Pregnancy Week by Week: Your Baby's Development
Pick any week to see baby's size, development and what is happening to your body, or enter your due date to jump straight to today.
Week 4
First trimester
Baby size: Poppy seed
Baby’s development
Implantation completes; the neural tube begins forming.
Your body & bumps
Often before a positive test; maybe spotting or tiredness.
All weeks at a glance
| Week | Baby size | Baby development | Your body & bumps |
|---|---|---|---|
| 4 | Poppy seed | Implantation completes; the neural tube begins forming. | Often before a positive test; maybe spotting or tiredness. |
| 6 | Lentil | Early heart tube beats; limb and face buds appear. | Positive test; nausea and breast tenderness may start. |
| 8 | Raspberry | Arms and legs lengthen; organs forming; moves but unseen. | Symptoms peak for many; frequent urination begins. |
| 10 | Prune | Vital organs in place; nails form; tail gone. | Wardrobe may feel tight; mood shifts as hormones climb. |
| 12 | Lime | Reflexes start; kidney makes urine; about 7 cm long. | Sickness often eases; energy lifts toward trimester's end. |
| 14 | Nectarine | Facial features settle; skeleton hardening; sucking motions. | Appetite returns; the bump may start to show. |
| 16 | Avocado | Skeleton more defined; tiny bones in the ear; sex visible. | Round-ligament twinges; congestion or mild swelling common. |
| 18 | Sweet potato | Fat forming; hearing begins; Vernix protects skin. | First flutters of movement possible, earlier if not first. |
| 20 | Banana | About 25 cm; lanugo hair; organs visible at the scan. | Mid-pregnancy anomaly scan; backache as the bump grows. |
| 24 | Corn cob | Lungs develop air sacs; skin reddens; sounds detected. | Swelling and heartburn may appear; Braxton Hicks can start. |
| 28 | Eggplant | Brain grows fast; lungs mature; blinking and dreaming. | Third trimester begins; frequent antenatal checks start. |
| 32 | Squash | Bones fully formed; practice breathing; gains weight. | Breathlessness as baby pushes up; more Braxton Hicks. |
| 36 | Honeydew | Lungs nearly ready; most are head-down now. | Nesting energy; pelvic pressure and drop; tired but eager. |
| 40 | Small pumpkin | Considered full term; ready to be born any day. | Due date reached; only a few babies arrive exactly on it. |
Sizes and milestones are typical averages, and babies vary a little week to week, so treat these as a general guide rather than a strict schedule. Your scan measurements and provider are the authority on how your particular pregnancy is progressing, and every description here counts weeks from the last period, about two before conception.
The formula
Current week = (today − (due date − 280 days)) ÷ 7, matched to the nearest listed week
- Week — A seven-day block of the 40-week pregnancy, counted from the first day of the last period.
- Crown to heel — The standard way a baby's length is measured in the womb and quoted week by week.
- Trimester — One of three roughly three-month phases: first to week 13, second to 27, third to birth.
- Milestone — A development landmark, such as a heartbeat or first movements, that tends to fall in a given week.
