Week-by-Week Pregnancy Development

Complete guide to all 40 weeks of pregnancy — baby size, development milestones, ultrasound visibility, and teratogen sensitivity windows.

Fetal Development Timeline

Week	Milestone	System	Detail
3-4	Neural tube forms	Nervous	The neural plate folds to form the neural tube, which becomes the brain and spinal cord. Failure to close properly causes neural tube defects (anencephaly, spina bifida). Folate is critical during this period.
4-5	Heart begins beating	Cardiovascular	The primitive heart tube forms and begins rhythmic contractions at ~22 days post-conception. Initially a simple tube, it loops and septates into 4 chambers by week 8.
5-6	Limb buds appear	Musculoskeletal	Upper limb buds appear at day 26, lower at day 28. Cartilaginous skeleton forms first, then ossifies. Fingers separate by week 8 via programmed cell death (apoptosis).
6-7	Eyes and ears form	Sensory	Optic cups and lens vesicles form. Otic placodes invaginate to form inner ear structures. Retinal layers differentiate.
7-8	Liver produces blood cells	Hematopoietic	The liver becomes the primary site of hematopoiesis (blood cell production), replacing the yolk sac. It continues this role until bone marrow takes over at ~28 weeks.
8-9	All major organs present	Multiple	End of embryonic period. Every major organ system is present in at least primitive form. The embryo is now called a fetus. Susceptibility to teratogens decreases (but does not disappear).
10-11	External genitalia differentiate	Reproductive	Under influence of testosterone (or its absence), the genital tubercle differentiates into male or female external genitalia. Ultrasound sex determination becomes possible by ~12-13 weeks.
12-13	Kidneys produce urine	Renal	Metanephric kidneys begin filtering blood and producing urine, which contributes to amniotic fluid volume. By term, the fetus produces 500-700 mL of urine daily.
16	Hearing develops	Sensory	The cochlea achieves adult configuration. The fetus begins responding to sound — maternal heartbeat, voice, and loud external noises. Studies show neonates recognize their mother's voice at birth.
20	Vernix and lanugo	Integumentary	Vernix caseosa (waxy coating of sebum and desquamated cells) protects skin from amniotic fluid maceration. Lanugo (fine downy hair) helps vernix adhere to skin.
24	Viability threshold	Respiratory	Type II pneumocytes begin producing surfactant, which reduces alveolar surface tension and prevents lung collapse. Survival at 24 weeks: ~60-70% with intensive NICU care. Each additional week improves outcomes significantly.
26	Eyes open	Sensory	Eyelids unseal and the fetus can open eyes. Pupillary light reflex present. Retinal layers are mature enough for light perception.
28	Brain folds develop	Nervous	Cerebral cortex develops sulci and gyri, dramatically increasing surface area. Brain growth accelerates — the brain will grow more in the third trimester than at any other time.
32	Fat deposition accelerates	Metabolic	Body fat increases from ~3% to ~8-15% by term. Brown fat (for temperature regulation) and white fat (energy storage, insulation) both accumulate rapidly.
34	Lungs approaching maturity	Respiratory	Surfactant levels reach functional threshold. Late preterm infants (34-36 weeks) have >95% survival but may still need respiratory support. Lecithin-to-sphingomyelin (L/S) ratio >2.0 indicates maturity.
37-39	Full maturity	Multiple	All organ systems functionally mature. Brain myelination continues (and will continue for years postnatally). Immune system receives final maternal IgG transfer. Liver stores iron for first 6 months.

First Trimester (Weeks 1-13)

The first trimester spans conception through week 13. During this critical period, all major organ systems begin forming (organogenesis), making this the highest-risk window for teratogenic exposure. Most miscarriages occur during this trimester.

Week 1-2: Conception & Implantation

Baby: Fertilization occurs in the fallopian tube. The zygote undergoes rapid cell division (cleavage) forming a morula, then blastocyst. By day 6-12, the blastocyst implants into the uterine lining (endometrium).

Mom: Most women are unaware of pregnancy at this stage. The luteal phase continues under progesterone from the corpus luteum. Basal body temperature remains elevated.

Symptoms: No noticeable symptoms yet; Possible implantation bleeding (light spotting, 25% of pregnancies); Mild cramping from implantation

Week 3-4: Missed Period & Early Detection

Baby: The embryo is ~1mm. Three germ layers form: ectoderm (skin, nervous system), mesoderm (muscles, bones, heart), endoderm (digestive tract, lungs). The neural tube begins forming. hCG production begins.

Mom: Missed period is typically the first recognized sign. hCG levels double every 48-72 hours. Home pregnancy tests become positive (sensitivity: 25 mIU/mL for most tests).

Symptoms: Missed period; Breast tenderness and swelling; Fatigue (progesterone-mediated); Frequent urination (hCG stimulates kidneys); Mild nausea beginning

Tests: Home pregnancy test (urine hCG); Serum beta-hCG (quantitative) if needed; Progesterone level if history of loss

Warning signs: Heavy bleeding with cramping (possible ectopic or miscarriage); Severe one-sided pain (ectopic pregnancy)

Week 5-6: Heartbeat Begins

Baby: Crown-rump length (CRL) ~4mm. The primitive heart tube begins beating at ~6 weeks (80-100 bpm initially, rising to 120-160 bpm). Limb buds appear. The neural tube closes (critical window for folate). Facial features begin forming.

