Week 17 · Second trimester
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Week 17 fetal development and pregnancy guide — length about 13.0cm, weight 约140g (about the size of a a pear)

Your baby is now about 13.0cm and 约140g, about the size of a a pear.

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

View each day of week 17 →

Week 17 fetal development milestones: length / weight / ultrasound data

Key developmental changes during week 17:

  • Subcutaneous fat begins to accumulate, preparing to keep the baby warm after birth.
  • The auditory system becomes more responsive to sound, but it still cannot truly hear outside sounds.
  • The heartbeat is strong, clearly audible with a Doppler fetal monitor (about 110-160 beats per minute).
  • The brain begins to control the body’s autonomous functions more effectively, and movements become more purposeful.
  • Bone calcification accelerates; the hardness of the long bones and ribs increases noticeably.
  • The gums and the primordia of the baby teeth develop quietly inside the jawbone, laying the groundwork for teeth after birth.
  • The eyes can move, but the eyelids remain closed.
  • About 13.0cm long, about the size of a pear, weighing about 140g.
  • The skin is thin and semi-transparent, with blood vessels visible.
  • The fingers can grasp and explore the body and the umbilical cord more flexibly.
Illustration of your baby about the size of a pear at week 17 (about 13.0cm/约140g)
Week 17 size: about the size of a a pear
BPD (biparietal diameter)36.9mmWidth across the head; reflects brain development
AC (abdominal circumference)114.5mmAbdomen circumference; reflects nutrient absorption
FL (femur length)22.4mmThigh bone length; tracks gestational age
Ultrasound parameters at week 17: BPD 36.9 mm, AC 114.5 mm, FL 22.4 mm (median reference)
Week 17 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 17 daily development

Your baby quietly grows every day of week 17. Tap any day to see fetal development, maternal changes, and daily guidance for that day.

113Day 1Your baby’s subcutaneous fat is starting to build up noticeably, gradually turning it from a wrinkled little figure into a plump, rounded one. These fat stores are not just for looks — they are crucial for maintaining body temperature, providing an energy reserve and supporting brain development after birth. The healthy fats you eat today (avocado, nuts, fish oil) directly become the building materials of the baby’s subcutaneous fat and brain tissue; every quality mouthful sculpts the little one.114Day 2Before bed, do 3 minutes of calf stretches plus half a glass of warm milk: face the wall with both hands on it, in a front-back lunge, keeping the back leg straight with the heel pressed to the floor to feel the back of the calf stretch, holding 30 seconds per leg then switching; then drink half a glass of warm milk (about 150ml). This combination noticeably cuts nighttime leg cramps — about half of pregnant women get calf cramps in mid-to-late pregnancy, and inadequate calcium/magnesium and muscle fatigue are the main causes.115Day 3Today find a warm, quiet time; lie on your side or half-reclining, gently place your hand on your lower belly and speak softly to the baby for a short while — "Baby, you’re 17 weeks now, your bones are getting harder" — in your most everyday tone. The baby can’t yet hear clear words, but this period of parent-child conversation is essentially about helping you build the psychological connection of becoming a mother. Ten minutes each time, lightly and steadily, is one of pregnancy’s quietest happy moments.116Day 4Prepare a few "pregnancy massage vouchers" for Dad-to-be — hand-write on coloured card "Redeem for 30-minute shoulder-and-neck massage ×1", "Redeem for a 3km accompany-walk ×1", "Redeem for doing tonight’s dishes ×1" — then give them to Dad-to-be. It’s light-hearted humour, and it also gives your partner a clear channel to take part in pregnancy care. Your partner can gently knead the shoulders and upper back with the palm, no oil, avoiding the lumbar spine and the inner acupoint of the ankle.117Day 5Today add quality fats to your diet: half an avocado sliced onto wholewheat toast, or a handful of walnuts (about 15g) as a morning snack, or olive-oil-dressed vegetable salad. About 60% of the baby’s brain is made of fat, and adequate DHA, linolenic acid and other unsaturated fats are the basic materials of brain-cell membranes and nerve myelin. A small handful of nuts a day is the "brick" of brain building.118Day 6Have Dad-to-be use his phone to record his voice gently reading a picture book (slow and warm with feeling), and save it. The baby can’t hear yet, but this recording has three values: it puts Dad into "father role" mode, it serves as your relaxing BGM, and — once the baby is born — a voice it has heard repeatedly can help Dad comfort it faster.119Day 7If the anomaly scan isn’t booked, you must call today: slots for weeks 20-24 are tighter than most people expect, and many hospitals are already full by weeks 14-16. If your preferred hospital has no slot, ask whether another branch or doctor has an alternative. At the same time confirm the mid-term screen or NIPT schedule — if you haven’t done it by weeks 16-18, this week is the last chance to catch up.

