Week 31 · Third trimester
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Week 31 fetal development and pregnancy guide — length about 41.1cm, weight 约1502g (about the size of a a coconut)

Your baby is now about 41.1cm and 约1502g, about the size of a a coconut.

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

View each day of week 31 →

Week 31 fetal development milestones: length / weight / ultrasound data

Key developmental changes during week 31:

  • Surfactant production approaches the threshold needed to sustain breathing on its own, laying the groundwork for a 34-week-later preterm baby to breathe on its own.
  • The central nervous system matures further and body-temperature regulation strengthens, laying the groundwork for adapting to outside-the-womb conditions after birth.
  • The baby starts storing large amounts of calcium, iron and phosphorus in the bones and liver, reserving them for rapid growth after birth.
  • The iris begins to produce pigment, and the foundation of the baby’s final eye colour (such as brown or blue) starts to take shape.
  • Subcutaneous fat keeps thickening, and the skin changes from thin and red to thicker and more substantial.
  • Regular REM sleep predominates — the baby may be "dreaming", with a clearer routine.
  • Hearing is mature, with a stable response to the parents’ voices and heartbeat rhythm.
  • Most babies’ positions gradually become fixed and turn head-down from this week.
  • Swallowing amniotic fluid is more forceful and hiccups are frequent; the respiratory muscles and digestion are "rehearsing".
  • About 41.1cm long, about the size of a coconut, weighing about 1502g.
Illustration of your baby about the size of a coconut at week 31 (about 41.1cm/约1502g)
Week 31 size: about the size of a a coconut
BPD (biparietal diameter)78.5mmWidth across the head; reflects brain development
AC (abdominal circumference)268mmAbdomen circumference; reflects nutrient absorption
FL (femur length)57.5mmThigh bone length; tracks gestational age
Ultrasound parameters at week 31: BPD 78.5 mm, AC 268 mm, FL 57.5 mm (median reference)
Week 31 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 31 daily development

Your baby quietly grows every day of week 31. Tap any day to see fetal development, maternal changes, and daily guidance for that day.

211Day 1Your baby is storing large amounts of calcium, iron and phosphorus in the bones and liver today — a "mineral strategic reserve" for independent metabolism once out of the womb. The fetal skeleton needs about 30g of calcium cumulatively across the whole pregnancy, depositing about 250-300mg a day in the third trimester (after week 28), all drawn from your diet and bone calcium. The milk and tofu you drink today not only protect your own bone density, but also save "bone deposits" for your baby.212Day 2Eat prune oat porridge — chop 5-6 prunes, add oatmeal and water, cook 10 minutes, no sugar. Prunes are rich in sorbitol (a natural laxative) and oats provide soluble fiber, which works better on a gut under heavy pressure from the uterus than plain honey water. Constipation is many mums’ "silent worry" in the third trimester — don’t squat long or strain, toilet at a set time daily, and don’t invite haemorrhoids.213Day 3Sit down with dad and write a birth plan (1-2 A4 pages is enough): preferred mode of delivery (natural/caesarean), whether to have epidural analgesia (when it’s given, who signs), whether episiotomy is acceptable (state your preference), whether activity and the birthing ball are allowed during labour, whether to delay cord clamping (ACOG recommends 30-60 seconds, WHO 1-3 minutes, with research-supported benefits for the baby), and whether to do rooming-in. Print one copy to put in the hospital bag and hand it to the midwife when entering the labour ward — it doesn’t mean "must be followed", but it makes your wishes heard.214Day 4Is your nipple secreting thick yellowish fluid? A little colostrum in the third trimester is very normal — it’s "liquid gold", rich in immunoglobulins (IgA) and lactoferrin, the baby’s first oral "vaccine" after birth. Don’t squeeze it (it may trigger contractions); just use cotton breast pads to absorb it. If there’s bleeding or continuous heavy leakage with breast redness, swelling and fever, seek care to rule out mastitis.215Day 5Your baby’s iris is starting to produce pigment and the base colour of its eyes is being determined — the final eye colour (brown, blue or grey) only fully appears between 6 months and 1 year after birth, because the melanocytes need light stimulation after birth to mature further. Today you can look at the colour of your own pupils in the mirror and guess whether the baby will take after you or dad.216Day 6To be clear again: the "light fetal education" of shining a torch on the belly is not recommended. Your baby’s visual system is very sensitive in the third trimester and can already sense the faint light through the abdominal wall, but deliberately stimulating with bright light as "visual training" has no scientific benefit and may instead disturb its rest. The best "visual stimulation" is your daily, regular exposure to natural light — getting 15 minutes of sun, sitting by a window to chat and read is already helping it tell day from night.217Day 7The small scan’s final window — if you haven’t done it, you must complete it this week (don’t let it pass week 32). It looks not only at fetal size and growth rate, but the more crucial indicators: whether the placental position is normal (low-lying placenta needs follow-up monitoring), the umbilical-artery blood-flow resistance value (judging placental transfer efficiency) and amniotic-fluid volume. These data are the navigation map for your third-trimester safety management.

