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Weeks pregnant calculator
How many weeks pregnant am I?
Count back from your due date. A pregnancy is dated as 40 weeks, so subtract the days still to go from 280 and divide by seven: 88 days left puts you 27 weeks and 3 days along. Weeks are counted from the first day of your last period rather than from conception, which is why you are two weeks pregnant before conception happens.
How far along am I?
Do not know your due date yet? The due date calculator estimates one from your last period. A dating ultrasound is more accurate than any calendar method — if your pregnancy care team has given you a date, use theirs.
Week by week, from 4 to 40
Every length below is the figure the NHS publishes for that week, alongside what published guidance describes developing. Up to week 19 a baby is measured from head to bottom, because the legs are curled up and cannot be seen; from week 20 the measurement is head to heel. That change of ruler is why the published length jumps from 15.3 cm to 25.6 cm between weeks 19 and 20 — the baby did nothing unusual that week.
Sizes shown in Things you already own. Change it in the calculator above and every week below changes with it.
- Week 4
Around 0.2 cm long — about the size of the head of a pin
First trimester · the head of a pin measures 0.2 cm across the head
The ball of cells has burrowed into the lining of the womb and split in two: one half becomes your baby, the other becomes the placenta. The amniotic sac is forming around it, and the yolk sac that feeds the embryo for the next few weeks is already in place. Nothing is recognisable as a baby yet — this is the fortnight of pure groundwork.
- Week 5
Around 0.2 cm long — about the size of a grain of sand
First trimester · a grain of sand measures 0.2 cm across a coarse grain
The neural tube, which becomes the brain and spinal cord, is closing along the length of the embryo. A tiny heart tube has begun to beat, far too faint for anyone to hear. This is the week most people get a positive test, because the pregnancy hormone hCG has finally built up enough to show.
- Week 6
Around 0.6 cm long — about the size of the hole a hole-punch makes
First trimester · the hole a hole-punch makes measures 0.6 cm across
A heartbeat can sometimes be seen on an early scan, flickering at around 110 beats a minute. Dark spots mark where the eyes will be and small pits where the ears are forming, and buds have appeared where the arms and legs will grow. The embryo is curled into a C shape, with a tail that will disappear over the next few weeks.
- Week 7
Around 1 cm long, from head to bottom — about the size of a single toy building brick
First trimester · a single toy building brick measures 1 cm a 1×1 brick, stud included
The brain is growing faster than anything else, which is why the head is so large compared with the rest of the body. The arm and leg buds are lengthening and flattening into paddles at the ends. The inner ear is taking shape, and the liver has started making red blood cells.
- Week 8
Around 1.6 cm long, from head to bottom — about the size of a dice
First trimester · a dice measures 1.6 cm along one edge
Fingers and toes are forming, still webbed together. The tail-like end of the spine has almost gone, and every major organ is under construction at once — heart, lungs, kidneys and gut. Nerve cells are branching out and beginning to connect to one another.
- Week 9
Around 2.2 cm long, from head to bottom — about the size of a 10-cent coin
First trimester · a 10-cent coin measures 2.4 cm across
The face is arriving: eyelids have grown over the developing eyes and will stay fused shut for months, and there is a nose, a mouth and a tongue. The heart has finished dividing into four chambers. The first movements begin this week, far too small to feel.
- Week 10
Around 3 cm long, from head to bottom — about the size of a 50-cent coin
First trimester · a 50-cent coin measures 3.2 cm across the flats
The embryo is now officially called a fetus. The vital organs are formed and starting to function, the fingers and toes have separated, and nails are beginning to appear. The cartilage of the skeleton is starting to harden into bone.
- Week 11
Around 4.1 cm long, from head to bottom — about the size of a toy building-block figure
First trimester · a toy building-block figure measures 4 cm head to feet
Bones are hardening, fingernails are appearing, and the baby can move freely in the amniotic fluid — stretching, rolling and sometimes hiccupping. External genitals are developing, though far too early to see on a scan. Tooth buds are forming in the gums.