Week 17 belly changes

Illustration of belly changes at week 17: uterine position and abdominal prominence

At week 17 (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 17 maternal body changes and common symptoms

  • The uterine fundus is about 2-3 finger-widths below the navel.
  • Cumulative weight gain is about 2.5-5kg (about 0.4-0.5kg a week for a standard BMI).
  • Leg cramps may appear — about half of pregnant women experience them during pregnancy, often related to inadequate calcium/magnesium, fatigue and circulation.
  • The breasts keep enlarging, so a suitable non-wired bra is needed.
  • Back pain and pelvic-zone discomfort grow more common (relaxin loosens the ligaments).
  • Residual early-pregnancy symptoms such as nasal congestion and gum bleeding are possible.
  • Pigmentation (pregnancy mask, abdominal line) becomes more obvious.
  • Energy is fairly good, entering a relatively comfortable phase.

Week 17 nutrition advice and recommended recipes

This week focuses on preventing leg cramps. ACOG recommends about 1000mg of calcium and good magnesium in the second trimester: for calcium, prioritise milk, soy products, tahini and dark-green vegetables; magnesium comes from nuts, whole grains and dark-green vegetables. Have milk plus soy products plus leafy greens daily, and a glass of warm milk before bed to help calcium absorption. Daily targets at the same time: iron 27mg, DHA 200-300mg, folate 400-800μg (recommended through the whole pregnancy), iodine 230μg, fiber 25-30g, about 70g of quality protein, and about 340kcal extra. Avoid alcohol, raw or unpasteurised milk products, high-mercury fish, and keep caffeine to ≤200mg a day.

DHA200-300mgBrain & vision development
Recommended recipeSpinach with sesame paste

Blanch the spinach to remove oxalic acid, then mix with sesame paste; calcium and iron work together, and it both supplements calcium and relieves constipation, also adding magnesium and dietary fibre. Sesame paste is fairly high in calories, so those with high blood sugar should have less; you can also substitute tofu mixed with spinach for a lighter version.

Week 17 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 stimulation makes the baby smarter. This week the baby’s hearing is developing and it is growing more sensitive to sound, but it still cannot hear clear outside sounds, so "the baby remembering Mom’s tone" is premature — treating your talking as a gentle conversation with the baby and with yourself is more fitting. What you can do: speak gently for 5-10 minutes at a fixed time daily and lightly stroke the belly (slow, gentle movements), which helps you relax and build the parent-child connection; and have Dad talk close to the belly at a fixed time daily, so the baby will be more familiar with Dad’s voice after birth. Don’t chase "remembering" — just enjoy the time together.

Week 17 partner's role

①The baby grows more sensitive to hearing; talk close to the belly at a fixed time daily, slowly and warmly, to build the parent-child connection. ②Help your partner prevent leg cramps: stretch the calves before bed, warm milk, and prepare support stockings. ③Accompany her to routine prenatal visits and record the fetal heartbeat. ④Prepare "pregnancy massage vouchers" — learn gentle shoulder/neck/waist massage (avoiding the lumbosacral zone and abdominal acupoints) to ease her aches.

