Week 21 · Second trimester
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Week 21 fetal development and pregnancy guide — length about 26.7cm, weight 约360g (about the size of a a carrot)

Your baby is now about 26.7cm and 约360g, about the size of a a carrot.

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

View each day of week 21 →

Week 21 fetal development milestones: length / weight / ultrasound data

Key developmental changes during week 21:

  • The way length is measured changes: from this week the body length is measured head-to-toe (about 26.7cm), whereas before it was crown-rump length; so the number suddenly looks larger, which is normal.
  • The strength of the hands and feet grows, kicking and punching are more forceful, fetal movement becomes increasingly obvious and gradually regular.
  • The eyebrows and the eyelid contours are fully formed.
  • The bone marrow begins to take on blood-making (previously mainly done by the liver and spleen), laying the groundwork for a steady supply of blood cells later.
  • The bronchial tree of the lungs branches further and the airway network becomes more complete, laying the groundwork for spontaneous breathing after birth.
  • The amount of swallowed amniotic fluid increases, and the intestine begins to absorb small amounts of water and sugar, rehearsing digestive function.
  • The taste buds develop further and it can sense different tastes in the amniotic fluid (related to the mother’s diet).
  • Brain neurons connect rapidly, and the baby begins to show early memory and habituation (weakening responses to repeated stimulation) — a prototype of early learning ability.
  • The external genitals have differentiated and taken shape (the female baby’s labia develop; the male baby’s penis and scrotum develop).
  • About 26.7cm long, about the size of a carrot, weighing about 360g.
Illustration of your baby about the size of a carrot at week 21 (about 26.7cm/约360g)
Week 21 size: about the size of a a carrot
BPD (biparietal diameter)49.2mmWidth across the head; reflects brain development
AC (abdominal circumference)161.1mmAbdomen circumference; reflects nutrient absorption
FL (femur length)33.8mmThigh bone length; tracks gestational age
Ultrasound parameters at week 21: BPD 49.2 mm, AC 161.1 mm, FL 33.8 mm (median reference)
Week 21 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 21 daily development

Your baby quietly grows every day of week 21. Tap any day to see fetal development, maternal changes, and daily guidance for that day.

141Day 1Your baby’s bone marrow begins making blood independently; from today the steady supply of blood cells is taken over by the bone marrow — meaning it now has a truly independent blood-making system, no longer relying on the liver and spleen. The baby’s kicks and punches grow more forceful, and you can feel increasingly obvious, gradually regular fetal movement. This evening, lie on your side quietly for 20 minutes with a hand on the lower belly to feel the baby’s most active time and record roughly what hours it is.142Day 2When heartburn flares up, try the "three-don’t" rule: don’t overeat (each meal 70% full, small frequent meals), don’t lie down (stay sitting or standing at least 30 minutes after a meal, no food within 2-3 hours before bed), don’t eat spicy (no chilli, chilli-bean paste or excess oil in today’s dishes). If heartburn already affects sleep, prop the pillows up 15-20cm so the upper body is tilted about 30 degrees, using gravity to stop acid reflux. In an urgent spell, sip a little warm water, hold it a few seconds, then swallow.143Day 3Your baby’s taste buds develop further; today the flavour components of every mouthful you eat enter the amniotic fluid, and it is "tasting" your menu — it can sense the basic tastes of sweet, sour and bitter. This also influences its food preferences after birth: eat more fruit, vegetables and natural ingredients, and don’t go heavy-flavoured every day — the baby’s "taste-culture dish" is the amniotic fluid, and every meal is early education.144Day 4Today switch to an ultra-soft maternity toothbrush, rinse with warm water, brush gently in small circles in a 45-degree angle along the gum line. Week 21 is high-risk for gingivitis: oestrogen makes the gums react more strongly and inflame to dental plaque, so the slightest touch can make them bleed. The second trimester is the safe window for cleaning and gum care; if you haven’t had a cleaning in a long time, book a 30-minute scaling — clearing the plaque and calculus works better than any toothpaste.145Day 5Dad-to-be, put your hand on your partner’s belly and wait quietly for 5-10 minutes — this week the baby’s kicks have grown strong enough for Dad to feel through the abdominal wall. If the first time you feel one kick, then it quiets down again, don’t rush — the baby has a 20-40 minute sleep cycle and will move again in the next awake round. Record the time; this is also a unique moment of building the father-child bond during pregnancy.146Day 6Is the anomaly scan done? If it isn’t booked, or was booked but missed — call immediately to reschedule, and never let the anomaly-scan window drag past week 24. If done, sort out the ultrasound report: check whether the three values of BPD (biparietal diameter), AC (abdominal circumference) and FL (femur length) are within normal range; take any marked abnormality to the obstetrician for a comprehensive interpretation, and don’t scare yourself over "suspected/possibly" wording on the report.147Day 7Today note the baby’s most active time: most fetuses are most alert after you eat (blood sugar up), around 9-11pm at night (when you lie still), and after a shower (a mildly warm, relaxed state). Circle these time slots in a small notebook to prepare for formal kick counting after week 28. If one day you feel no fetal movement at all in a period when it used to move, first lie on your side, drink a cup of warm water and wait 15 minutes; if still nothing, contact your doctor.

