Week 40 · Third trimester
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Week 40 fetal development and pregnancy guide — length about 51.2cm, weight 约3462g (about the size of a a watermelon)

Your baby is now about 51.2cm and 约3462g, about the size of a a watermelon.

Medical references: WHO INTERGROWTH-21st · China Prenatal & Pregnancy Care Guide · ACOG

View each day of week 40 →

Week 40 fetal development milestones: length / weight / ultrasound data

Key developmental changes during week 40:

  • The due date arrives, but only about 5% of babies are born exactly then.
  • All body systems meet the "shipping standard", ready to be born at any time.
  • Fetal-movement amplitude is very small (limited space), but the frequency should not clearly decrease.
  • Maternal antibodies (IgG) are ample, laying a base for protection after birth.
  • Fat reserves are very ample and the body is full.
  • All systems are mature and can operate independently.
  • Hearing is mature; it can recognise and prefer Mum’s heartbeat and voice.
  • The nails are long and need trimming after birth to prevent scratching.
  • Meconium accumulates in the bowel, to be passed after birth.
  • The hair is fairly long.
  • The cranial sutures are not yet fully closed, leaving the anterior and posterior fontanelles; the head can override and reshape moderately in labour to pass the birth canal.
  • The gut flora isn’t established yet; after birth it gradually colonises through breast milk and the birth-canal flora, influencing early immunity.
Illustration of your baby about the size of a watermelon at week 40 (about 51.2cm/约3462g)
Week 40 size: about the size of a a watermelon
BPD (biparietal diameter)93mmWidth across the head; reflects brain development
AC (abdominal circumference)353.3mmAbdomen circumference; reflects nutrient absorption
FL (femur length)72.4mmThigh bone length; tracks gestational age
Ultrasound parameters at week 40: BPD 93 mm, AC 353.3 mm, FL 72.4 mm (median reference)
Week 40 three core ultrasound parameters (median; varies by individual)

* Values are medians from WHO/China standards; individual variation is normal. References: WHO INTERGROWTH-21st, China Prenatal & Pregnancy Care Guide, ACOG guidelines.

Week 40 daily development

Your baby quietly grows every day of week 40. Tap any day to see fetal development, maternal changes, and daily guidance for that day.

274Day 1Today is the due date — but only about 5% of babies are born exactly then. If it hasn’t started, don’t feel you "missed it". The due date is the midpoint of a statistical range, not a delivery deadline. Today you could buy yourself a bunch of favourite flowers, have an unhurried meal with your partner and watch a movie you’ve wanted to see. You have perfectly completed the whole pregnancy — the day the baby chooses hardly matters now; what matters is that you have stored up all your love and preparation for it.275Day 2Take one last pregnancy-bump photo — stand before the same mirror, in the same clothes, in the same pose, and line it up beside the first photo at week 12, the halfway one at week 20 and the mid-term one at week 32. Four time points, four photos, a 40-week journey through space and time — this is the most silent, most magnificent visual record you write to your body and your baby.276Day 3Watch a light comedy movie with the father tonight, with sugar-free popcorn and lemonade. The joy of laughing and relaxing lets your parasympathetic nervous system dominate and promotes natural oxytocin secretion — the safest "induction" that costs nothing and has no side effects. A relaxed mood positively affects birth, and if not "metaphysics", it is at least a "last exercise" of pregnancy worth trying.277Day 4Keep counting fetal movements every day without slackening — even after the due date. If today’s count is less than half of yesterday’s, or fewer than 10 in 2 hours, don’t reassure yourself with "the baby just has less space" — go to hospital for an NST to confirm. Better to make an extra trip on a false alarm than to miss a signal because of "it’s probably fine".278Day 5Today ask your prenatal doctor: if it still hasn’t started after week 41, what is the exact induction date? What is the plan (cervical balloon or oxytocin)? Will you need to fast beforehand? What date and time, and which floor, do you check in? Write these questions one by one in your phone notes and set an alarm the day before induction. You aren’t "passively waiting for notice" — you know the timetable and can actively own the final stretch of waiting.279Day 6Write the last pregnancy diary — look back on taking folic acid every day from week 1, seeing the first heartbeat ultrasound at week 8, feeling butterfly-like fetal movement for the first time at week 18, starting to count movements at week 28, the lungs basically maturing at weeks 35-36, full term at week 37 — draw a small star beside each milestone as you finish it. When all the stars join in a line, that line spanning weeks 1-40 is the greatest journey you have walked.280Day 7Today walk no more than 20 minutes and don’t climb stairs — after the due date your centre of gravity is extremely unsteady, ligaments are slack and fatigue accumulates, so the fall risk is higher than at any other week. Choose flat, quiet ground, wear non-slip sports shoes and walk holding your partner’s arm or a light trekking pole. The aim of exercise isn’t to "induce labour" but to "let the body relax while waiting" — relaxation linking to oxytocin secretion is the scientific logic.