Week 17 checkup reminders

If the week 16-18 second-trimester screening result is abnormal, further confirmation is needed: the first choice is NIPT (non-invasive DNA, >99% detection) or amniocentesis (the diagnostic gold standard, roughly done at weeks 16-22). A high-risk result does not mean it is definitely abnormal — follow the genetic-counselling process with your obstetrician and don’t judge it yourself. Low-risk mothers also continue with routine prenatal care and book the week-20-24 anomaly scan.

Week 17 key concerns

Prevent leg cramps (calcium/magnesium)Increase quality fatsPrepare for fetal-movement monitoringStart prenatal massage30 minutes of exercise daily (ACOG recommends 150 minutes a week across pregnancy)

Signs that need immediate care at week 17

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 17 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, different from ordinary cramping); ⑧ sudden obvious oedema or a sudden week of rapid weight gain.

Week 17 frequently asked questions

How to relieve leg cramps at week 17?
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 standing too long. About half of pregnant women get cramps; if they’re frequent with weakness or numbness, check blood calcium, magnesium and electrolytes at a prenatal visit.
Is fetal movement still not obvious at week 17 normal?
Yes. Most first-time mothers clearly feel movement only at 18-22 weeks; mothers who’ve given birth before feel it a little earlier; heavier builds, placenta praevia or an anterior placenta also delay the sensation. The week-20 ultrasound can confirm, so don’t worry about "not feeling anything".
Can I hear the fetal heartbeat at week 17?
Most Doppler fetal monitors pick up a heartbeat after weeks 10-12; at week 17 the heartbeat is strong, about 110-160 beats per minute. A home fetal-heart monitor can be used occasionally but you don’t need to check daily; don’t panic if you can’t hear it (affected by position and amniotic fluid) — rely on prenatal checks for any abnormality.
Is my weight gain at week 17 normal?
A cumulative gain of about 2.5-5kg by week 17 is common, with about 0.4-0.5kg a week in the second trimester for a standard BMI. Watch the trend — you can use a weight-gain calculator to manage it by BMI — and talk to the doctor if it’s too fast or too slow.
How to read the week-17 ultrasound report?
The three measurements at 17 weeks are BPD (biparietal diameter) about 37mm, AC (abdominal circumference) about 115mm, and FL (femur length) about 22mm, 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 the doctor’s overall assessment.
Can I get a massage during pregnancy?
Yes; the second trimester is a good window for massage. Choose a therapist with pregnancy-massage qualifications, use the side-lying position (not face-down), avoid direct pressure on the abdomen and the acupoints near the ankles, and avoid deep leg massage (clot risk). It can relieve back pain, reduce oedema and improve sleep.
Sleeping position at week 17?
Practise sleeping on your left side to increase uterine-placental blood flow, using a pregnancy pillow; comfort is the priority, brief right-side or flat positions are fine, and avoid lying face-down on the belly.
Do I need calcium and magnesium supplements at week 17?
ACOG recommends about 1000mg of calcium daily in the second trimester; magnesium is also important (nuts, whole grains, dark-green vegetables). Give priority to food sources; if prenatal checks show a deficiency or cramps are obvious, add calcium/magnesium on the doctor’s advice rather than in blind large doses.
Is there a recommended exercise duration in pregnancy?
ACOG advises 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 mother and baby, and it 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 17?
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.
Back pain and pelvic pain at week 17?
Relaxin loosening the ligaments and the increased load on the pelvis make lumbar soreness and pubic-bone pain common. Avoid standing or sitting too long, wear low supportive shoes, sleep on your side with a pregnancy pillow, and do prenatal yoga and pelvic-floor exercises. Severe pain on one side, or with bleeding or fever, needs medical attention.
Constipation at week 17?
Eat more fiber-rich fruit and vegetables, drink 1.5-2L of water daily, walk after meals, and toilet on a regular schedule. 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 17?
Avoid raw or undercooked meat, fish and eggs, high-mercury large fish, unpasteurised dairy, alcohol, caffeine over 200mg/day, and excess liver.
Can I fly during pregnancy at week 17?