Week 31 belly changes

Illustration of belly changes at week 31: uterine position and abdominal prominence

At week 31 (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 31 maternal body changes and common symptoms

  • The uterine fundus is about 11cm above the navel.
  • Cumulative weight gain is about 9.5-13.5kg (about 0.4-0.5kg a week in the third trimester for a standard BMI).
  • The breasts may secrete more colostrum (yellow, sticky "liquid gold", which is normal).
  • Constipation may worsen again (the uterus presses the intestines and progesterone slows bowel movement).
  • Bloating and heartburn may persist.
  • Leg swelling, cramps and varicose veins may become more obvious.
  • Sleep difficulty and Braxton Hicks false contractions may increase.
  • Itchy abdominal skin, stretch marks and lower-back pain may worsen.

Week 31 nutrition advice and recommended recipes

Constipation recurs easily in the third trimester — eat more of the naturally laxative sorbitol-rich foods (prunes, pears, apples, kiwi), and keep up dietary fiber (25-30g a day) and water (1.5-2L). Daily targets: quality protein about 80-90g, iron about 27-29mg, calcium 1000mg, DHA 200-300mg, fiber 25-30g, iodine 230μg. Keep controlling sugar (strict for GDM). Avoid alcohol, raw or unpasteurised milk products, high-mercury fish, caffeine ≤200mg a day.

DHA200-300mgBrain & vision development
Recommended recipePrune oat porridge

The sorbitol in prunes is a natural laxative, oats add dietary fiber, and the warm porridge relieves third-trimester constipation. You can add a few walnuts for DHA; if blood sugar runs high, control the total amount and eat it with protein for a steadier rise.

Week 31 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. A special note: the "light fetal education" (shining a torch on the belly to guide vision) circulating online has limited evidence and isn’t recommended — although the baby’s eyelids can open and close, light through the abdominal wall is faint with no proven benefit, and bright light may disturb its rest. What you can do: talk gently or read a picture book 5-10 minutes at a set time daily, combined with gentle belly stroking; choose soothing music or white noise at a volume ≤60 decibels for 15-20 minutes a day to relax yourself. Treating fetal education as a ritual of caring for mum’s mood is the most practical.

Week 31 partner's role

①Discuss with your partner and write a "birth plan": preferred mode of delivery, whether to have epidural analgesia, whether to accompany the birth, whether to delay cord clamping, rooming-in, breastfeeding intention, etc., print it and take it to the hospital. ②Keep filling the hospital bag and confirm baby products are in place. ③Talk or sing close to the belly at a set time daily to build a father’s voice the baby recognises. ④Help relieve her constipation: prepare laxative foods such as prunes or kiwi, remind her to drink water, and walk with her after meals.

Week 31 checkup reminders

Weeks 30-32: the small scan (anomaly-late ultrasound, assessing growth, amniotic-fluid volume, placental position, umbilical blood flow and fetal position) plus a routine visit (blood pressure, weight, fundal height, abdominal circumference, fetal heart and urine protein). If not done, complete it this week. Since week 28, visits are every two weeks with daily fetal-movement counting. You can also learn about the 35-37-week GBS screening and the 27-36-week Tdap vaccine.

Week 31 key concerns

Complete the small scan (30-32 weeks)Prevent constipationDiscuss and write a birth planColostrum care30 minutes of moderate exercise daily (ACOG recommends 150 minutes a week across pregnancy)

Signs that need immediate care at week 31

If any of the following occurs, contact your obstetrician or seek care promptly: ① abnormal fetal movement (under 10 in 2 hours or a drop of more than 50% from usual) → seek care immediately; ② sudden vaginal leakage of fluid (possible premature rupture of membranes); ③ regular contractions (once every 10 minutes or more often) with lumbar heaviness/downward pressure (watch for preterm labour); ④ persistent severe headache, blurred vision or upper-abdominal pain with raised blood pressure or sudden swelling (watch for preeclampsia); ⑤ itching all over, especially palms and soles with no rash (watch for ICP); ⑥ a fever over 38℃ that does not come down; ⑦ swelling, warmth and pain in one leg or sudden breathing difficulty and chest pain (watch for a clot/pulmonary embolism); ⑧ sudden obvious swelling or persistent abdominal pain.