Week 40 belly changes

Illustration of belly changes at week 40: uterine position and abdominal prominence

At week 40 (third trimester) the uterine fundus keeps rising closer to the sternum. Your belly is clearly rounded and your waistline and fundal height grow quickly, so pay more attention to balance and safety in daily movement.

Week 40 maternal body changes and common symptoms

  • If it still hasn’t started, don’t be anxious — before 41 weeks is all normal.
  • Keep monitoring fetal movement and counting every day.
  • Contractions may increase but aren’t yet regular enough for labour.
  • Bloating, heartburn and constipation may persist as the baby hasn’t been born; keep the diet light.
  • Lower-limb swelling, cramps and varicose veins continue; walk to promote return flow and limit salt.
  • Sleep difficulty and back or pubic-bone pain are clearer from overdue anxiety; mindful relaxation helps.
  • Itchy abdominal skin, stretch marks and urine leaking, with frequent false contractions; still count fetal movements past the due date.
  • The breasts enlarge and colostrum is secreted; breastfeeding may start at any time, so have nursing items ready.

Week 40 nutrition advice and recommended recipes

If labour still hasn’t started on the due date, continue a normal balanced diet and adequate fluids; no need to deliberately "supplement" or "induce". Eat a nutritious, good-looking meal (mood matters too) — a cheerful mood lets the body secrete oxytocin, which favours natural onset. Daily targets still follow authority guidance: quality protein about 80-90g, iron about 27-29mg, calcium 1000mg, DHA 200-300mg, fiber 25-30g, iodine 230μg. Avoid alcohol, raw or unpasteurised milk products, high-mercury fish, caffeine ≤200mg a day.

DHA200-300mgBrain & vision development
Recommended recipeChicken-soup noodles

Steaming hot, warm and satisfying — chicken soup is easily absorbed and the noodles are soft without filling the stomach. The baby isn’t in a hurry and neither are you; eat well to keep your strength; skim the fat and don’t over-salt. Gout sufferers eat moderately.

Week 40 prenatal education guide

The core of "fetal education" is still to help the mother-to-be relax and go with the flow; there is no strong evidence that any specific stimulation makes the baby smarter. This week you can do a final "pregnancy keepsake" with the father: take one last pregnancy-bump photo, speak one last speech to the baby, write one last pregnancy diary — a precious family keepsake that helps you sort out your feelings and stay cheerful; its value lies in caring for your own mood, not "teaching" the baby anything.

Week 40 partner's role

①On the due date, stay relaxed and don’t be tense. If it hasn’t started, take your partner on a walk, watch a light movie and make a warm dinner. ②Most important: help her relax — a cheerful mood lets the body secrete oxytocin, a benign catalyst for natural onset. ③Talk or sing close to the belly at a set time daily to build a father’s voice the baby recognises. ④Be on stand-by: confirm the hospital bag, the route and a full phone charge.