Pregnancy week by week
| Week | Baby size | Baby development | Your body & bumps |
|---|---|---|---|
| 4 | Poppy seed | Implantation completes; the neural tube begins forming. | Often before a positive test; maybe spotting or tiredness. |
| 6 | Lentil | Early heart tube beats; limb and face buds appear. | Positive test; nausea and breast tenderness may start. |
| 8 | Raspberry | Arms and legs lengthen; organs forming; moves but unseen. | Symptoms peak for many; frequent urination begins. |
| 10 | Prune | Vital organs in place; nails form; tail gone. | Wardrobe may feel tight; mood shifts as hormones climb. |
| 12 | Lime | Reflexes start; kidney makes urine; about 7 cm long. | Sickness often eases; energy lifts toward trimester's end. |
| 14 | Nectarine | Facial features settle; skeleton hardening; sucking motions. | Appetite returns; the bump may start to show. |
| 16 | Avocado | Skeleton more defined; tiny bones in the ear; sex visible. | Round-ligament twinges; congestion or mild swelling common. |
| 18 | Sweet potato | Fat forming; hearing begins; Vernix protects skin. | First flutters of movement possible, earlier if not first. |
| 20 | Banana | About 25 cm; lanugo hair; organs visible at the scan. | Mid-pregnancy anomaly scan; backache as the bump grows. |
| 24 | Corn cob | Lungs develop air sacs; skin reddens; sounds detected. | Swelling and heartburn may appear; Braxton Hicks can start. |
| 28 | Eggplant | Brain grows fast; lungs mature; blinking and dreaming. | Third trimester begins; frequent antenatal checks start. |
| 32 | Squash | Bones fully formed; practice breathing; gains weight. | Breathlessness as baby pushes up; more Braxton Hicks. |
| 36 | Honeydew | Lungs nearly ready; most are head-down now. | Nesting energy; pelvic pressure and drop; tired but eager. |
| 40 | Small pumpkin | Considered full term; ready to be born any day. | Due date reached; only a few babies arrive exactly on it. |
Sizes are approximate fruit comparisons for the average baby; only a minority of pregnancies land exactly on these figures. Your scans and provider give the real measurements for your baby. Weeks are counted from the last period.
Watching a pregnancy unfold week by week is one of the most satisfying parts of expecting a baby, and the questions arrive on a weekly rhythm too. Is a heartbeat there yet, when will I feel movement, how big is the baby now, and what is happening to my own body this week? This tracker answers those at a glance.
Pick any milestone week and it shows the baby's approximate size, what they are developing right then, and what you might be noticing in your body and bump. Enter your due date instead and it jumps to the week you are in today, using the standard forty-week count from your last period.
It is a friendly map rather than a strict timetable. Every baby varies slightly and every pregnancy has its own pace, so the sizes and milestones here are reassuring averages, not deadlines, and your scan measurements and provider remain the real guide to how your particular pregnancy is progressing.
How the week-by-week count works
Pregnancy is measured in weeks from the first day of your last menstrual period, not from conception, so the count starts about two weeks before the egg is actually fertilised. That is why a pregnancy described as four weeks along is only about two weeks past conception.
The whole journey is traditionally forty weeks, grouped into three trimesters. The first runs to the end of week thirteen and is where the earliest loss risk and the fiercest early symptoms sit, the second to week twenty-seven is often the most comfortable stretch, and the third to birth is the final growth push.
This tracker keeps to that convention throughout, so the week you pick, the milestones shown, and any due-date jump all line up with the dating your midwife, doctor and scan reports use, which avoids the confusion of counting from two different starting points.
The first trimester, weeks one to thirteen
The opening weeks are quietly dramatic. Around weeks four to six the neural tube that becomes the brain and spine forms and a heart begins to beat, and by week eight the tiny embryo has arm and leg buds lengthening into limbs and is about the size of a raspberry, or roughly two centimetres.
By week twelve the baby has grown reflexes, formed fingernails and is moving though you cannot feel it yet, reaching around six or seven centimetres crown to heel. Most organs that will carry on maturing later are laid down in this window, which is why early care and folic acid matter so much.
For the parent this is often the hardest stretch for symptoms, with nausea, tiredness and tender breasts peaking as hormones surge, balanced against the joy of a positive test and a first scan. Many find energy and appetite begin to lift toward the end of the trimester.
The second trimester, weeks fourteen to twenty-seven
Often called the honeymoon phase, the second trimester brings more energy as early sickness fades, and a range of lovely firsts. Around weeks eighteen to twenty many people feel first flutters of movement, quicker to spot in a second or later pregnancy than a first.
The week-twenty anomaly scan is a highlight, checking anatomy and often revealing baby's sex if you want to know, by which point the baby is about twenty-five centimetres long and covered in protective vernix and fine lanugo hair that will later shed.
Your body is changing too: the bump becomes visible, round ligament twinges can appear as the uterus grows, and things like nasal congestion or mild swelling are common as blood volume rises. It is a stretch to book the scan, plan a nest and enjoy feeling generally your best.
