Week 33 · Third trimester
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Week 33 fetal development and pregnancy guide — length about 43.7cm, weight 约1918g (about the size of a a pineapple)

Your baby is now about 43.7cm and 约1918g, about the size of a a pineapple.

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

View each day of week 33 →

Week 33 fetal development milestones: length / weight / ultrasound data

Key developmental changes during week 33:

  • The skull is linked by the unfused fontanelles and sutures, so it can slightly overlap during delivery to ease passage through the birth canal.
  • The immune system matures faster, obtaining large amounts of antibodies (IgG) from the mother, and protection steadily strengthens.
  • Subcutaneous fat makes the skin no longer translucent, changing from red to pink.
  • Swallowing and breathing movements are more coordinated.
  • The nails have grown past the fingertips and need trimming after birth.
  • The lungs keep maturing — the alveoli branch further and surfactant increases.
  • The cerebral cortex sulci and gyri grow more complex and the neural connections are denser.
  • The auditory cortex is fully developed, with a startle reflex to sudden sounds.
  • Fetal movement has clear active and quiet periods.
  • The bones keep hardening, but the skull remains soft to aid delivery.
  • About 43.7cm long, about the size of a pineapple, weighing about 1918g.
Illustration of your baby about the size of a pineapple at week 33 (about 43.7cm/约1918g)
Week 33 size: about the size of a a pineapple
BPD (biparietal diameter)83mmWidth across the head; reflects brain development
AC (abdominal circumference)289.1mmAbdomen circumference; reflects nutrient absorption
FL (femur length)61.4mmThigh bone length; tracks gestational age
Ultrasound parameters at week 33: BPD 83 mm, AC 289.1 mm, FL 61.4 mm (median reference)
Week 33 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 33 daily development

Your baby quietly grows every day of week 33. Tap any day to see fetal development, maternal changes, and daily guidance for that day.

225Day 1Your baby’s skin has completely changed from translucent to pink baby skin — the subcutaneous fat is finally thick enough to cover the blood vessels, no longer showing the purplish-red through. Another function of this fat layer is to store energy for maintaining body temperature after birth — leaving your 37°C womb, your baby faces rapid heat loss, and the fat is its "built-in warmer".226Day 2Today’s calcium goal: a cup of milk in the morning (about 300mg calcium), tofu or dark leafy greens at lunch, and a cup of yoghurt at dinner, with calcium-rich food arranged across the three meals. ACOG advises about 1000mg a day in pregnancy — you don’t "reach for calcium tablets when food isn’t enough", but first look at whether your three meals systematically include milk and soy products. Take calcium and iron at least 2 hours apart, or they compete for absorption.227Day 3Today upgrade your Kegel exercise to the advanced version — contract the pelvic-floor muscles for 10 seconds → fully relax 10 seconds, repeating 10 rounds, 3 sets a day. Whether sitting typing or side-lying scrolling the phone, you can do it; the key is that you feel the "muscles around the anus and vagina pulling inward and upward", not your legs and buttocks tightening. People who persist with Kegels have a clearly lower incidence of postpartum stress incontinence (about 30% in evidence-based studies) — this is a real "leak-proofing investment".228Day 4When your baby kicks on the belly, gently tap the spot near the kick twice and say in a gentle voice "Baby, it’s me" — your familiar voice brings a settling response. This isn’t "teaching" anything but building an in-womb interaction ritual. Note that when the baby is quiet, don’t actively poke, tap or rock the belly to "wake it" — babies’ sleep is important for brain development.229Day 5This week narrow the baby-name choices to 2-3 with dad, write each on a small card and read it aloud several times — read loudly, softly, and in the tone you’ll use to wake it up, and see which is most natural and brings the quickest smile to your face. Check the names for bad homophones and write the final candidate on a newborn birth card to have ready.230Day 6Today’s hospital-bag check is the last "full inventory": empty everything onto the bed and check quantities, expiry dates (pads/care sheets/nappies), and any gaps (is the power bank charged? is the straw cup in? is the newborn going-home swaddle thick enough?), then repack, zip and lock the bag by the door. Tick off each correct item on the list — you don’t need "roughly ready"; you need "anyone could grab this bag and take me to hospital".231Day 7The Tdap whooping-cough vaccine is still in its vaccination window this week (weeks 27-36); if you haven’t had it, call the community health centre or maternity hospital today to book it. After vaccination your body produces Pertussis antibody that passes through the placenta to the fetus — newborn protection in the first few weeks relies mainly on it; it’s the only defence before the baby is vaccinated itself. Slight local soreness within 24 hours is a normal reaction.