Week 31 frequently asked questions

How do I write a birth plan?
Include: preferred mode of delivery (the medical indications for natural/caesarean), whether to have epidural analgesia, whether episiotomy is acceptable, delivery-position preferences, whether to delay cord clamping, rooming-in, and breastfeeding intention. Print a copy to take to the hospital so staff understand your wishes, but with maternal-fetal safety as the priority, adjust per the doctor’s advice.
What’s the latest time for the small scan?
The small scan is generally arranged at weeks 30-32, so at week 31 you’re still in the window — complete it as soon as possible and don’t let it pass week 32. It assesses fetal growth, amniotic-fluid volume, placental position and maturity, fetal position and umbilical blood flow.
What is colostrum at week 31 — should I express it?
Colostrum is the thick yellow milk secreted first after birth, rich in antibodies and nutrients, known as "liquid gold". A little colostrum in pregnancy is normal — don’t deliberately squeeze or stimulate it (it may trigger contractions); after birth, let the baby suckle as early as possible.
Constipation has worsened again at week 31 — what should I do?
A growing uterus pressing the intestines and progesterone slowing bowel movement make constipation worse. Eat more sorbitol-rich foods such as prunes, pears and kiwi, coarse grains and vegetables for fiber, drink 1.5-2L of water daily, walk after meals, and toilet regularly without squatting long. Iron supplements can worsen constipation — discuss an adjustment with your doctor; in serious cases choose a pregnancy-safe laxative rather than a stimulant laxative.
How much weight gain is normal at week 31?
A cumulative gain of about 9.5-13.5kg by week 31 is common. For a standard BMI, the third trimester is about 0.4-0.5kg a week and the whole pregnancy recommends about 11.5-16kg. Watch the overall trend and manage by BMI with a weight-gain calculator.
Is the baby storing calcium and iron at week 31?
Yes. In the third trimester the baby starts storing large amounts of calcium, iron and other minerals in preparation for birth, so you should keep calcium (1000mg a day) and iron (about 27-29mg a day) ample — food first, supplements per the doctor’s prescription when insufficient.
Should I do "light fetal education" at week 31?
It’s not recommended. Shining a torch on the belly for "light fetal education" has limited evidence and no proven benefit; light through the belly is faint and may disturb the baby’s rest. Talking, reading picture books and listening to soothing music are steadier choices for fetal education.
How do I read the week-31 ultrasound report?
The three measurements at 31 weeks: BPD (biparietal diameter) about 78.5mm, AC (abdominal circumference) about 268.0mm, and FL (femur length) about 57.5mm, reflecting head, abdomen and leg development. If the three are in step, growth is good; a single slight deviation isn’t necessarily abnormal — go by your obstetrician’s overall assessment.
Are the fetal movements regular at week 31?
Most babies have obvious active periods. Since week 28 you should count fetal movements daily: one hour in the morning, afternoon and evening each; ≥10 within 2 hours is normal. If it’s clearly under 10 in 2 hours or down by more than half from usual, seek care immediately.
Is there a recommended exercise duration in pregnancy?
ACOG advises women without complications about 150 minutes of moderate-intensity exercise accumulated weekly (walking, prenatal yoga, swimming), split over 5 days at about 30 minutes a day. In the third trimester focus on walking and stretching; avoid intense exercise and pressure on the abdomen; stop and seek care for pain or bleeding.
Do I still need a vaccine in the third trimester?
Yes. The CDC and ACOG recommend the Tdap whooping-cough vaccine at weeks 27-36 to prevent Pertussis in the baby; the inactivated flu vaccine can also be given in flu season. Both are pregnancy-safe vaccines; tell them you’re pregnant before vaccination.
Can I have sex at week 31?
If there’s no placenta praevia, bleeding, cervical insufficiency or threatened preterm labour, gentle sex is fine using positions that don’t press on the belly and with gentle movements. The closer to term, the more caution in the third trimester; pause if unwell or the doctor advises against it; seek care for post-sex bleeding, pain or leaking fluid.
What about leg cramps and swelling at week 31?
When a cramp hits, flex the foot and massage the calf; prevention relies on supplementing calcium and magnesium, drinking water and stretching before bed. For swelling, raise your legs, lie on your left side, walk to promote return flow, and moderately limit salt. If swelling suddenly worsens, spreads to the face and hands, or comes with headache or vision changes, check blood pressure to rule out preeclampsia.
What should I avoid eating at week 31?
Avoid raw or undercooked meat, fish and eggs, high-mercury large fish, unpasteurised dairy, alcohol, caffeine over 200mg a day, and excess animal liver. If you have GDM, strictly control refined sugar and sugary drinks as well.
What about heartburn and bloating at week 31?
Eat smaller meals more often and chew slowly, sit or stand for 30 minutes after meals before lying down, avoid eating in the 2-3 hours before bed, avoid spicy, greasy, overly sweet food and carbonated drinks, and prop up your upper body when sleeping. In serious cases, use a pregnancy-safe antacid under a doctor’s guidance.
What about lower-back pain at week 31?
It comes from a forward centre of gravity and ligament relaxation. Avoid long standing or sitting, wear supportive low-heeled shoes, use a belly-support belt, side-sleep with a pregnancy pillow, do prenatal yoga and pelvic-floor exercise, and apply local warmth. Sharp pain on one side or with bleeding and fever needs care.
What if I sleep poorly at week 31?
A growing belly, frequent urination and trouble turning over affect sleep. Use a pregnancy pillow for support, sleep on your left side, soak your feet before bed to relax, reduce evening fluids to cut night urination, and exercise moderately during the day. Avoid screens before bed and caffeinated drinks.
What should go in the hospital bag?
It sorts into three: mum’s supplies (maternity pads, nursing bra, toiletries), baby’s supplies (NB nappies, swaddle, going-home outfit), and documents (ID card, social-security card, prenatal records). Prepare it in batches before week 34 and keep the list by the door so you can leave at a moment’s notice.
⚡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 31 daily focus