- Week 12
Around 5.4 cm long, from head to bottom — about the size of a wireless earbud case
First trimester · a wireless earbud case measures 5.4 cm across the lid
The kidneys are working, and the baby can pass small amounts of urine into the amniotic fluid. The digestive system is practising the squeezing motions it will use to move food along. For most people this is the week of the dating scan — the first proper look.
- Week 13
Around 7.4 cm long, from head to bottom — about the size of a sticky note
Second trimester · a sticky note measures 7.6 cm along one side
The baby can suck and swallow, and fine downy hair called lanugo begins to grow over the body. Vocal cords are forming. The intestines, which have been developing inside the umbilical cord, move into the abdomen where they belong.
- Week 14
Around 8.5 cm long, from head to bottom — about the size of a credit card
Second trimester · a credit card measures 8.6 cm along the long edge
The baby can squint, frown and grimace, and may start sucking a thumb. The neck is lengthening, so the head sits more upright instead of resting on the chest. The spleen has started producing red blood cells of its own.
- Week 15
Around 10.1 cm long, from head to bottom — about the size of a computer mouse
Second trimester · a computer mouse measures 10.5 cm front to back
The baby can sense bright light through closed eyelids and may move away from it. The ears have shifted into position at the sides of the head, and the tiny bones inside them are hardening enough to begin carrying sound. Taste buds are forming.
- Week 16
Around 11.6 cm long, from head to bottom — about the size of a can of soft drink
Second trimester · a can of soft drink measures 11.5 cm top to bottom
The nervous system is developing quickly, so movements are becoming coordinated rather than random — arms and legs now move together. The eyes can move behind closed lids. The baby has begun making the breathing motions that draw amniotic fluid in and out of the lungs.
- Week 17
Around 12 cm long, from head to bottom — about the size of a passport
Second trimester · a passport measures 12.5 cm top to bottom
Fat is starting to form under the skin, which is what will keep the baby warm after birth. The umbilical cord is thickening and strengthening. The fingerprints are set this week, and there will never be another set like them.
- Week 18
Around 14.2 cm long, from head to bottom — about the size of the phone in your hand
Second trimester · the phone in your hand measures 14.7 cm top to bottom
The baby can hear — a heartbeat, the rumble of digestion, and voices from outside, though muffled. Myelin, the fatty sheath that insulates nerves and speeds their signals up, is beginning to coat them. Yawning and hiccupping become regular.
- Week 19
Around 15.3 cm long, from head to bottom — about the size of a 15 cm ruler
Second trimester · a 15 cm ruler measures 15 cm end to end
Vernix, a greasy white coating, forms over the skin to protect it through months of soaking in amniotic fluid. In female babies the ovaries already hold their lifetime supply of eggs. Many first-time parents notice movement around now, often mistaking it for wind.
- Week 20
Around 25.6 cm long, from head to heel — about the size of a dinner plate
Second trimester · a dinner plate measures 26 cm across
Halfway. The anomaly scan usually happens between 18 and 21 weeks, checking growth, the position of the placenta and how the organs are developing. The baby is now producing meconium — the dark first bowel movement — and swallowing amniotic fluid regularly.
- Week 21
Around 26.7 cm long, from head to heel — about the size of a drink bottle
Second trimester · a drink bottle measures 27 cm top to bottom
The baby swallows amniotic fluid daily and the digestive system rehearses absorbing it. Rapid eye movements have begun beneath the closed lids. Movements are becoming strong enough that other people can sometimes feel them from the outside.
- Week 22
Around 27.8 cm long, from head to heel — about the size of a frying pan
Second trimester · a frying pan measures 28 cm across
Touch, taste and hearing are all sharpening. The baby has a working grip and will take hold of the umbilical cord. The lips and eyebrows are more distinct, and the skin is still wrinkled and translucent with blood vessels visible beneath it.
- Week 23
Around 28.9 cm long, from head to heel — about the size of a sheet of A4 paper
Second trimester · a sheet of A4 paper measures 29.7 cm top to bottom
The lungs are building the tiny air sacs they will need to breathe, along with the blood vessels that will serve them. The skin is loose and wrinkled with the fat yet to fill it out. The inner ear is now developed enough for the baby to sense which way up they are.