Week 40 checkup reminders

Week 40: weekly prenatal visit + NST fetal-heart monitoring. If it still hasn’t started after week 41, the doctor will arrange an induction plan (generally before 41 weeks to reduce the risk of a post-term pregnancy). Keep counting fetal movements daily. Seek care immediately for regular 511 contractions, rupture of membranes, bloody show or reduced movement.

Week 40 key concerns

Keep counting fetal movements dailyPlan induction at week 41 (if needed)Keep your mindset relaxedBe on stand-by at any time30 minutes of moderate exercise daily (ACOG recommends 150 minutes a week across pregnancy)

Signs that need immediate care at week 40

If any of the following occurs, contact your obstetrician or seek medical help promptly: ① follow the 511 rule (regular contractions every 5 minutes, each over 1 minute, for over 1 hour) → go to hospital; ② rupture of membranes → lie flat immediately with the hips raised and seek care; ③ clearly reduced fetal movement (fewer than 10 in 2 hours or less than half the usual); ④ severe headache, blurred vision or severe upper-abdominal pain with raised blood pressure or sudden oedema (watch for preeclampsia); ⑤ itching all over, especially the palms and soles with no rash (watch for ICP); ⑥ a fever over 38℃ that does not come down; ⑦ one leg swollen, hot and painful, or sudden breathlessness and chest pain (watch for a clot/pulmonary embolism); ⑧ a large amount of bright-red vaginal bleeding (watch for placenta praevia). Don’t ignore anything that feels "off" — an extra trip is better than missing a signal.