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

Day 1Your baby is storing large amounts of calcium, iron and phosphorus in the bones and liver today — a "mineral strategic reserve" for independent metabolism once out of the womb. The fetal skeleton needs about 30g of calcium cumulatively across the whole pregnancy, depositing about 250-300mg a day in the third trimester (after week 28), all drawn from your diet and bone calcium. The milk and tofu you drink today not only protect your own bone density, but also save "bone deposits" for your baby.Day 2Eat prune oat porridge — chop 5-6 prunes, add oatmeal and water, cook 10 minutes, no sugar. Prunes are rich in sorbitol (a natural laxative) and oats provide soluble fiber, which works better on a gut under heavy pressure from the uterus than plain honey water. Constipation is many mums’ "silent worry" in the third trimester — don’t squat long or strain, toilet at a set time daily, and don’t invite haemorrhoids.Day 3Sit down with dad and write a birth plan (1-2 A4 pages is enough): preferred mode of delivery (natural/caesarean), whether to have epidural analgesia (when it’s given, who signs), whether episiotomy is acceptable (state your preference), whether activity and the birthing ball are allowed during labour, whether to delay cord clamping (ACOG recommends 30-60 seconds, WHO 1-3 minutes, with research-supported benefits for the baby), and whether to do rooming-in. Print one copy to put in the hospital bag and hand it to the midwife when entering the labour ward — it doesn’t mean "must be followed", but it makes your wishes heard.Day 4Is your nipple secreting thick yellowish fluid? A little colostrum in the third trimester is very normal — it’s "liquid gold", rich in immunoglobulins (IgA) and lactoferrin, the baby’s first oral "vaccine" after birth. Don’t squeeze it (it may trigger contractions); just use cotton breast pads to absorb it. If there’s bleeding or continuous heavy leakage with breast redness, swelling and fever, seek care to rule out mastitis.Day 5Your baby’s iris is starting to produce pigment and the base colour of its eyes is being determined — the final eye colour (brown, blue or grey) only fully appears between 6 months and 1 year after birth, because the melanocytes need light stimulation after birth to mature further. Today you can look at the colour of your own pupils in the mirror and guess whether the baby will take after you or dad.Day 6To be clear again: the "light fetal education" of shining a torch on the belly is not recommended. Your baby’s visual system is very sensitive in the third trimester and can already sense the faint light through the abdominal wall, but deliberately stimulating with bright light as "visual training" has no scientific benefit and may instead disturb its rest. The best "visual stimulation" is your daily, regular exposure to natural light — getting 15 minutes of sun, sitting by a window to chat and read is already helping it tell day from night.Day 7The small scan’s final window — if you haven’t done it, you must complete it this week (don’t let it pass week 32). It looks not only at fetal size and growth rate, but the more crucial indicators: whether the placental position is normal (low-lying placenta needs follow-up monitoring), the umbilical-artery blood-flow resistance value (judging placental transfer efficiency) and amniotic-fluid volume. These data are the navigation map for your third-trimester safety management.

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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