Week 21 belly changes

Illustration of belly changes at week 21: uterine position and abdominal prominence

At week 21 (second trimester) the uterus rises from the pelvis into the abdomen and most mums start to "show" — a gentle bump. First-time mums usually show a little later than mums who've been pregnant before; the fundal height sits between the navel and pubic bone.

Week 21 maternal body changes and common symptoms

  • The uterine fundus is about level with, or slightly above, the navel (rises about 1cm a week from now on).
  • Fetal movement becomes increasingly obvious and gradually regular; you can start to note the baby’s active times (systematic kick counting usually starts from week 28).
  • Heartburn may appear — pregnancy hormone relaxes the cardiac sphincter and the enlarging uterus pushes up into the stomach, so stomach acid easily refluxes.
  • The gums may be more sensitive and bleed easily (pregnancy gingivitis in about 60-75% of pregnant women); keep up the soft toothbrush plus floss.
  • Weight grows steadily, about 0.4-0.5kg a week in the second trimester (ACOG, standard BMI).
  • The breasts keep enlarging; some women secrete a little colostrum (normal, don’t squeeze).
  • Mild leg oedema or varicose veins may appear (from the enlarged uterus pressing the inferior vena cava and increased blood volume).
  • Pigmentation (pregnancy line, areolae, pregnancy mask) becomes more obvious.

Week 21 nutrition advice and recommended recipes

This week focuses on easing heartburn while keeping nutrition. To ease heartburn: eat smaller, more frequent meals (5-6 a day, each about 70-80% full), stay sitting or standing for 30 minutes after a meal before lying down, don’t eat within 2-3 hours before bed, and avoid spicy, greasy, overly sweet foods and fizzy drinks. Daily nutrition targets still follow authority guidance: about 340kcal extra (ACOG, roughly a glass of milk plus an egg plus a small handful of nuts), calcium 1000mg, iron 27mg, DHA 200-300mg, folate 400-800μg (recommended through the whole pregnancy), iodine 230μg, fiber 25-30g, about 70g of quality protein. Avoid alcohol, raw or unpasteurised milk products, high-mercury fish, and keep caffeine to ≤200mg a day.

DHA200-300mgBrain & vision development
Recommended recipePumpkin millet congee

Pumpkin pectin gently protects the stomach lining and millet soothes the stomach; congee is easy to digest and doesn’t easily trigger acid reflux, making it a good snack during the heartburn phase. If your blood sugar runs high, control the amount and eat it with an egg or soy product for steadier sugar.