Week 40 frequently asked questions

Worried if not in labour by the due date?
No need to worry at all — only about 5% of babies are born exactly on the due date; 40-41 weeks are both within the normal full-term range. Stay relaxed, keep counting fetal movements daily and attend weekly visits + NST.
What can I do at week 40 if it won’t start?
Safe practices: walking (promotes engagement and cervical ripening), staying relaxed and keeping a regular routine. "Stair-climbing", "eating spicy food" and "nipple stimulation to induce labour" lack solid evidence or carry risk and need doctor guidance — don’t try them on your own. Patience plus proper prenatal care is the safest.
How much weight gain is normal at week 40?
A cumulative gain of about 12-16kg by week 40 is common. Weight may plateau or dip a little at term — normal; the recommended total gain is about 11.5-16kg (standard BMI), so watch the overall trend.
How to read the week-40 ultrasound report?
The three measurements at 40 weeks: BPD (biparietal diameter) about 93.0mm, AC (abdominal circumference) about 353.3mm, and FL (femur length) about 72.4mm. If the three are in step, growth is good; a single slight deviation isn’t necessarily abnormal — go by your obstetrician’s overall assessment.
Is a small fetal-movement amplitude normal at week 40?
After term, limited space makes a smaller amplitude (force/range) normal, but the count should not clearly drop. Count daily: one hour morning, noon and evening, with ≥10 in 2 hours normal; if clearly under 10 in 2 hours or less than half the usual, seek care immediately.
Must I be induced past the due date?
Not "definitely", but induction before 41 weeks is generally advised (some hospitals 40⁺⁶-41 weeks) to lower the risks of a post-term pregnancy (≥42 weeks), such as reduced fluid and deteriorating placental function. The timing is decided by the doctor with your cervical condition, baby and placental assessment.
Can I be induced at week 40?
With a medical indication or on reaching week 41, the doctor arranges a standard induction (such as a cervical balloon or oxytocin), fully monitored in hospital. Don’t try folk methods on your own. Whether and when to induce follows the doctor’s arrangements.
Is there a recommended exercise duration in pregnancy?
ACOG recommends about 150 minutes of moderate-intensity exercise accumulated weekly. At week 40, if it hasn’t started, favour walking and stretching; moderate walking helps engagement and relaxes you, but avoid stair-climbing or intense exercise; seek care immediately for regular 511 contractions, rupture of membranes, bleeding or clearly reduced movement; induction may be arranged before week 41.
Can I have sex at week 40?
After term, sex may trigger contractions and is generally advised against. With no contraindications and the doctor’s permission, very gentle sex could be acceptable, but most doctors advise pausing; see a doctor immediately for bleeding, pain or fluid leakage after sex.
What about leg cramps and oedema at week 40?
For a cramp, flex the foot and massage the calf; prevention centres on calcium, magnesium, water and stretching before bed. Continued oedema and cramps: raise the legs, lie on your left side, walk to promote return flow and limit salt; if the swelling suddenly worsens, spreads to the face and hands or comes with headache/vision changes, check blood pressure to rule out preeclampsia.
What should I avoid eating at week 40?
Still avoid raw or undercooked meat, fish and eggs, high-mercury large fish, unpasteurised dairy, alcohol, caffeine up to 200mg a day, and excess animal liver; GDM mums strictly control sugar. Don’t try folk induction like castor oil, spicy food or nipple stimulation — the risk outweighs the benefit; real induction is done in hospital with standard methods (balloon, oxytocin).
What about heartburn and bloating at week 40?
With the baby not yet born, the stomach is still pressed and heartburn/bloating may persist. Eat small frequent meals, chew slowly, stay sitting 30 minutes after a meal before lying down, don’t eat within 2-3 hours before bed, avoid spicy, greasy, overly sweet food and fizzy drinks, and prop up the upper body; in severe cases use a pregnancy-safe antacid as directed — don’t eat randomly just to "get labour going".
What about constipation and haemorrhoids at week 40?
Eat more coarse grains, fruit and vegetables for fiber, drink 1.5-2L of water daily, walk after meals and toilet on a regular schedule without squatting long; warm-water sitz baths help. Iron supplements can worsen constipation — discuss adjusting them with your doctor; for severe constipation choose a pregnancy-safe medication and avoid stimulating laxatives.
What about lower-back and pubic-bone pain at week 40?
At term, forward centre of gravity and relaxin loosening the pelvic joints, plus overdue anxiety, make back/pubic pain clearer. Avoid long standing or sitting, wear supportive low shoes, use a belly-support belt, side-sleep with a pregnancy pillow, do prenatal yoga and pelvic-floor exercises and apply local heat; sharp pain on one side, or with bleeding or fever, needs medical care.
Can’t sleep well and anxious at week 40?
Anticipation and anxiety around the due date are common, and the more anxious the harder to sleep. Use a pregnancy pillow for support, side-sleep, soak your feet before bed, walk in the day and communicate more with your partner; remember a relaxed mood helps natural oxytocin secretion; if anxiety is marked, talk to the doctor and an NST confirming all is well reassures you.
Are frequent false contractions at week 40 normal?
Frequent false contractions at week 40 are fairly common: brief tightening, painless, irregular, resolving with rest. If they become regular, painful or come with lumbar heaviness (meeting 511), seek care to rule out labour; even past the due date keep counting movements daily and seek care promptly for anything that feels off.
What should the hospital bag contain?
Three categories: maternal items (postnatal pads, nursing bra, toiletries, straw cup), baby items (NB nappies, swaddling, going-home clothes, small hat) and documents (ID, social-security card, prenatal record). By week 40 it should have stood by the door for a while, ready to go; if induction is set for week 41, re-check it against the admission date.
What can I do at week 40 to move things along?
There is no reliable "natural induction" trick. What you can do: moderate walking, stay relaxed and ensure rest and energy. When induction is truly needed, the hospital uses standard methods (balloon, oxytocin) — safe and controlled.
Do I still have NST at week 40?
Yes. After week 40 NST fetal-heart monitoring is still advised weekly (or more) to assess the baby’s wellbeing until delivery; after 41 weeks monitoring may be intensified. This is an important way to keep the baby safe, so don’t skip appointments just because labour hasn’t started.
⚡The gestational-development data on this tool is compiled from the WHO INTERGROWTH-21st fetal growth standards, ACOG (American College of Obstetricians and Gynecologists), and the Chinese Medical Association's Guidelines for Pre-conception and Prenatal Care, and is reviewed for consistency by obstetrics consultants against those authoritative standards. Daily developmental tips, maternal changes, and nutrition advice follow an evidence-based approach. Note that fetal length is measured differently by week: crown–rump length (CRL) at or before week 20, and crown–heel length (CHL) from week 21 — values either side of week 21 are not directly comparable, which is a normal measurement change.

