Week 13 · First trimester
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Week 13 fetal development and pregnancy guide — length about 7.4cm, weight 约23g (about the size of a a lemon)

Your baby is now about 7.4cm and 约23g, about the size of a a lemon.

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

View each day of week 13 →

Week 13 fetal development milestones: length / weight / ultrasound data

Key developmental changes during week 13:

  • Fingerprint ridges begin forming! Fingerprint patterns are determined by genes and the intrauterine environment together; weeks 13-19 are the critical period for establishing the ridge patterns
  • Bones begin to harden (ossify), starting from the collarbones and long bones, gradually replacing cartilage
  • The intestines have fully withdrawn into the abdomen, and the liver begins secreting bile to take part in digestion
  • The baby has a grasp reflex and can clench its little fists
  • The vocal cords begin to form, but no sound can be made yet (the lungs are not mature)
  • The eyes have moved from the sides toward the front of the face, but the eyelids remain closed
  • About 7.4cm — the size of a lemon — weighing about 23g
  • Movements are more coordinated; the baby can roll over and stretch in the amniotic fluid
Illustration of your baby about the size of a lemon at week 13 (about 7.4cm/约23g)
Week 13 size: about the size of a a lemon

* Values are medians from WHO/China standards; individual variation is normal. References: WHO INTERGROWTH-21st, China Prenatal & Pregnancy Care Guide, ACOG guidelines.

Week 13 daily development

Your baby quietly grows every day of week 13. Tap any day to see fetal development, maternal changes, and daily guidance for that day.

85Day 1The baby’s fingerprints are being etched on its fingertips — genes and the intrauterine environment together shape this set of “biometric keys,” and even identical twins have different prints. Look down at your own ten spirals — equally unique. Today your baby clenches and unclenches its little printed hands, rolls over in the amniotic fluid, and its bones begin gradually turning from cartilage to hard bone — every day more solid.86Day 2Welcome formally to the second-trimester honeymoon! HCG has clearly fallen and the placenta has fully taken over hormone production, so your nausea, sleepiness and fatigue will basically disappear over the next 1-2 weeks. Pour that recovered energy into gentle exercise: walking, prenatal yoga and swimming all work, about 150 minutes a week total (ACOG), 30 minutes a day over 5 days. But don’t get carried away into marathon training — go from light to moderate and stay within your limits.87Day 3Book the mid-trimester screen (weeks 16-18) or go straight to NIPT. The mid-term screen is the lower-cost option (about 70-80% detection for trisomy 21); NIPT has a higher detection rate (>99%) but higher cost. If your NT was abnormal, you’re ≥35, or there’s a family history of chromosome problems, NIPT is the more recommended route. Call your hospital now to learn the process and schedule — don’t wait until week 15.88Day 4Switch to a comfortable wire-free maternity bra! The breasts are expanding for breastfeeding and are more than a cup size bigger than before pregnancy — wearing your old regular bra not only squeezes uncomfortably but can ache at night. Choose pure-cotton seamless designs with wide straps and adjustable back hooks; they can last until the end of nursing. Investing in comfort is the most worthwhile pregnancy expense.89Day 5Do a nutrition self-check today: against your three meals, do you have protein (egg/milk/fish/meat/soy), vegetables (dark green first), fruit (2-3 kinds), dairy (1-2 portions) and whole grains (at least 1-2 servings)? In mid-pregnancy you only need about 340 extra calories a day (ACOG, ≈ a glass of milk + an egg + a small handful of nuts) — not “eating for two”; the point is variety, not quantity.90Day 6A better appetite in mid-pregnancy comes with a “sweet trap”: milk tea, cake and sugary drinks become more tempting, but pregnancy hormones blunt insulin sensitivity, and lots of refined sugar can swing blood sugar and raise GDM risk. Crave something sweet? Choose low-GI fruit (strawberries, blueberries, cherries, kiwi), sweet potato, or a small square of dark chocolate — satisfying and still healthy.91Day 7At week 13 you can plan the whole-pregnancy and postpartum buying rhythm: research the big items now through week 20 (crib, stroller, car seat), start buying baby clothes and daily goods in batches from weeks 20-28, and prepare the hospital bag and nursing supplies at weeks 28-36. Don’t try to buy everything at once — while your energy is best in mid-pregnancy, shop around and compare, buy rationally, and don’t be pushed by marketing.

Week 13 belly changes

Illustration of belly changes at week 13: uterine position and abdominal prominence

At week 13 (first trimester) your uterus still sits low in the pelvis, so there is little visible change and your abdomen circumference barely moves. Don't worry if you don't "show" yet.