- Week 24
Around 30 cm long, from head to heel — about the size of a 30 cm ruler
Second trimester · a 30 cm ruler measures 30 cm end to end
The lungs begin making surfactant, the substance that stops the air sacs collapsing between breaths. Twenty-four weeks is the point from which many health services describe a pregnancy as viable. The face is fully formed, eyelashes and eyebrows included.
- Week 25
Around 34.6 cm long, from head to heel — about the size of a shoebox
Second trimester · a shoebox measures 34 cm along the long side
The baby responds to familiar voices and may startle at a sudden noise — the startle reflex has appeared. The hands are fully developed, and the nerves around the mouth and lips are becoming sensitive. Fat is starting to fill out the wrinkles.
- Week 26
Around 35.6 cm long, from head to heel — about the size of a 16-inch laptop
Second trimester · a 16-inch laptop measures 35.6 cm across the lid
The eyes begin to open for the first time, and the baby can blink. Brain activity in response to sight and sound can now be detected. The lungs carry on developing, and the baby practises drawing amniotic fluid in and out.
- Week 27
Around 36.6 cm long, from head to heel — about the size of a garden gnome
Third trimester · a garden gnome measures 36 cm hat to boots
Sleep and wake cycles settle into a rhythm, which is why the kicks start arriving at consistent times of day. The brain is growing rapidly and developing its characteristic folds. The baby may answer a loud sound with a kick or a jump.
- Week 28
Around 37.6 cm long, from head to heel — about the size of a bowling pin
Third trimester · a bowling pin measures 38.1 cm top to bottom
The third trimester. The baby can open the eyes properly and may turn towards a bright light held against the bump. Fat is building steadily, smoothing out the skin. Measurable REM sleep — the stage where dreaming happens — is now under way.
- Week 29
Around 38.6 cm long, from head to heel — about the size of a games console, standing up
Third trimester · a games console, standing up measures 39 cm top to bottom
Muscles and lungs are maturing, and the head is growing to make room for the developing brain. Movements feel stronger and more crowded, with more shoving than somersaulting as space runs short. The bones are absorbing a great deal of calcium.
- Week 30
Around 39.9 cm long, from head to heel — about the size of a rolling pin
Third trimester · a rolling pin measures 40 cm end to end
Bone marrow has taken over production of red blood cells from the liver and spleen. The fine lanugo hair is starting to disappear. The grip is strong, and the eyes can track a light source moved across the bump.
- Week 31
Around 41.1 cm long, from head to heel — about the size of an oven tray
Third trimester · an oven tray measures 41 cm along the long side
The baby is putting on weight quickly, and all five senses are now working. The brain can process information arriving from all of them at once. Much of the day and night is spent asleep, a good deal of it in REM sleep.
- Week 32
Around 42.4 cm long, from head to heel — about the size of a record player
Third trimester · a record player measures 42 cm across the plinth
Many babies settle head-down around now, though plenty change their mind later. The toenails have finished growing and the fingernails have reached the fingertips. Practice breathing continues, and the lungs are nearly ready.
- Week 33
Around 43.7 cm long, from head to heel — about the size of a bike pump
Third trimester · a bike pump measures 44 cm end to end
The bones are hardening — all except the skull, which stays soft with gaps between the plates so it can move through the birth canal. The immune system is developing as antibodies cross the placenta. The pupils can now widen and narrow in response to light.
- Week 34
Around 45 cm long, from head to heel — about the size of a traffic cone
Third trimester · a traffic cone measures 45 cm top to bottom
The central nervous system and the lungs are maturing steadily. The vernix thickens to protect the skin through the last weeks. The sleep and wake pattern is well established, and the baby has a clear preference about when to be awake.
- Week 35
Around 46.2 cm long, from head to heel — about the size of a cat
Third trimester · a cat measures 46 cm nose to base of tail
The kidneys are fully developed and the liver can process some waste products. Most of the growth from here is weight rather than length, at roughly 200 grams a week. Space is tight enough that big movements become rolls and stretches.