Week 33 belly changes

Illustration of belly changes at week 33: uterine position and abdominal prominence

At week 33 (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 33 maternal body changes and common symptoms

  • The uterine fundus is about 13cm above the navel.
  • Cumulative weight gain is about 10.5-14.5kg (about 0.4-0.5kg a week in the third trimester for a standard BMI).
  • There may be slight urine leaking (stress incontinence when coughing, laughing or sneezing).
  • Bloating, heartburn and constipation may persist.
  • Leg swelling, cramps and varicose veins may become more obvious.
  • Sleep difficulty and lower-back or pubic-bone pain may worsen.
  • Itchy abdominal skin and stretch marks.
  • The breasts enlarge, colostrum is secreted, and Braxton Hicks false contractions may increase.

Week 33 nutrition advice and recommended recipes

The calcium need in the third trimester is about 1000mg a day (ACOG view; China’s current DRIs 2023 edition is 800mg a day) — food first: milk and dairy, soy products, tahini and dark-green vegetables, arranging calcium-rich food in each meal (milk at breakfast, tofu and leafy greens at lunch, yoghurt at dinner). At the same time, daily targets: quality protein about 80-90g, iron about 27-29mg, DHA 200-300mg, fiber 25-30g, iodine 230μg. If calcium deficiency is obvious, add a calcium supplement per the doctor (taken separate from iron). Avoid alcohol, raw or unpasteurised milk products, high-mercury fish, caffeine ≤200mg a day.

DHA200-300mgBrain & vision development
Recommended recipeMilk steamed egg custard

Milk + egg is a fine combination of quality calcium and protein, steamed to a silky texture that doesn’t irritate the gut — good for a third-trimester snack. If lactose-intolerant, swap to low-lactose milk or yoghurt; if blood sugar runs high, control the sugar amount.

Week 33 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, combined with gentle belly stroking and gentle talking, is a good way to relax and connect — gently tapping different spots on the belly and responding gently to the baby’s kicks is a warm interaction that helps you relax, but don’t interpret it as "spatial-cognition training". What you can do: talk gently or read a picture book 5-10 minutes at a set time daily, combined with gentle stroking; choose soothing music or white noise at a volume ≤60 decibels for 15-20 minutes a day to relax yourself.

Week 33 partner's role

①Help and accompany your partner in practising Kegel exercises: 3 sets of 10 a day, contract the pelvic-floor muscles 5 seconds → relax 5 seconds, to help relieve urine leaking and lay the foundation for delivery and postpartum recovery (dad can also do them, good for the prostate). ②Keep checking 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 urine leaking and swelling: prepare panty liners, remind her of Kegels, raise her legs and soak her feet in warm water.

Week 33 checkup reminders

Weeks 28-36: routine visits every two weeks (weekly after 36), plus NST fetal-heart monitoring (generally from week 34, earlier for high-risk mums). This week you can confirm the Tdap vaccination (weeks 27-36) and book the 35-37-week GBS screening. Keep counting fetal movements daily. See a doctor any time if movement, blood pressure or blood sugar is abnormal.

Week 33 key concerns

Keep up Kegel exercisesFood-first calcium supplementation (about 1000mg a day)Manage urine leakingFinal hospital-bag check30 minutes of moderate exercise daily (ACOG recommends 150 minutes a week across pregnancy)

Signs that need immediate care at week 33

If any of the following occurs, contact your obstetrician or seek care promptly: ① a large amount of bright-red vaginal bleeding (watch for placenta-praevia bleeding) → lie flat immediately and seek care; ② clearly reduced fetal movement (under 10 in 2 hours or down by more than half from usual); 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 and a tender uterus.