Week 21 prenatal education guide

The core of "fetal education" is still to help the mother-to-be relax and go with the flow; there is currently no strong evidence that any specific stimulation makes the baby smarter. This week fetal movement becomes more obvious, and many mothers like to pat or respond with words when they feel it — this is a warm parent-child interaction that helps you relax and build a connection, but it needn’t be understood as "the baby talking back to you" or as "developing intelligence". What you can do: talk gently for 5-10 minutes at a fixed time daily and lightly stroke the belly (slow, gentle movements); choose soothing music or natural white noise at ≤60 decibels for 15-20 minutes a day, in order to soothe yourself. Enjoy the process; don’t chase "the baby remembering".

Week 21 partner's role

①The baby’s movement grows more and more obvious; put your hand on your partner’s belly at a fixed time daily to quietly feel the kicks, and record the active times together. ②Help your partner ease heartburn: prepare light, easy-to-digest small frequent meals, avoid spicy and greasy food, and help prop up the pillows before bed. ③Talk gently or sing the same song close to the belly at a fixed time daily to build a father’s voice the baby recognises. ④Accompany your partner through the anomaly scan (weeks 20-24), record the check time and results, and give her psychological support during the check.

Week 21 checkup reminders

The anomaly scan (weeks 20-24 systematic ultrasound) is underway; if you haven’t done it, complete it as soon as possible — it is the most comprehensive structural malformation screen of pregnancy, covering the brain, spine, heart, limbs, internal organs, cleft lip/palate and more; popular hospitals have tight slots and need advance booking. The anomaly scan needs no fasting or full bladder; it takes about 30-60 minutes, and if the baby’s position is poor and things can’t be seen clearly, you may need to walk and re-check. At the same time look ahead: the GDM glucose-tolerance screen (OGTT) is usually done at weeks 24-28; start sugar-control habits this week.

Week 21 key concerns

Complete/follow up the anomaly scanNotice and record the fetal-movement patternEase heartburn (small frequent meals)Oral care to prevent gingivitis30 minutes of moderate exercise daily (ACOG recommends 150 minutes a week across pregnancy)

Signs that need immediate care at week 21

If any of the following occurs, contact your obstetrician or seek medical help promptly: ① persistent spotting or brown discharge; ② sudden leakage of fluid from the vagina (possible preterm rupture of membranes); ③ persistent or severe abdominal pain, with fever, or regular contraction-like tightening (watch for preterm labour); ④ a fever over 38℃ that does not come down; ⑤ severe headache, blurred vision or severe upper-abdominal pain (watch for pregnancy high blood pressure/preeclampsia — the risk begins to rise after week 20; if it appears, have blood pressure and urine protein checked); ⑥ painful urination, frequent/urgent urination or blood in the urine (urinary-tract infection); ⑦ swelling, warmth or pain in one leg (watch for a clot); ⑧ clearly reduced fetal movement or regular contractions (more than once every 10 minutes).