- Week 36
Around 47.4 cm long, from head to heel — about the size of a pillow
Third trimester · a pillow measures 48 cm across the short side
The lungs are almost fully developed and would very likely work on their own. Around now many babies drop lower into the pelvis, which can make breathing easier and walking harder. Most of the remaining lanugo is being shed.
- Week 37
Around 48.6 cm long, from head to heel — about the size of a board game board, opened out
Third trimester · a board game board, opened out measures 49 cm along one side
Early term. The baby is practising breathing movements, sucking and blinking, and could arrive any time from here. The brain and lungs keep maturing right up to birth — the last weeks are not idle. Firm, regular movements should continue.
- Week 38
Around 49.8 cm long, from head to heel — about the size of a microwave
Third trimester · a microwave measures 50 cm side to side
The organs are ready. The baby is shedding vernix and lanugo and swallowing them, which becomes the contents of the first nappy. The head may be engaged in the pelvis, and the lungs are producing more surfactant by the day.
- Week 39
Around 50.7 cm long, from head to heel — about the size of a ukulele
Third trimester · a ukulele measures 53 cm end to end
Full term. The baby keeps laying down fat, mostly to hold their own body temperature once they are out in the air. Antibodies are still crossing the umbilical cord to build the immune system. The chest is becoming more prominent.
- Week 40
Around 51.2 cm long, from head to heel — about the size of a newborn baby
Third trimester · a newborn baby measures 51.2 cm head to heel
The estimated due date. Only about 1 in 20 babies arrive on it — most come somewhere between 37 and 42 weeks. The baby carries on growing and laying down fat right up to labour, and the skull bones stay unfused so the head can mould on the way out.
Where each theme's measurements come from
- Things you already own
- Everyday objects at their standard or typical size, and the theme with the least to worry about: a credit card is 85.6 mm under the ISO/IEC 7810 ID-1 standard, an Australian 50-cent coin 31.6 mm across the flats, a school ruler exactly what it says on it.
- Sport
- Match-regulation equipment sizes: a golf ball is 42.67 mm across at the legal minimum, a shuttlecock 85 mm end to end, a basketball hoop 45.7 cm, an Olympic weight plate 45 cm.
- Sweet treats
- Typical retail sizes for confectionery and baking, described by what a thing is rather than by who makes it: a four-finger wafer bar is 10.5 cm end to end, a frog-shaped chocolate 5.5 cm, and the triangular bar they sell at duty-free a genuine 26 cm.
- Australian animals
- Head-and-body or beak-to-tail lengths as field guides publish them. Where a species spans a range, the figure is a typical adult rather than the largest one anybody has measured.
- The 90s
- Retail dimensions of the actual products, described by what they were rather than by brand: the era-defining handheld console is 148 mm tall, a digital pet keyring 55 mm, a cassette 102 mm, and the indestructible mobile everyone had 113 mm.
- Gaming
- Manufacturer dimensions, described by what each thing is rather than by brand: the hybrid handheld is 242 mm wide with its controllers attached, the full-size console 390 mm standing up, and the tower-shaped one 301 mm.
- Fast food
- Menu-standard sizes, which are mostly just inches wearing a hat: a twelve-inch pizza is 30.5 cm across because twelve inches is 30.5 cm.
- Toys
- Boxed retail sizes, described generically: a building-block figure is 4 cm head to feet, a fashion doll 29.2 cm, and a twelve-inch action figure exactly what its name suggests.
Why pregnancy is counted in weeks and days
A pregnancy is dated from the first day of the last menstrual period and runs for 40 weeks, written as completed weeks plus days: 24+3 means twenty-four full weeks finished and three days into the twenty-fifth. It is counted from the period rather than from conception for a practical reason — that is the date almost everyone can name, while the day of ovulation usually is not known. The side effect is the one that confuses everybody: on the standard convention you are counted as two weeks pregnant at the moment of conception, so a "40-week" pregnancy contains about 38 weeks of baby.