Week 13 maternal body changes and common symptoms

  • Early-pregnancy reactions basically disappear, entering the relatively comfortable “honeymoon phase” — about 60% of women see morning sickness clearly ease after week 12 and 90% before week 20
  • Energy and appetite gradually return; nausea, sleepiness and fatigue clearly ease
  • The breasts keep enlarging; more than half of women need suitable, wire-free maternity bras
  • The uterus keeps growing and leaves the pelvis; some women begin to show faintly
  • Dizziness and fatigue mostly improve, but occasional discomfort from blood-pressure adjustment may remain; some women have occasional palpitations
  • Constipation and bloating may persist; about a third of women are bothered by pregnancy hormones slowing intestinal movement
  • Pigmentation (melasma, areolas, abdominal midline) may become more obvious; more than half of women have it
  • Mood tends to stabilize, entering a lighter stage of pregnancy

Week 13 nutrition advice and recommended recipes

Week 13 is still the tail end of the first trimester, so extra calorie needs are very small — the point is “eating well,” not “eating more”; from the second trimester (week 14 onward) ACOG recommends about 340 extra calories a day. This week’s key nutrient targets: ①folic acid — ACOG recommends 400-800μg (0.4-0.8mg) daily, ideally continuing through the whole pregnancy; after that, whether to switch to a multivitamin follows your doctor; ②calcium — about 1000mg a day (ACOG) after mid-pregnancy, from milk, yogurt, soy products and dried shrimp; ③iron — about 27mg a day (ACOG) from the second trimester, from red meat, liver (≤50g/week) and spinach paired with vitamin C; ④DHA — about 200-300mg a day, from low-mercury fish 2-3 times a week or algae oil; ⑤iodine — about 230μg a day in pregnancy, use iodized salt; ⑥quality protein — eggs, fish, lean meat and soy products, about 60-70g a day; ⑦vitamin D — get sun. Avoid alcohol, cold-raw unpasteurized milk and high-mercury fish; caffeine ≤200mg/day.

DHA200-300mgBrain & vision development
Recommended recipeSalmon quinoa salad

Salmon provides DHA and quality protein, quinoa is a whole-grain powerhouse, and mixed vegetables keep it fresh and light — good for weight control and nutrition. Pan-sear the salmon with little oil until done, cook the quinoa and let it cool, toss with the vegetables and drizzle with lemon juice; non-fish eaters can use tofu or chicken breast instead.

Week 13 prenatal education guide

At week 13 the baby still has no established hearing (it gradually starts after about week 16), so the “music + story + touch” combo isn’t received yet. Don’t treat combined prenatal care as training, and avoid pressing the abdomen hard or shining a flashlight on it (pointless and it may stimulate the uterus). Entering the “honeymoon phase,” what truly works is: ①a good mood, regular routine and moderate exercise to give the baby a safe, comfortable environment; ②talking to your belly, reading picture books or playing soft music is of course fine — it relaxes the mother and builds the parent-child bond; ③staying away from tobacco, alcohol and harmful factors. Understanding prenatal care as “taking care of yourself” is most practical.

Week 13 partner's role

①The second trimester is a great time for the father to get involved — read aloud and talk to the baby at a fixed time each evening so the baby (eventually) gets used to dad’s voice and the mother feels supported; ②Go along and learn about the mid-trimester screen (about weeks 16-18) or NIPT; ③Help choose suitable maternity bras and healthy snacks, and take walks/exercise together; ④Keep quitting smoking, share chores, and keep the prenatal file and due date.

Week 13 checkup reminders

If the NT/early-screening result is abnormal or you are older (≥35), you can do NIPT (cell-free DNA) from this week or after week 12 for further screening; NIPT has a high detection rate but is still screening — a positive result still needs amniocentesis to confirm. For low risk, you move into the second trimester as planned; from next week focus on the mid-trimester screen (about weeks 16-18) and book the later anatomy scan (the “big malformation scan,” usually weeks 20-24, which often needs booking well in advance).

Week 13 key concerns

Weight management (about 0.4-0.5kg per week)Gradually resume / keep up gentle exerciseSwitch to comfortable maternity brasBook the mid-trimester screen or NIPTKeep a balanced diet — don’t overfeed blindly

Signs that need immediate care at week 13

Though you’ve entered a relatively lower-risk period, seek care promptly for any of the following: ①vaginal bleeding (bright red or brown, light or heavy) with abdominal pain (be alert for threatened miscarriage); ②persistent or severe abdominal pain, bloating or vomiting; ③fever above 38°C with chills; ④severe headache or blurred vision (be alert for blood-pressure problems); ⑤clearly abnormal vaginal discharge with odor or itching (be alert for infection); ⑥heavy gush of fluid from the vagina (be alert for premature rupture of membranes); ⑦painful urination, frequent/urgent urination or blood in the urine (urinary tract infection); ⑧sudden fainting or pale face.

