Week 16 · Second trimester
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Week 16 fetal development and pregnancy guide — length about 11.6cm, weight 约100g (about the size of a an avocado)

Your baby is now about 11.6cm and 约100g, about the size of a an avocado.

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

View each day of week 16 →

Week 16 fetal development milestones: length / weight / ultrasound data

Key developmental changes during week 16:

  • The nervous system refines rapidly, with denser neural pathways and more coordinated limb movement.
  • The sucking reflex appears: the baby puts its fingers into its mouth to suck, practicing for breastfeeding after birth.
  • The fingernails begin to grow, not yet reaching the fingertips.
  • The external genitals have developed enough that ultrasound can usually distinguish the sex fairly clearly (but this is affected by the baby’s position and the equipment, not 100% accurate).
  • The placenta has now fully taken over from the corpus luteum as the main source of hormones and nutrients (it is not yet fully mature; maturity grading usually reaches grade II-III only in late pregnancy).
  • It begins trying to swallow amniotic fluid and occasionally hiccups; digestive and breathing movements keep being rehearsed.
  • Hearing is still developing and it cannot yet truly hear outside sounds.
  • Fat begins to deposit in small amounts, laying the groundwork for keeping warm after birth.
  • The eyeballs can make slight movements while the eyelids remain closed.
  • About 11.6cm long, about the size of an avocado, weighing about 100g.
Illustration of your baby about the size of an avocado at week 16 (about 11.6cm/约100g)
Week 16 size: about the size of a an avocado
BPD (biparietal diameter)33.6mmWidth across the head; reflects brain development
AC (abdominal circumference)102.5mmAbdomen circumference; reflects nutrient absorption
FL (femur length)19.1mmThigh bone length; tracks gestational age
Ultrasound parameters at week 16: BPD 33.6 mm, AC 102.5 mm, FL 19.1 mm (median reference)
Week 16 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 16 daily development

Your baby quietly grows every day of week 16. Tap any day to see fetal development, maternal changes, and daily guidance for that day.

106Day 1Today the baby is sucking its thumb and its fingernails are growing — seeing this on ultrasound is adorable. Sucking is the foundation practice for feeding well after birth, and the baby is already rehearsing it in the womb. If you have a mid-term screen or a regular ultrasound booked, you can chat with the doctor — the external genitals are usually fairly clear at week 16, and while sex determination isn’t done for non-medical reasons in China, it’s still a meaningful full look at the baby’s development.107Day 2Today eat more heme-iron beef or lean pork, paired with bell pepper, broccoli or tomato rich in vitamin C. ACOG recommends about 27mg of iron daily in the second trimester — iron is key for the baby’s blood-making and for preventing your own anaemia. Best to have red meat at lunch or dinner, and don’t drink strong tea or coffee within two hours after a meal (tannic acid interferes with iron absorption); use warm water or lemon water instead. If you can, take a 10-minute walk after lunch to help digestion.108Day 3Book a dental check-up and cleaning! Weeks 14-27 are the safe window for dental treatment in pregnancy — pregnancy gingivitis can make gums red, swollen, bleeding, and can even form pregnancy epulis (a gum tumour). Left untreated it only gets worse. Tell the dentist your week; they’ll use pregnancy-safe anaesthesia and cleaning and won’t do X-rays or pull wisdom teeth. Stick to the everyday trio of a soft-bristle toothbrush plus floss plus warm-water rinses.109Day 4Watch for faint fetal movement at a fixed time daily. This week a mother who has given birth before may feel slight sensations like a tiny fish bubbling or a butterfly fanned wing; first-time mothers usually need 2-4 more weeks. The best time to feel it is after dinner, sitting quietly on the sofa or lying on your side with a hand on the lower-belly area, staying still and sensing for 5-10 minutes. If you feel nothing, don’t worry — mark this "detection day" as a baseline to compare against later when sensation arrives.110Day 5Today review the pregnancy "red-list" and stick it on the fridge: banned — any alcohol, undercooked meat/fish/eggs (listeria), high-mercury big fish (shark, swordfish, tilefish), unpasteurised dairy and fresh-squeezed milk; limit — caffeine to ≤200mg a day (about a medium Americano or two light teas) and animal liver to no more than 50g a week (vitamin A excess). The whole family can follow it together, avoiding the grey zone of "one taste can’t hurt".111Day 6Research and book the anomaly scan (weeks 20-24 systematic ultrasound)! This is the most important structural screening of pregnancy, checking from brain to toes. Slots at popular hospitals are very tight and often need booking 4-8 weeks or more ahead; call this morning — any later and you may miss it. Also confirm the exact date of the mid-term screen or NIPT and put both in your phone calendar with a one-day-before reminder.112Day 7Weigh yourself on a fixed day each week and keep a pregnancy diary. Write a few short sentences into the diary: the week-16 belly photo, today’s weight, and the body changes you noticed today (e.g. the first gum bleed, the first "maybe-fetal-movement" signal). It doesn’t need to be long — one or two genuine lines are what look most precious in hindsight. If you haven’t started before, starting this week as the milestone is not too late at all.

