Week 17
about the size of a pear
BabyA layer of fat starts developing under the skin to help regulate temperature after birth, and baby may already recognize the sound of your voice.
YouLigament stretching continues; occasional sciatic twinges are common.
🥥
18 Week 18
about the size of a bell pepper
BabyEars have moved into position -- baby may start responding to sound.
YouThis is often when "quickening" happens -- the first time you feel baby move, like small flutters or bubbles. The anatomy ultrasound is often scheduled around this window too.
🍏
19 Week 19
about the size of a mango
BabyVernix -- the protective waxy coating -- starts forming on the skin.
YouMovement becomes more noticeable and regular for many.
🍌
20 Week 20
about the size of a banana
BabyHalfway there. Baby is swallowing more and producing meconium.
YouThe detailed anatomy scan typically happens this week if not already done.
PlacentaThe placenta is now fully mature and working at capacity.
🥕
21 Week 21
about the size of a carrot
BabyEyebrows and eyelids are fully formed.
YouAppetite often increases as baby's growth accelerates.
🍫
22 Week 22
about the size of a small squash
BabyEars are now fully functioning, so baby may startle at sudden sounds or respond to music and familiar voices.
YouBraxton Hicks practice contractions may begin -- irregular and painless.
🍊
23 Week 23
about the size of a grapefruit
BabyBlood vessels in the lungs are developing to prepare for breathing.
YouSkin changes like stretch marks may become visible.
🌽
24 Week 24
about the size of an ear of corn
BabyA major milestone: the lungs begin producing surfactant, a substance essential for breathing. Baby is now considered viable -- meaning survival is possible with medical support if born early.
YouGestational diabetes screening usually happens between 24 and 28 weeks.
PlacentaThe placenta keeps pace with baby's accelerating growth.
🌿
25 Week 25
about the size of a cauliflower
BabyA regular sleep-wake pattern is starting to develop.
YouBackaches are common as posture shifts to balance the belly.
🥒
26 Week 26
about the size of a green onion
BabyEyes begin to open; baby may respond to light.
YouPractice contractions may become more noticeable.
🍀
27 Week 27
about the size of a head of lettuce
BabyBrain tissue develops rapidly, with more distinct sleep cycles.
YouThe end of trimester two -- the golden window winds down.
Chapter Four

Trimester 3

Weeks 28-40 · the home stretch
🍆
28 Week 28
about the size of an eggplant
BabyEyes can now blink, and REM sleep -- baby may be dreaming -- has been detected.
YouTrimester three begins; appointments become more frequent.
PlacentaThe placenta matures further to support rapid weight gain ahead.
🍫
29 Week 29
about the size of a butternut squash
BabyMuscles and lungs continue maturing as baby gains weight quickly.
YouShortness of breath may increase as baby pushes on the diaphragm.
🍀
30 Week 30
about the size of a cabbage
BabyBaby is now producing its own red blood cells in the bone marrow, and movements are getting stronger and more predictable -- a good stretch to start noticing kick patterns.
YouBraxton Hicks contractions may increase in frequency.
🥨
31 Week 31
about the size of a coconut
BabyRapid brain development -- baby can track light and process information.
YouA good week to read up on perineal massage ahead of week 34.
🍒
32 Week 32
about the size of a jicama
BabyBaby is settling into more regular sleep-wake patterns, and toenails are now fully formed.
YouFrequent urination often returns as baby settles lower.
🍍
33 Week 33
about the size of a pineapple
BabyBaby's immune system is developing, receiving antibodies from you.
YouSleep may get harder simply from belly size alone.
🍈
34 Week 34
about the size of a cantaloupe
BabyFingernails reach the fingertips; the central nervous system matures.
YouPerineal massage is commonly recommended starting this week.
🍈
35 Week 35
about the size of a honeydew melon
BabyKidneys are fully developed -- most physical development is complete.
YouThe Group B strep test usually happens between weeks 35 and 37.
🌿
36 Week 36
about the size of a head of romaine
BabyBaby is nearly early-term and gaining about half a pound a week.
YouWeekly appointments typically begin around now.
🥒
37 Week 37
about the size of a bunch of chard
BabyBaby is considered full term. Breathing movements are being practiced.
YouBraxton Hicks practice contractions often become more frequent now -- usually painless, but they can feel intense.
🌽
38 Week 38
about the size of a leek
BabyBrain and lungs keep fine-tuning right up to the very end.
YouThe nesting instinct often kicks in hard around now.
🍉
39 Week 39
about the size of a mini watermelon
BabyBaby is ready for birth -- organs are fully mature.
YouAny day now. Watch for the 5-1-1 contraction pattern.
Chapter Five

Birth

Ready to meet you
🍫
40 Week 40
about the size of a small pumpkin
BabyFull term and ready to meet you.
YouDue date! Many babies arrive within about two weeks of it, either way.
🍫
41 Week 41
about the size of still about a small pumpkin
BabyBaby is considered post-term now -- still growing a little and continuing to practice breathing.
YouExtra monitoring is common at this point, checking on the placenta and fluid levels. Conversations about induction options often start here.

Want the story behind the science?

The Pregnancy track pairs each trimester with real symptoms, real tests, and what actually helped -- from someone who kept notes.

Read the Pregnancy track →

Sizes and milestones are approximate and vary between pregnancies -- informed by general clinical references (ACOG, Mayo Clinic, and similar sources). Educational only, not medical advice. Icons are placeholders; custom illustrations swap in via the WEEKS list in generate.py.