Week 13 frequently asked questions

How much weight should I gain in the second trimester?
By pre-pregnancy BMI: a standard weight (BMI 18.5-24.9) gains about 0.4-0.5kg a week; slightly more if underweight, slightly less if overweight. Gaining too fast raises the risk of gestational diabetes and a large baby; too slowly may affect fetal development. Use a pregnancy weight-gain calculator to manage within your BMI’s recommended range.
Is NIPT (cell-free DNA) really necessary?
Not mandatory but strongly recommended, especially for older mothers (≥35), NT abnormalities, or a history of chromosome problems. NIPT tests fetal free DNA in maternal blood to screen for common trisomies — it has a high detection rate and no miscarriage risk; but it is screening, and a positive result still needs amniocentesis to confirm. Whether to do it combines age, risk and your doctor’s advice.
What does entering the “honeymoon phase” at week 13 mean?
It means the early-pregnancy reactions (nausea, sleepiness) basically disappear, energy and appetite return, and the risk of miscarriage relatively drops — the stage most women find most comfortable. But it’s still pregnancy, so don’t let your guard down — keep regular prenatal visits and good habits.
Can I have sex at week 13?
If there is no bleeding, abdominal pain, history of miscarriage, low-lying placenta or other contraindications, gentle sex is usually fine — move carefully and avoid pressure on the abdomen. If you feel unwell or your doctor advises against it, pause; bleeding or pain after sex warrants care.
Can I resume exercise at week 13?
Yes. Walking, prenatal yoga, swimming and light aerobic activity all suit; about 30 minutes each time, 3-5 times a week. Avoid strenuous activity, abdominal pressure and high-risk sports (skiing, horseback riding). If you had no baseline, build up gradually.
Will I be showing at week 13?
Some women with looser abdominal muscles or twins may begin to show faintly; most first-time mothers only show clearly after week 16. Not showing doesn’t mean abnormal — there’s a wide range of variation.
Should I start calcium and iron at week 13?
Second-trimester calcium need is about 1000mg a day — get it from milk, yogurt, soy products and dried shrimp, adding tablets as directed if low; iron need rises from the second trimester — get it from red meat, liver and spinach, adding targeted supplements if a check shows anemia. Folic acid is generally taken through week 12; after that, whether to switch to a multivitamin follows your doctor.
Can I do the mid-trimester screen at week 13?
The mid-trimester screen is generally arranged at weeks 15-20 (commonly 16-18); this week is still a little early. You can book it now — don’t miss the week range. Unlike early screening/NT, the mid-term screen is also a blood test combined with markers to assess risk.
Can I fly at week 13?
For uncomplicated pregnancies, the second trimester is relatively comfortable for travel; move around often, drink water to prevent blood clots, and place the seatbelt under the abdomen. If you have bleeding, pain, placental problems, or a history of miscarriage or preterm birth, consult your doctor before traveling and learn the airline’s pregnancy rules.
What about constipation and bloating at week 13?
Eat more whole grains, fruit and vegetables for fiber, drink 1.5-2L of water a day, walk after meals, and develop a regular toilet habit. If severe, don’t buy irritant laxatives on your own — ask a doctor for a pregnancy-safe laxative.
Is vaginal bleeding at week 13 serious?
Bleeding at any week of pregnancy should be evaluated. Small amounts of brown discharge with a sensitive cervix can be observed, but bright red, increased bleeding or bleeding with pain needs immediate care.
How many months pregnant am I at week 13? How big is the baby?
Week 13 is about a full 3 months and approaching the second trimester. The baby is about 7.4cm, like a lemon; fingerprints have formed and bones are beginning to harden — entering a phase of rapid growth.
Can I drink coffee or milk tea at week 13?
A little is okay, but keep caffeine ≤200mg/day (about one medium americano or two cups of weak tea). Milk tea is high in sugar and trans fats, so limit it. Avoid alcohol entirely.
Can I take medicine for a cold or fever at week 13?
Do not self-medicate. Rest and drink plenty of fluids; if fever is above 38°C or symptoms are obvious, see a doctor promptly and tell them you are pregnant so they can choose relatively safe medications.
What about dark or spotted skin at week 13?
Pregnancy hormones cause pigmentation (cheek mask, darker abdominal midline), which is common. Use sun protection (physical sunscreen preferred), gentle skincare and enough sleep; most of it fades after childbirth. If a patch darkens or spreads rapidly, let your doctor look at a prenatal visit.
Can I wear makeup, use skincare, or dye my hair at week 13?
Basic cleansing, moisturizing and sun protection are fine; choose gentle products and avoid ingredients like retinoic acid (retinol), salicylic acid and hydroquinone. Hair dyeing/perming is best avoided in early pregnancy; in the second trimester an occasional treatment is relatively lower risk but still prefer gentle products and good ventilation.
Can I get a massage or soak my feet at week 13?
Gentle shoulder/neck/low-back massage and warm foot soaks (without blood-activating herbs, not too hot, not too long) are usually fine, avoiding the sacral area, abdominal points and blood-activating treatments; ask your doctor first if you have bleeding, pain, or placental problems.
Is the risk of a stopped pregnancy still high at week 13?
After entering the second trimester the risk of miscarriage/stopped pregnancy is clearly lower than in the first trimester, but not zero. Keep regular visits, watch fetal movement (gradually appearing after week 18), and seek care promptly for bleeding or pain — no need to be overly anxious.
Do I need to book the anatomy scan (systematic ultrasound) at week 13?
Yes, and book early. The systematic scan (“big malformation scan”) is usually done at weeks 20-24; popular hospitals have tight slots and often need booking weeks or more ahead, so start paying attention and booking from this week to avoid missing the best week range.
Is there a recommended amount of exercise for pregnancy?
Yes. ACOG recommends about 150 minutes of moderate-intensity activity per week (walking, prenatal yoga, swimming, etc.) for pregnant women without complications, split over 5 days at about 30 minutes each. Entering the comfortable phase at week 13, exercise regularly; avoid strenuous activity and abdominal pressure — stop and seek care if pain or bleeding occurs.
Should I get a flu shot in pregnancy?
Yes, recommended. The CDC and ACOG both recommend the inactivated flu vaccine for pregnant women — pregnant women have a higher risk of severe outcomes, the vaccine protects both mother and baby, and it is safe at all stages. You can get it during flu season (roughly September-April); tell the vaccinator you are pregnant.
How much weight gain is normal in pregnancy?
By pre-pregnancy BMI: a standard weight (18.5-24.9) gains about 0.4-0.5kg a week, slightly more if underweight and slightly less if overweight (IOM/ACOG ranges). In the first trimester staying level or dropping a little is normal — focus on the overall trend rather than any single week, use a pregnancy weight-gain calculator by your BMI, and talk to your doctor if gain is too fast or too slow.
⚡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 13 daily focus