Week 16 belly changes

Illustration of belly changes at week 16: uterine position and abdominal prominence

At week 16 (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 16 maternal body changes and common symptoms

  • The uterine fundus is roughly midway between the navel and the pubic symphysis, slightly above.
  • Some women (especially those who have given birth before) start to feel the earliest fetal movements, like a butterfly flapping its wings or small bubbles.
  • Back pain may appear, linked to the forward shift in the centre of gravity and to relaxin loosening the ligaments.
  • The breasts keep enlarging and you need a suitable non-wired bra.
  • The appetite clearly increases, entering a phase of steady weight gain.
  • Nasal congestion (pregnancy rhinitis) and gum bleeding (pregnancy gingivitis in about 60-75%) are possible.
  • Skin pigment changes (areolae, abdominal line, pregnancy mask) become more obvious.
  • A few women feel round-ligament tugging pain (brief sharp lower-abdominal twinges).

Week 16 nutrition advice and recommended recipes

This week focuses on iron. ACOG recommends about 27mg of iron daily in the second trimester: give priority to heme iron — red meat and animal liver (no more than 50g a week) are highly absorbable, and pairing them with vitamin C vegetables (tomato, bell pepper, orange) promotes absorption; plant iron (spinach, legumes) absorbs less and needs even more vitamin C. Daily targets at the same time: calcium 1000mg, DHA 200-300mg, folate 400-800μg (recommended through the whole pregnancy), iodine 230μg, fiber 25-30g, and about 70g of quality protein, plus about 340kcal extra (ACOG). Avoid alcohol, raw or unpasteurised milk products, high-mercury fish, and keep caffeine to ≤200mg a day.

DHA200-300mgBrain & vision development
Recommended recipeStir-fried broccoli with beef

Beef provides highly absorbable heme iron, and broccoli’s vitamin C promotes iron absorption while also supplying dietary fiber and folate; a quick, low-oil stir-fry keeps the nutrients. Women who are anaemic or vegetarian can substitute lean pork plus dark-green vegetables, and if needed add an iron supplement on the doctor’s advice (taken at a different time from calcium tablets).

Week 16 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 particular stimulation makes the baby smarter. This week the baby’s hearing is still developing and it cannot yet hear clear outside sounds, so "playing white noise to calm the baby" mostly helps soothe your own nerves. What you can do: talk gently for 5-10 minutes at a fixed time daily to build the parent-child connection; choose soothing natural sounds (rain, ocean waves) or light music at a volume of ≤60 decibels for 15-20 minutes a day as a relaxation ritual for you and your partner. Don’t chase "the baby remembering"; the most rewarding thing is simply enjoying the time together.

Week 16 partner's role

①Accompany your partner to the second-trimester screening; if the ultrasound shows the sex, express "the baby’s health is what matters most" either way, to avoid pressure from preference. ②Keep sharing household chores and prepare high-iron foods (beef, lean meat) to help her replenish iron. ③Talk or sing gently close to the belly at a fixed time daily so the baby (in the future) becomes familiar with Dad’s voice. ④Help relieve her back pain: remind her to change positions slowly, prepare a pregnancy pillow, and join her in prenatal yoga.

Week 16 checkup reminders

Weeks 16-18: the second-trimester screening blood test (checks AFP/hCG/uE3/Inhibin A, assessing the risk of trisomy 21, trisomy 18 and neural-tube defects). If you didn’t have NIPT before and want higher detection, you can switch to NIPT (>99%). The week’s ultrasound can also often distinguish the external genital sex fairly clearly, but it is affected by position so there can be error — follow what the doctor tells you. Older mothers or those at high risk are advised to go straight to NIPT or to ask about amniocentesis.