Week 33 frequently asked questions

Is urine leaking in pregnancy normal?
Stress incontinence is common in the third trimester, related to the pelvic-floor muscles being pressed by the growing uterus and relaxin loosening them. Persisting with Kegel exercises (contract 5 seconds → relax 5 seconds, 3 sets of 10 a day) is most effective; you can wear a panty liner. If urine leaking comes with pain, blood in the urine or fever, a urinary-tract infection needs to be ruled out.
How much calcium should I take in the third trimester?
The calcium need in the third trimester is about 1000mg a day (ACOG view; China’s current DRIs 2023 edition is 800mg a day). Food first: milk and dairy, soy products, tahini and dark-green vegetables; if calcium deficiency is obvious, add a calcium supplement per the doctor, taken separate from iron for better absorption.
How much weight gain is normal at week 33?
A cumulative gain of about 10.5-14.5kg by week 33 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.
How do I read the week-33 ultrasound report?
The three measurements at 33 weeks: BPD (biparietal diameter) about 83.0mm, AC (abdominal circumference) about 289.1mm, and FL (femur length) about 61.4mm. If the three are in step, growth is good; a single slight deviation isn’t necessarily abnormal — go by your obstetrician’s overall assessment.
How are Kegel exercises done?
Like holding your urine, contract your pelvic-floor muscles (around the anus and vagina) for 5 seconds, relax 5 seconds as one rep, 3 sets of 10 a day. Avoid contracting your abdominal and buttock muscles and breathe normally. Persisting helps relieve urine leaking and promotes postpartum recovery.
Are the fetal movements regular at week 33?
Most babies have obvious active periods. Since week 28, 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.
When is the GBS screening?
GBS (group-B streptococcus) screening is generally arranged at weeks 35-37, sampling with vaginal and rectal swabs. If positive, intravenous antibiotics are given during delivery, effectively preventing newborn infection and not affecting vaginal delivery.
Do I need the Tdap vaccine in the third trimester?
The CDC and ACOG recommend the Tdap whooping-cough vaccine at weeks 27-36 so the baby gets maternal antibody and is protected against Pertussis; you’re still in the window this week. Tell them you’re pregnant before vaccination.
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.
Can I have sex at week 33?
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 33?
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 33?
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 33?
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 constipation and haemorrhoids at week 33?
Eat more coarse grains and vegetables for dietary fiber, drink 1.5-2L of water daily, walk after meals, and toilet regularly without squatting long; soak in warm water for haemorrhoids. Iron supplements can worsen constipation — discuss an adjustment with your doctor; in serious cases choose a pregnancy-safe medication rather than a stimulant laxative.
What about lower-back and pubic-bone pain at week 33?
It comes from a forward centre of gravity and relaxin loosening the ligaments and pelvic joints. 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 33?
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 basically complete by week 34 and keep the list by the door so you can leave at a moment’s notice.
Is it normal to have many false contractions at week 33?
More frequent false contractions in the third trimester are common: brief tightening of the belly, painless and irregular, disappearing with rest — the uterus "warming up". If they become regular, with pain or lumbar heaviness down, seek care to rule out preterm labour.
⚡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 33 daily focus

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

Day 1Your baby’s skin has completely changed from translucent to pink baby skin — the subcutaneous fat is finally thick enough to cover the blood vessels, no longer showing the purplish-red through. Another function of this fat layer is to store energy for maintaining body temperature after birth — leaving your 37°C womb, your baby faces rapid heat loss, and the fat is its "built-in warmer".Day 2Today’s calcium goal: a cup of milk in the morning (about 300mg calcium), tofu or dark leafy greens at lunch, and a cup of yoghurt at dinner, with calcium-rich food arranged across the three meals. ACOG advises about 1000mg a day in pregnancy — you don’t "reach for calcium tablets when food isn’t enough", but first look at whether your three meals systematically include milk and soy products. Take calcium and iron at least 2 hours apart, or they compete for absorption.Day 3Today upgrade your Kegel exercise to the advanced version — contract the pelvic-floor muscles for 10 seconds → fully relax 10 seconds, repeating 10 rounds, 3 sets a day. Whether sitting typing or side-lying scrolling the phone, you can do it; the key is that you feel the "muscles around the anus and vagina pulling inward and upward", not your legs and buttocks tightening. People who persist with Kegels have a clearly lower incidence of postpartum stress incontinence (about 30% in evidence-based studies) — this is a real "leak-proofing investment".Day 4When your baby kicks on the belly, gently tap the spot near the kick twice and say in a gentle voice "Baby, it’s me" — your familiar voice brings a settling response. This isn’t "teaching" anything but building an in-womb interaction ritual. Note that when the baby is quiet, don’t actively poke, tap or rock the belly to "wake it" — babies’ sleep is important for brain development.Day 5This week narrow the baby-name choices to 2-3 with dad, write each on a small card and read it aloud several times — read loudly, softly, and in the tone you’ll use to wake it up, and see which is most natural and brings the quickest smile to your face. Check the names for bad homophones and write the final candidate on a newborn birth card to have ready.Day 6Today’s hospital-bag check is the last "full inventory": empty everything onto the bed and check quantities, expiry dates (pads/care sheets/nappies), and any gaps (is the power bank charged? is the straw cup in? is the newborn going-home swaddle thick enough?), then repack, zip and lock the bag by the door. Tick off each correct item on the list — you don’t need "roughly ready"; you need "anyone could grab this bag and take me to hospital".Day 7The Tdap whooping-cough vaccine is still in its vaccination window this week (weeks 27-36); if you haven’t had it, call the community health centre or maternity hospital today to book it. After vaccination your body produces Pertussis antibody that passes through the placenta to the fetus — newborn protection in the first few weeks relies mainly on it; it’s the only defence before the baby is vaccinated itself. Slight local soreness within 24 hours is a normal reaction.

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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