Data sources & notes

This tool provides algorithm-based estimates from the authoritative standards above for pregnancy time planning and general reference. It does not replace a doctor's diagnosis or individualized medical advice; follow your obstetrician for all prenatal care decisions.

Week 40 daily focus

Key daily tips for this week; tap any item to view the full development and guidance for that day.

Day 1Today is the due date — but only about 5% of babies are born exactly then. If it hasn’t started, don’t feel you "missed it". The due date is the midpoint of a statistical range, not a delivery deadline. Today you could buy yourself a bunch of favourite flowers, have an unhurried meal with your partner and watch a movie you’ve wanted to see. You have perfectly completed the whole pregnancy — the day the baby chooses hardly matters now; what matters is that you have stored up all your love and preparation for it.Day 2Take one last pregnancy-bump photo — stand before the same mirror, in the same clothes, in the same pose, and line it up beside the first photo at week 12, the halfway one at week 20 and the mid-term one at week 32. Four time points, four photos, a 40-week journey through space and time — this is the most silent, most magnificent visual record you write to your body and your baby.Day 3Watch a light comedy movie with the father tonight, with sugar-free popcorn and lemonade. The joy of laughing and relaxing lets your parasympathetic nervous system dominate and promotes natural oxytocin secretion — the safest "induction" that costs nothing and has no side effects. A relaxed mood positively affects birth, and if not "metaphysics", it is at least a "last exercise" of pregnancy worth trying.Day 4Keep counting fetal movements every day without slackening — even after the due date. If today’s count is less than half of yesterday’s, or fewer than 10 in 2 hours, don’t reassure yourself with "the baby just has less space" — go to hospital for an NST to confirm. Better to make an extra trip on a false alarm than to miss a signal because of "it’s probably fine".Day 5Today ask your prenatal doctor: if it still hasn’t started after week 41, what is the exact induction date? What is the plan (cervical balloon or oxytocin)? Will you need to fast beforehand? What date and time, and which floor, do you check in? Write these questions one by one in your phone notes and set an alarm the day before induction. You aren’t "passively waiting for notice" — you know the timetable and can actively own the final stretch of waiting.Day 6Write the last pregnancy diary — look back on taking folic acid every day from week 1, seeing the first heartbeat ultrasound at week 8, feeling butterfly-like fetal movement for the first time at week 18, starting to count movements at week 28, the lungs basically maturing at weeks 35-36, full term at week 37 — draw a small star beside each milestone as you finish it. When all the stars join in a line, that line spanning weeks 1-40 is the greatest journey you have walked.Day 7Today walk no more than 20 minutes and don’t climb stairs — after the due date your centre of gravity is extremely unsteady, ligaments are slack and fatigue accumulates, so the fall risk is higher than at any other week. Choose flat, quiet ground, wear non-slip sports shoes and walk holding your partner’s arm or a light trekking pole. The aim of exercise isn’t to "induce labour" but to "let the body relax while waiting" — relaxation linking to oxytocin secretion is the scientific logic.

Content sources & notes

This guide references WHO INTERGROWTH-21st standards, the China Prenatal & Pregnancy Care Guide, ACOG clinical guidelines and BabyCenter (medically reviewed).

This page is educational pregnancy information for general reference and does not replace the diagnosis or advice of a licensed healthcare provider.

View the full fetal BPD/AC/FL reference table (weeks 14–42) →

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