Week 16 key concerns

Second-trimester screening / NIPTFocus on iron supplementationWatch for early fetal movementsRelieve back pain30 minutes of exercise daily (ACOG recommends 150 minutes a week across pregnancy)

Signs that need immediate care at week 16

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; 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, which usually occurs after week 20 — extremely rare at week 16 but must be checked if it appears); 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 sudden obvious oedema or a sudden week of rapid weight gain.

Week 16 frequently asked questions

Second-trimester screening vs. NIPT — how to choose?
The second-trimester screen costs less and detects about 70-80% of trisomy 21; NIPT detects >99% and only needs maternal blood with no miscarriage risk but costs more. It is recommended more strongly for older mothers (≥35), NT abnormalities or a history of chromosome problems; which to choose depends on age, NT results and the doctor’s advice.
Can I know the baby’s sex at week 16?
By this week the external genitals are usually fairly clear on ultrasound, but it is not 100% accurate due to the baby’s position, the equipment and the doctor’s experience, and sex determination for non-medical reasons is not compliant in China. Go by what the prenatal doctor tells you, and don’t put sex pressure on your partner.
Do I need to supplement DHA at week 16?
Yes. DHA is important for the fetal brain and retina; ACOG/authoritative advice is about 200-300mg a day. Eating low-mercury ocean fish (salmon, sardines, cod) 2-3 times a week is enough; vegetarians can choose algal-oil DHA supplements.
Is it normal to feel fetal movement at week 16?
Mothers who have given birth before may feel slight movements at 16-18 weeks (like bubbles or butterflies); first-time mothers usually feel them at 18-22 weeks. It’s also normal to feel nothing at all — don’t worry; the week-20 ultrasound can confirm.
How to relieve back pain at week 16?
The forward centre of gravity and relaxed ligaments from relaxin are the main causes. Avoid standing or sitting too long, wear low supportive shoes, sleep on your side with a pregnancy pillow, do prenatal yoga and pelvic-floor exercises, and apply local heat. Severe pain on one side, or pain with bleeding or fever, needs medical attention.
Should I watch my sleeping position at week 16?
You can practise sleeping on your left side to increase uterine-placental blood flow, using a pregnancy pillow for support; you don’t need to stay that way all night — comfort matters, and avoid lying face-down on your belly.
How to read the week-16 ultrasound report?
Three measurements at 16 weeks: BPD (biparietal diameter) about 34mm, AC (abdominal circumference) about 103mm, and FL (femur length) about 19mm — all are the median for the gestational age. If the three are in step, development is fine; a single slight deviation isn’t necessarily abnormal — go by the doctor’s overall assessment.
Can I see the dentist at week 16?
The second trimester (weeks 14-27) is the safe dental window. You can have cleanings, fillings and treatment of gingivitis; tell the dentist you’re pregnant, avoid X-rays (with abdominal shielding if truly needed), and lidocaine is acceptable for local anaesthesia.
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. 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 are at higher risk of severe flu, the vaccine protects both mother and baby, and the inactivated vaccine is safe at every stage. During flu season (September to April) you can get vaccinated; tell them you’re pregnant.
Can I have sex at week 16?
The second trimester is relatively stable; if there’s no placenta praevia, bleeding or cervical insufficiency, gentle sex is fine using positions that don’t press on the belly and with gentle movements. Pause if you feel unwell or the doctor advises against it; see a doctor for post-sex bleeding or pain.
How much weight should I gain by week 16?
For a standard BMI, about 0.4-0.5kg a week in the second trimester, with a cumulative gain of roughly 2-5kg by week 16 being common. Watch the trend — you can use a weight-gain calculator to manage it by BMI — and ask your doctor if it’s too fast or too slow.
What can I do about constipation/haemorrhoids at week 16?
Eat more fiber-rich fruit and vegetables, drink 1.5-2L of water daily, walk after meals, and toilet on a regular schedule without squatting long. For haemorrhoids, try warm sitz baths; in serious cases ask a doctor for pregnancy-safe medication rather than buying stimulant laxatives yourself.
What should I avoid eating at week 16?
Avoid raw or undercooked meat, fish and eggs, high-mercury large fish, unpasteurised dairy, alcohol, caffeine over 200mg/day, and excess liver (vitamin A overdose).
Can I fly during pregnancy?
Flying is more comfortable in the second trimester; most airlines don’t require a certificate before 32 weeks and need one after 32 weeks (most prohibit flying after 36 weeks). Choose an aisle seat, stay hydrated, and use compression stockings to prevent clots; ask your doctor first if you have bleeding, pain or placenta problems.
What about iron-deficiency anaemia at week 16?
First build it up through food — red meat, liver (≤50g a week) and spinach with vitamin C; once prenatal screening confirms iron deficiency/anaemia the doctor will add an iron supplement, which absorbs better when taken apart from calcium tablets and dairy. Don’t self-medicate with large doses.
Is increased discharge normal at week 16?
Higher oestrogen increases white discharge; milky-white, clear and odourless is normal. Yellow-green, foul-smelling, itchy discharge or large amounts of watery leakage needs medical attention; wear cotton underwear and don’t douche.
Should I book the anomaly scan now at week 16?
The anomaly scan (structural ultrasound) is done at weeks 20-24 and slots are tight, often needing to be booked weeks ahead. When you have the second-trimester screen/ultrasound this week, book it as soon as possible so you don’t miss the best window.
Are calf cramps normal at week 16?
From mid-pregnancy cramps become more common, usually related to inadequate calcium/magnesium and fatigue. Supplement calcium and magnesium, drink water, stretch before bed, and wear support stockings; when a cramp hits, flex the foot and massage the calf.
Is abdominal tightening normal at week 16?
Occasional brief tightening is usually a sensitive uterus or ligament tugging and is common if it resolves on its own in seconds to tens of seconds and disappears with rest. If the tightening becomes regular (e.g. every 10 minutes), or comes with pain/bleeding/leakage, seek medical evaluation.
⚡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 16 daily focus