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

Day 1The baby’s fingerprints are being etched on its fingertips — genes and the intrauterine environment together shape this set of “biometric keys,” and even identical twins have different prints. Look down at your own ten spirals — equally unique. Today your baby clenches and unclenches its little printed hands, rolls over in the amniotic fluid, and its bones begin gradually turning from cartilage to hard bone — every day more solid.Day 2Welcome formally to the second-trimester honeymoon! HCG has clearly fallen and the placenta has fully taken over hormone production, so your nausea, sleepiness and fatigue will basically disappear over the next 1-2 weeks. Pour that recovered energy into gentle exercise: walking, prenatal yoga and swimming all work, about 150 minutes a week total (ACOG), 30 minutes a day over 5 days. But don’t get carried away into marathon training — go from light to moderate and stay within your limits.Day 3Book the mid-trimester screen (weeks 16-18) or go straight to NIPT. The mid-term screen is the lower-cost option (about 70-80% detection for trisomy 21); NIPT has a higher detection rate (>99%) but higher cost. If your NT was abnormal, you’re ≥35, or there’s a family history of chromosome problems, NIPT is the more recommended route. Call your hospital now to learn the process and schedule — don’t wait until week 15.Day 4Switch to a comfortable wire-free maternity bra! The breasts are expanding for breastfeeding and are more than a cup size bigger than before pregnancy — wearing your old regular bra not only squeezes uncomfortably but can ache at night. Choose pure-cotton seamless designs with wide straps and adjustable back hooks; they can last until the end of nursing. Investing in comfort is the most worthwhile pregnancy expense.Day 5Do a nutrition self-check today: against your three meals, do you have protein (egg/milk/fish/meat/soy), vegetables (dark green first), fruit (2-3 kinds), dairy (1-2 portions) and whole grains (at least 1-2 servings)? In mid-pregnancy you only need about 340 extra calories a day (ACOG, ≈ a glass of milk + an egg + a small handful of nuts) — not “eating for two”; the point is variety, not quantity.Day 6A better appetite in mid-pregnancy comes with a “sweet trap”: milk tea, cake and sugary drinks become more tempting, but pregnancy hormones blunt insulin sensitivity, and lots of refined sugar can swing blood sugar and raise GDM risk. Crave something sweet? Choose low-GI fruit (strawberries, blueberries, cherries, kiwi), sweet potato, or a small square of dark chocolate — satisfying and still healthy.Day 7At week 13 you can plan the whole-pregnancy and postpartum buying rhythm: research the big items now through week 20 (crib, stroller, car seat), start buying baby clothes and daily goods in batches from weeks 20-28, and prepare the hospital bag and nursing supplies at weeks 28-36. Don’t try to buy everything at once — while your energy is best in mid-pregnancy, shop around and compare, buy rationally, and don’t be pushed by marketing.

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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