The third trimester, weeks twenty-eight to forty
The final trimester is about growth and maturing for life outside. The lungs are among the last to finish, the brain develops rapidly, and the baby gains weight quickly, from around a kilogram at twenty-eight weeks to a typical three to four kilograms by term.
Movements become strong and regular, and the standard advice is to learn your baby's own pattern and report any clear reduction the same day. Positions settle too, with most babies turning head-down in the final weeks, and you may feel bracing kicks to the ribs.
For the parent, the last stretch brings frequent antenatal checks, Braxton Hicks practice contractions, heartburn, and often a nesting burst of energy. By week forty the due date arrives, though only a minority of babies are born on it and going a little past is normal.
Reading your size and milestone estimates
The comparison of a baby to a fruit or vegetable, from a poppy seed to a pumpkin, is a fun way to grasp growth, but it is illustrative rather than clinical. Medically, what matters is the crown-to-heel length and the head-circumference and femur measurements taken at scans.
Those measurements chart growth along a centile and can differ from your dated week, since babies come in all shapes just like adults. A measurement a few days ahead or behind an expected date is ordinary and not, on its own, something to worry about.
So enjoy the week-by-week snapshots for the sense of progress they give, but let your provider's scans set the real picture of growth, and treat any single figure here as the average it is rather than a precise report on your baby.
Key terms
- Trimester
- One of three phases of pregnancy; first to week 13, second to 27, third to birth.
- Crown to heel
- The standard length of a baby in the womb, measured head to bottom of foot.
- Vernix
- The creamy protective coating on a baby's skin in the womb.
- Braxton Hicks
- Practice contractions that can begin mid-pregnancy and are usually irregular.
Pro tips
- Count weeks from your last period, matching how your clinic dates the pregnancy.
- Enter your due date here to jump straight to the week you are in today.
- Treat fruit sizes as fun illustrations, and scans as the real growth measure.
- In the third trimester, learn baby's movement pattern and report clear changes.
Common mistakes
- Expecting your baby's measurements to match an average week exactly.
- Counting from conception when every care setting counts from the last period.
- Worrying that a scan a few days off the dated week means something is wrong.
- Assuming a due date is a deadline rather than the centre of a birth window.
Week-by-week pregnancy — your questions
How are pregnancy weeks counted?
Pregnancy is counted in weeks from the first day of your last menstrual period, not from conception, so it starts about two weeks before the egg is fertilised. That means at four weeks pregnant you conceived around two weeks ago. The journey runs to forty weeks, split into three trimesters, and every scan, appointment and week-by-week guide, including this one, uses that same last-period convention.
When do I start showing a bump?
Usually somewhere between the end of the first trimester and the middle of the second, commonly around weeks twelve to sixteen for a first pregnancy, though it varies a lot. A bump can appear earlier in a second or later pregnancy as the abdominal muscles are already softened, and body type, height and even whether it is front or back positioned all shift the timing, so there is no single right week.
When will I feel the baby move?
First movements, often felt as light flutters, typically arrive between weeks eighteen and twenty in a first pregnancy, and a little earlier, sometimes from around sixteen weeks, in later pregnancies when you recognise the sensation. Movement becomes stronger and more regular through the second and third trimesters, and from around twenty-eight weeks it is worth learning your baby's own pattern and reporting any clear reduction the same day.
Are the fruit sizes accurate?
They are friendly illustrations rather than clinical measurements, meant to convey roughly how growth accelerates week by week. Medically, what matters is the crown-to-heel length and the head and thigh-bone measurements taken at scans, which chart your baby along a growth centile. A baby can be a few days ahead or behind an average and be perfectly healthy, so treat the fruit comparison as context, not a report card.
Is a forty-week pregnancy exact?
Forty weeks is the conventional full-term span, but it is a centre rather than a deadline. Only a small minority of babies are born on the exact estimated due date, and birth anywhere within a window of a few weeks either side is normal, which is why providers speak of term as roughly weeks thirty-seven to forty-two. Being a little past the due date is common and usually closely monitored rather than alarming.