Week 21 frequently asked questions

How much fetal movement is normal at week 21?
This week you don’t yet need to count systematically; feeling the baby move regularly each day with its own active times is enough (mostly after meals and in the quiet at night). Systematic kick counting usually starts at week 28. If you feel the movement clearly drop by more than half one day, seek medical care promptly.
Heartburn keeps getting worse at week 21 — what should I do?
Eat small frequent meals, stay sitting for at least 30 minutes after a meal, don’t eat within 2-3 hours before bed, avoid spicy, greasy, overly sweet food and fizzy drinks, and prop up the upper body when sleeping. If symptoms are severe, you can use pregnancy-safe antacids such as calcium carbonate under a doctor’s guidance — don’t self-medicate long term.
Can the anomaly scan detect everything?
It can screen for most structural malformations (brain, spine, heart, limbs, internal organs, cleft lip/palate), but cannot detect all abnormalities (such as small heart defects, functional problems, metabolic diseases). The overall detection rate is about 60-80%, depending on the type and the hospital’s level; low risk is not zero risk.
What preparation does the anomaly scan need?
No fasting or full bladder needed. Wear loose two-piece clothes to expose the abdomen; the test takes about 30-60 minutes; you can bring a small snack — when the baby doesn’t cooperate, eat something sweet or move around to stimulate movement. If nervous, your partner can accompany you.
How to read the week-21 ultrasound report?
The three measurements at 21 weeks: BPD (biparietal diameter) about 49.2mm (head width), AC (abdominal circumference) about 161.1mm (nutrition and organ development), and FL (femur length) about 33.8mm (thigh-bone length). If all three lie within the ranges for that gestational age, development is coordinated; a single slight deviation isn’t necessarily abnormal — go by your obstetrician’s overall assessment.
How much weight gain is normal at week 21?
For a standard BMI, about 0.4-0.5kg a week in the second trimester, with a cumulative gain of roughly 4-7kg by week 21 being common. Watch the overall trend rather than a single week; discuss with the prenatal doctor if weight is rising too fast or too slowly, and use a pregnancy weight-gain calculator to manage it by BMI.
What about bleeding gums at week 21?
About 60-75% of pregnant women have pregnancy gingivitis; pregnancy hormones make the gums more reactive to dental plaque. Keep up the soft toothbrush plus floss, rinse with warm water and clean after meals; the second trimester is the safe dental window, so you can have a cleaning and gingivitis treatment. If bleeding is heavy or bad breath is obvious, see a dentist.
Should I start counting fetal movements at week 21?
Not yet systematically. This week you can start familiarising yourself with the baby’s active rhythm to prepare for formal counting after week 28. If you feel the movement suddenly drop clearly, seek care any time.
Is there a recommended exercise duration in pregnancy?
Yes. 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. Avoid intense exercise and pressure on the abdomen; stop and seek care for pain or bleeding.
Should I get the flu vaccine in pregnancy?
It’s recommended. The CDC and ACOG recommend that pregnant women receive the inactivated flu vaccine: pregnant women infected with flu are at higher risk of severe illness, and the vaccine protects both mother and baby; the inactivated vaccine is safe at every stage of pregnancy. During flu season (September to April) you can get vaccinated; tell them you’re pregnant.
Can I have sex at week 21?
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. If pain, bleeding or contractions occur after sex, pause immediately and consult a doctor; follow the doctor’s advice if they advise against it.
What should I note about my sleeping position at week 21?
Practise sleeping on your left side to increase uterine-placental blood flow and improve the baby’s oxygen supply. Use a pregnancy pillow to support the waist, abdomen and legs; you don’t need to stay that way all night — comfort and good sleep matter; avoid prolonged lying on your back and face-down on the belly.
What about constipation at week 21?
Pregnancy hormone slows intestinal movement and the uterus presses on the intestine, so constipation is common. Drink 1.5-2L of water daily, eat more coarse grains, fruit and vegetables for dietary fiber, walk after meals, and toilet on a regular schedule without squatting long. In serious cases consult a doctor for a pregnancy-safe laxative rather than buying a stimulant laxative yourself.
What should I avoid eating at week 21?
Avoid raw or undercooked meat, fish and eggs (toxoplasma, listeria), high-mercury large fish (shark, swordfish etc.), unpasteurised dairy, alcohol (no amount is safe), caffeine over 200mg a day, and excess animal liver (vitamin A excess). Fully cooked, well-heated food is safer.
Can I fly during pregnancy at week 21?