Weeks stay the unit all the way through because the whole of pregnancy care is scheduled against them. The dating scan sits around 12 weeks, the anomaly scan between 18 and 21, and the checks that matter later are named for weeks too. Months vary in length and nobody agrees whether a pregnancy is nine of them or ten; weeks do not have that problem.
What this calculator will not tell you
It reports what published guidance says about a week of pregnancy, and it does that the same way for everybody. It does not assess your pregnancy, and nothing on the page appears, changes or escalates because of the date you entered. If a scan has measured your baby and the number differs from the population figure in the table above, that is a conversation for your midwife or doctor — a published median is what a guide can print about babies in general, not a measurement of yours.
Questions parents ask about counting the weeks
- Why am I two weeks pregnant before I conceived?
- Because pregnancy is counted from the first day of your last menstrual period, not from conception. That is the one date almost everyone can pin down, whereas the day of ovulation usually is not known. On the standard 28-day cycle, conception happens around day 14 — so by the time it occurs you are already counted as two weeks pregnant, and a 40-week pregnancy contains about 38 weeks of actual baby.
- What does "24 weeks and 3 days" mean, and why not just months?
- Pregnancy is counted in completed weeks plus days, written as 24+3. It means you have finished 24 weeks and are three days into your 25th. Weeks are used rather than months because every scan, screening test and appointment is scheduled against a week number — the 12-week dating scan, the 18-to-21-week anomaly scan, the 28-week checks — and because months vary in length while weeks do not.
- How big is my baby right now?
- The NHS publishes a length for every week of pregnancy: around 5.4 cm at 12 weeks, 25.6 cm at 20 weeks, 37.6 cm at 28 weeks and 51.2 cm at 40 weeks. Two things about those figures matter. The measurement changes at week 20 — before that a baby is measured head to bottom because the legs are curled up out of sight, and from week 20 head to heel, which is why the published length jumps between week 19 and week 20 without the baby doing anything unusual. And they are population figures rather than a measurement of any individual baby.
- Why does this calculator not compare my baby to a fruit?
- Because nobody knows how big a rutabaga is, and produce lists are copied between pregnancy sites rather than measured. This tool offers eight rulers instead — everyday objects, sport, sweet treats, Australian animals, the 90s, gaming, fast food and toys — and every comparison in every theme is checked against the published length automatically, so the joke never comes at the cost of the number.
- Can I change what my baby is compared to?
- Yes. Pick a theme in the calculator and every week changes with it, all the way down the page: at 20 weeks your baby is a dinner plate, a basketball, the big triangular chocolate bar from duty-free, a rainbow lorikeet, a reflex toy that barks orders at you, a hybrid handheld console, a slice of large pizza or a small teddy bear. The choice is remembered on your device. The measurement never changes — only the thing it is held up against.
- When do the trimesters start and end?
- The commonly published split is weeks 1 to 12 for the first trimester, 13 to 26 for the second, and 27 until birth for the third. They are conventions for talking about pregnancy rather than biological switches — nothing changes overnight at week 13, and some sources draw the second and third trimester boundary at 28 weeks instead.
- Does the calculator remember my due date?
- Yes. The date is saved in your browser on the device you entered it on, so coming back later shows how far along you are without asking again. It is never sent anywhere, never attached to analytics and never shared. Entering a different date replaces it, clearing your browser storage removes it, and a different device or browser will ask you for the date again.
Where these numbers come from
- NHS — Week-by-week guide to pregnancy — The published length of a baby at each week of pregnancy — measured head to bottom until week 20 and head to heel from week 20 — and what is developing at that stage.
- Pregnancy, Birth and Baby — Pregnancy week by week — The Australian Government health service’s week-by-week account of how a pregnancy is counted and what develops at each stage.
- Pregnancy, Birth and Baby (Australian Government) — How an estimated due date is calculated, and pregnancy week-by-week guidance.
- NHS — Pregnancy — Week-by-week pregnancy guidance, including how pregnancies are dated.
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Stop doing this maths in your head
The counting does not stop when the baby arrives — it just changes units, to feeds, sleeps and nappies. BubSync keeps that record shared across everyone caring for your baby, from day one.