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

Day 1Today the baby is sucking its thumb and its fingernails are growing — seeing this on ultrasound is adorable. Sucking is the foundation practice for feeding well after birth, and the baby is already rehearsing it in the womb. If you have a mid-term screen or a regular ultrasound booked, you can chat with the doctor — the external genitals are usually fairly clear at week 16, and while sex determination isn’t done for non-medical reasons in China, it’s still a meaningful full look at the baby’s development.Day 2Today eat more heme-iron beef or lean pork, paired with bell pepper, broccoli or tomato rich in vitamin C. ACOG recommends about 27mg of iron daily in the second trimester — iron is key for the baby’s blood-making and for preventing your own anaemia. Best to have red meat at lunch or dinner, and don’t drink strong tea or coffee within two hours after a meal (tannic acid interferes with iron absorption); use warm water or lemon water instead. If you can, take a 10-minute walk after lunch to help digestion.Day 3Book a dental check-up and cleaning! Weeks 14-27 are the safe window for dental treatment in pregnancy — pregnancy gingivitis can make gums red, swollen, bleeding, and can even form pregnancy epulis (a gum tumour). Left untreated it only gets worse. Tell the dentist your week; they’ll use pregnancy-safe anaesthesia and cleaning and won’t do X-rays or pull wisdom teeth. Stick to the everyday trio of a soft-bristle toothbrush plus floss plus warm-water rinses.Day 4Watch for faint fetal movement at a fixed time daily. This week a mother who has given birth before may feel slight sensations like a tiny fish bubbling or a butterfly fanned wing; first-time mothers usually need 2-4 more weeks. The best time to feel it is after dinner, sitting quietly on the sofa or lying on your side with a hand on the lower-belly area, staying still and sensing for 5-10 minutes. If you feel nothing, don’t worry — mark this "detection day" as a baseline to compare against later when sensation arrives.Day 5Today review the pregnancy "red-list" and stick it on the fridge: banned — any alcohol, undercooked meat/fish/eggs (listeria), high-mercury big fish (shark, swordfish, tilefish), unpasteurised dairy and fresh-squeezed milk; limit — caffeine to ≤200mg a day (about a medium Americano or two light teas) and animal liver to no more than 50g a week (vitamin A excess). The whole family can follow it together, avoiding the grey zone of "one taste can’t hurt".Day 6Research and book the anomaly scan (weeks 20-24 systematic ultrasound)! This is the most important structural screening of pregnancy, checking from brain to toes. Slots at popular hospitals are very tight and often need booking 4-8 weeks or more ahead; call this morning — any later and you may miss it. Also confirm the exact date of the mid-term screen or NIPT and put both in your phone calendar with a one-day-before reminder.Day 7Weigh yourself on a fixed day each week and keep a pregnancy diary. Write a few short sentences into the diary: the week-16 belly photo, today’s weight, and the body changes you noticed today (e.g. the first gum bleed, the first "maybe-fetal-movement" signal). It doesn’t need to be long — one or two genuine lines are what look most precious in hindsight. If you haven’t started before, starting this week as the milestone is not too late at all.

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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