The second trimester is a better window for flying; most airlines don’t require a doctor’s certificate before 32 weeks and need one after 32 weeks (most prohibit flying after 36 weeks). Choose an aisle seat for movement, stay hydrated, and wear compression stockings to prevent clots. If you have bleeding, pain or placenta problems, ask your doctor first before travelling.
What about leg cramps at week 21?
When a cramp hits, flex the foot and massage the calf. Prevention: supplement calcium and magnesium, drink water, stretch before bed, wear support stockings, and avoid long standing or sitting. If cramps are frequent with weakness and numbness, check blood calcium, magnesium and electrolytes at a prenatal visit.
Is increased discharge normal at week 21?
Higher oestrogen increases white discharge; milky-white or clear and odourless is normal. Watch out for turning yellow-green, foul-smelling, with vulvar itching or burning (infection), or a sudden large amount of watery leakage (rupture of membranes) — seek care the moment these appear. Wear cotton underwear and don’t douche.
Is leg oedema normal at week 21?
Mild oedema after long standing or sitting is common; raising your legs, lying on your left side and walking to promote return flow can ease it. If the swelling spreads from the ankle up the calf and won’t ease with rest, or comes with headache or vision changes, have your blood pressure checked to rule out preeclampsia.
⚡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 21 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 bone marrow begins making blood independently; from today the steady supply of blood cells is taken over by the bone marrow — meaning it now has a truly independent blood-making system, no longer relying on the liver and spleen. The baby’s kicks and punches grow more forceful, and you can feel increasingly obvious, gradually regular fetal movement. This evening, lie on your side quietly for 20 minutes with a hand on the lower belly to feel the baby’s most active time and record roughly what hours it is.Day 2When heartburn flares up, try the "three-don’t" rule: don’t overeat (each meal 70% full, small frequent meals), don’t lie down (stay sitting or standing at least 30 minutes after a meal, no food within 2-3 hours before bed), don’t eat spicy (no chilli, chilli-bean paste or excess oil in today’s dishes). If heartburn already affects sleep, prop the pillows up 15-20cm so the upper body is tilted about 30 degrees, using gravity to stop acid reflux. In an urgent spell, sip a little warm water, hold it a few seconds, then swallow.Day 3Your baby’s taste buds develop further; today the flavour components of every mouthful you eat enter the amniotic fluid, and it is "tasting" your menu — it can sense the basic tastes of sweet, sour and bitter. This also influences its food preferences after birth: eat more fruit, vegetables and natural ingredients, and don’t go heavy-flavoured every day — the baby’s "taste-culture dish" is the amniotic fluid, and every meal is early education.Day 4Today switch to an ultra-soft maternity toothbrush, rinse with warm water, brush gently in small circles in a 45-degree angle along the gum line. Week 21 is high-risk for gingivitis: oestrogen makes the gums react more strongly and inflame to dental plaque, so the slightest touch can make them bleed. The second trimester is the safe window for cleaning and gum care; if you haven’t had a cleaning in a long time, book a 30-minute scaling — clearing the plaque and calculus works better than any toothpaste.Day 5Dad-to-be, put your hand on your partner’s belly and wait quietly for 5-10 minutes — this week the baby’s kicks have grown strong enough for Dad to feel through the abdominal wall. If the first time you feel one kick, then it quiets down again, don’t rush — the baby has a 20-40 minute sleep cycle and will move again in the next awake round. Record the time; this is also a unique moment of building the father-child bond during pregnancy.Day 6Is the anomaly scan done? If it isn’t booked, or was booked but missed — call immediately to reschedule, and never let the anomaly-scan window drag past week 24. If done, sort out the ultrasound report: check whether the three values of BPD (biparietal diameter), AC (abdominal circumference) and FL (femur length) are within normal range; take any marked abnormality to the obstetrician for a comprehensive interpretation, and don’t scare yourself over "suspected/possibly" wording on the report.Day 7Today note the baby’s most active time: most fetuses are most alert after you eat (blood sugar up), around 9-11pm at night (when you lie still), and after a shower (a mildly warm, relaxed state). Circle these time slots in a small notebook to prepare for formal kick counting after week 28. If one day you feel no fetal movement at all in a period when it used to move, first lie on your side, drink a cup of warm water and wait 15 minutes; if still nothing, contact your doctor.

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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