Week-by-Week Pregnancy Development Guide (International Standards)
Weeks 1-4
Weeks 5-8
Weeks 9-12
Weeks 13-16
Weeks 17-20
Weeks 21-24
Weeks 25-28
Weeks 29-32
Weeks 33-36
Weeks 37-40
Weeks 1-4: Conception and Implantation
No obvious pregnancy symptoms appear yet, but the ovary has released an egg. If fertilization is successful, the fertilized egg will move to the uterus and implant. Clinically, the first day of the last menstrual period is typically used as the starting point for calculating gestational age.
- Ovulation occurs around week 2, and sperm fertilizes the egg to form a zygote
- Implantation occurs at weeks 3-4, and hCG (human chorionic gonadotropin) secretion begins
- Pregnancy can be detected with a home pregnancy test (10-14 days after intercourse)
- Recommendation: Start taking folic acid (0.8mg/day) once pregnancy is confirmed
Common symptoms: Fatigue, mild abdominal cramps, breast tenderness, missed period.
Weeks 5-8: Rapid Embryo Development
The embryo takes shape, the heart starts beating, and major organs begin to differentiate. It is recommended to have the first ultrasound scan at weeks 6-8 to confirm intrauterine pregnancy and detect fetal heartbeat and gestational sac.
- Fetal heartbeat can be detected via ultrasound at weeks 6-7 (normal: ≥120 beats per minute)
- Brain, spinal cord, and limb buds form, and facial features begin to appear
- At week 8, the embryo is about 2cm long and weighs about 1g
- Prenatal care: Establish pregnancy record, basic tests including blood count, urine analysis, thyroid function
Common symptoms: Morning sickness, loss of appetite, frequent urination, mood swings.
Weeks 9-12: End of First Trimester
The embryo develops into a fetus with all major organs formed, and the risk of miscarriage decreases significantly. Key prenatal test: NT scan (11-13⁺⁶ weeks) to screen for Down syndrome risk.
- Fingers and toes are fully differentiated and can make small movements
- Genitalia begins to develop; at week 12, the fetus is about 6cm long and weighs about 14g
- NT scan: Normal nuchal translucency thickness is <2.5mm
- Recommendation: Continue folic acid until week 12, start calcium supplementation (1000mg/day)
Common symptoms: Reduced morning sickness, persistent breast tenderness, bloating, constipation.
Weeks 13-16: Entering Second Trimester
The second trimester is a relatively comfortable stage of pregnancy with accelerated fetal growth. It is recommended to undergo second-trimester Down syndrome screening at weeks 15-20.
- Fetal gender can be preliminarily determined via ultrasound (gender identification for non-medical purposes is prohibited)
- The fetus can hear external sounds, and lanugo hair begins to grow on the skin
- At week 16, the fetus is about 16cm long and weighs about 110g
- Screening results: Low risk is normal; high risk requires further non-invasive DNA testing or amniocentesis
Common symptoms: Increased appetite, backache, gradually protruding abdomen, first perception of fetal movement (earlier for multiparous women).
Weeks 17-20: Obvious Fetal Movement
Primiparous women usually feel fetal movement at weeks 18-20. It is recommended to undergo anomaly scan ultrasound (systematic ultrasound) at weeks 20-24 to detect fetal structural abnormalities.
- Fetal heartbeat can be heard with a Doppler fetal monitor (normal: 110-160 beats per minute)
- Fetal skin is covered with vernix caseosa, can swallow amniotic fluid, and has sleep/wake cycles
- At week 20, the fetus is about 25cm long and weighs about 320g
- Prenatal care: Measure fundal height and abdominal circumference, monitor blood pressure and blood sugar (screen for gestational hypertension/diabetes)
Common symptoms: Ligament pain from uterine enlargement, heartburn, lower extremity edema, increased vaginal discharge.
Weeks 21-24: Critical Period for Lung Development
The fetal lungs start producing surfactant in preparation for breathing. Key prenatal test: Anomaly scan ultrasound (20-24 weeks), the most important malformation screening during pregnancy.
- The fetus can respond to light and sound, and eyelids can open and close
- At week 24, the fetus is about 30cm long and weighs about 630g
- Anomaly scan: Covers all major organs including skull, spine, heart, limbs
- Recommendation: Undergo gestational diabetes screening (OGTT test) at weeks 24-28
Common symptoms: Braxton Hicks contractions, leg cramps, insomnia, hemorrhoids.
Weeks 25-28: End of Second Trimester
The fetal brain develops rapidly and may experience dreaming. It is recommended to start counting fetal movements at week 28 (count for 1 hour in the morning, noon, and evening; total ×4 ≥30 movements is normal).
- Fetal eyes can focus and recognize mother's voice
- At week 28, the fetus is about 35cm long and weighs about 1000g
- Prenatal care: OGTT test (screen for gestational diabetes), repeat blood count (screen for anemia)
- If preterm birth occurs, survival rate of 28-week fetus is about 80%
Common symptoms: Chest tightness and shortness of breath, increased frequent urination, hand and foot edema, back pain.
Weeks 29-32: Rapid Fetal Weight Gain
The fetus gains weight rapidly with increased subcutaneous fat. It is recommended to have prenatal checkups every 2 weeks to monitor fetal position and perform non-stress tests (NST).
- Fetal bones harden (skull remains soft to facilitate delivery), fetal movement amplitude decreases but strength increases
- At week 32, the fetus is about 40cm long and weighs about 1700g
- Prenatal care: Non-stress test (NST) to assess fetal well-being
- Fetal position fixed: Cephalic presentation is normal; breech presentation requires medical evaluation for correction
Common symptoms: Pelvic pressure, difficulty sleeping, heartburn, nipple discharge.
Weeks 33-36: Preparing for Delivery
The fetus descends into the pelvis (more obvious in primiparous women), and lungs are basically mature. It is recommended to have weekly prenatal checkups, with NST and ultrasound to assess amniotic fluid/placenta starting at week 36.
- Fetal immune system matures, storing antibodies to adapt to postnatal environment
- At week 36, the fetus is about 45cm long and weighs about 2500g
- Prenatal care: Group B streptococcus screening (35-37 weeks) to determine if antibiotics are needed during delivery
- Placental grading: 0-Ⅰ grade is normal; Ⅲ grade indicates placental aging
Common symptoms: Increased Braxton Hicks contractions, vaginal pressure, fatigue, prenatal anxiety.
Weeks 37-40: Full Term and Ready for Delivery
37 weeks is considered full term, and delivery between 37-42 weeks is normal clinically. If no labor starts by 40 weeks, cervical condition and placental function are evaluated to decide if induction is needed.
- Full-term fetus weighs about 2500-4000g (normal range) and is 48-52cm long
- Fetal lungs are fully mature with autonomous breathing ability
- Prenatal care: Weekly NST and ultrasound to assess amniotic fluid index (8-25cm is normal)
- Signs of labor: Bloody show, rupture of membranes, regular contractions (every 5-10 minutes)
Common symptoms: Mucus plug discharge, rupture of membranes, regular contractions, bloody discharge.