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; ask your doctor first if you have bleeding, pain or placenta problems.
Should I book the anomaly scan at week 17?
The anomaly scan is done at weeks 20-24 with tight slots, so book this week to avoid missing the best window.
Increased discharge at week 17?
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.
Iron-deficiency anaemia at week 17?
Build it up with red meat, liver (≤50g a week) and spinach paired with vitamin C; once diagnosed, the doctor will prescribe an iron supplement, taken apart from calcium tablets and dairy for better absorption.
Pregnancy mask getting darker at week 17?
Pigmentation from pregnancy hormones is common; use sun protection (physical first), gentle skincare and adequate sleep, and most of it fades after delivery. If it darkens and spreads quickly, have the doctor look at it during a prenatal visit.
Abdominal tightening at week 17?
Occasional brief tightening is usually a sensitive uterus or ligament tugging and is common if it resolves on its own. If it becomes regular (every 10 minutes), or comes with pain/bleeding/leakage, seek medical attention.
⚡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 17 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 subcutaneous fat is starting to build up noticeably, gradually turning it from a wrinkled little figure into a plump, rounded one. These fat stores are not just for looks — they are crucial for maintaining body temperature, providing an energy reserve and supporting brain development after birth. The healthy fats you eat today (avocado, nuts, fish oil) directly become the building materials of the baby’s subcutaneous fat and brain tissue; every quality mouthful sculpts the little one.Day 2Before bed, do 3 minutes of calf stretches plus half a glass of warm milk: face the wall with both hands on it, in a front-back lunge, keeping the back leg straight with the heel pressed to the floor to feel the back of the calf stretch, holding 30 seconds per leg then switching; then drink half a glass of warm milk (about 150ml). This combination noticeably cuts nighttime leg cramps — about half of pregnant women get calf cramps in mid-to-late pregnancy, and inadequate calcium/magnesium and muscle fatigue are the main causes.Day 3Today find a warm, quiet time; lie on your side or half-reclining, gently place your hand on your lower belly and speak softly to the baby for a short while — "Baby, you’re 17 weeks now, your bones are getting harder" — in your most everyday tone. The baby can’t yet hear clear words, but this period of parent-child conversation is essentially about helping you build the psychological connection of becoming a mother. Ten minutes each time, lightly and steadily, is one of pregnancy’s quietest happy moments.Day 4Prepare a few "pregnancy massage vouchers" for Dad-to-be — hand-write on coloured card "Redeem for 30-minute shoulder-and-neck massage ×1", "Redeem for a 3km accompany-walk ×1", "Redeem for doing tonight’s dishes ×1" — then give them to Dad-to-be. It’s light-hearted humour, and it also gives your partner a clear channel to take part in pregnancy care. Your partner can gently knead the shoulders and upper back with the palm, no oil, avoiding the lumbar spine and the inner acupoint of the ankle.Day 5Today add quality fats to your diet: half an avocado sliced onto wholewheat toast, or a handful of walnuts (about 15g) as a morning snack, or olive-oil-dressed vegetable salad. About 60% of the baby’s brain is made of fat, and adequate DHA, linolenic acid and other unsaturated fats are the basic materials of brain-cell membranes and nerve myelin. A small handful of nuts a day is the "brick" of brain building.Day 6Have Dad-to-be use his phone to record his voice gently reading a picture book (slow and warm with feeling), and save it. The baby can’t hear yet, but this recording has three values: it puts Dad into "father role" mode, it serves as your relaxing BGM, and — once the baby is born — a voice it has heard repeatedly can help Dad comfort it faster.Day 7If the anomaly scan isn’t booked, you must call today: slots for weeks 20-24 are tighter than most people expect, and many hospitals are already full by weeks 14-16. If your preferred hospital has no slot, ask whether another branch or doctor has an alternative. At the same time confirm the mid-term screen or NIPT schedule — if you haven’t done it by weeks 16-18, this week is the last chance to catch up.

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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