Mom: Morning sickness often intensifies (affects 70-80% of pregnancies). Progesterone causes smooth muscle relaxation leading to bloating and constipation. Blood volume begins increasing.

Symptoms: Morning sickness (nausea/vomiting, can occur any time of day); Food aversions and heightened smell sensitivity; Extreme fatigue; Bloating and gas; Mood swings (hormonal fluctuation)

Tests: First prenatal visit (schedule for weeks 8-10); Transvaginal ultrasound if indicated (confirm intrauterine pregnancy, rule out ectopic)

Warning signs: No heartbeat detected at 6+ weeks (may need repeat scan); Severe vomiting unable to keep fluids down (hyperemesis gravidarum)

Week 7-8: Rapid Organ Development

Baby: CRL ~16mm. All major organs are forming. Fingers and toes begin separating. Eyes develop pigment. The intestines start forming in the umbilical cord. Brain develops distinct regions (forebrain, midbrain, hindbrain).

Mom: Uterus grows from fist-sized to grapefruit-sized. Blood volume increases by ~10% already. Ligaments begin softening (relaxin hormone). Increased vaginal discharge (leukorrhea) is normal.

Symptoms: Continued nausea (peaks weeks 8-10); Breast growth and darkening areolas; Visible veins on breasts and abdomen; Headaches; Constipation (progesterone slows GI motility)

Tests: First prenatal visit: complete blood count (CBC), blood type & Rh, antibody screen, rubella immunity, hepatitis B/C, HIV, syphilis (RPR/VDRL), urinalysis, urine culture, Pap smear if due, STI screening

Warning signs: Fever >101.5F (38.6C) — potential teratogenic risk; Vaginal bleeding with tissue passage

Week 9-10: Embryo to Fetus Transition

Baby: At week 10, the embryonic period ends and the fetal period begins. CRL ~31mm. All essential organs are present. External genitalia begin differentiating. Fingers and toes are fully separated. Spontaneous movements begin (too small to feel).

Mom: Waistline may begin thickening though not yet "showing." Heart rate increases 10-20 bpm above baseline to support increased blood volume. Emotional lability is common.

Symptoms: Nausea may begin improving; Round ligament pain (sharp groin/lower abdomen pain with movement); Vivid dreams (progesterone effect); Nasal congestion (pregnancy rhinitis — increased blood flow to mucous membranes); Acne or skin changes

Tests: NIPT (non-invasive prenatal testing) available from 10 weeks — cell-free fetal DNA screens for trisomies 21, 18, 13 and sex chromosome aneuploidies (sensitivity >99% for trisomy 21); Chorionic villus sampling (CVS) available 10-13 weeks if indicated

Warning signs: Sudden loss of pregnancy symptoms (may indicate missed miscarriage — but symptom fluctuation is also normal); Persistent sharp abdominal pain

Week 11-12: Nuchal Translucency Window

Baby: CRL ~54mm. Bones begin hardening (ossification). Tooth buds form. Kidneys begin producing urine. Red blood cells form in the liver. Vocal cords develop. The fetus can open and close hands.

Mom: First trimester fatigue may begin lifting. Appetite often returns as nausea subsides. The uterus rises above the pubic symphysis. Hair and nails may grow faster.

Symptoms: Decreased nausea for many women; Increased appetite; Heartburn (lower esophageal sphincter relaxation); Dizziness (blood pressure drops in early pregnancy); Darkening linea nigra may appear

Tests: NT scan (nuchal translucency ultrasound) 11-14 weeks — measures fluid at back of fetal neck; combined with maternal serum markers (PAPP-A, free beta-hCG) for first-trimester combined screening; Detection rate for trisomy 21: ~85% with 5% false-positive rate

Warning signs: NT measurement >3.5mm warrants further evaluation; Vaginal bleeding — affects 25% of first trimester pregnancies; 50% of those continue normally

Week 13: End of First Trimester

Baby: CRL ~76mm, weight ~23g. Fingerprints forming. Vocal cords complete. Intestines migrate from umbilical cord into abdomen. The placenta is now fully functional, taking over hormone production from the corpus luteum.

Mom: Miscarriage risk drops significantly (from 15-20% to <1% after confirmed heartbeat at 12 weeks). Energy typically returns. The placenta takes over progesterone production from the corpus luteum (luteal-placental shift).

Symptoms: Energy returning ("second trimester glow" approaching); Reduced nausea and vomiting; Visible "baby bump" for some (especially second+ pregnancies); Increased libido for some women; Occasional headaches

Tests: Cell-free DNA results returned if drawn at 10-12 weeks; First-trimester screening results available

Warning signs: Persistent vomiting beyond 14 weeks (evaluate for hyperemesis gravidarum or molar pregnancy); Any heavy bleeding or passage of tissue

Second Trimester (Weeks 14-27)

Often called the "honeymoon trimester" — nausea typically resolves, energy returns, and the belly visibly grows. Critical screening tests occur during this period including the anatomy scan. The fetus reaches viability (~24 weeks) by the end of this trimester.

Week 14-15: The Honeymoon Begins

Baby: Length ~87mm, weight ~43g. Facial muscles allow squinting and frowning. Lanugo (fine body hair) develops. The liver produces bile. The spleen begins producing red blood cells. Thyroid gland begins functioning.

Mom: Energy levels improve markedly. Appetite increases — second trimester requires ~340 extra calories/day above pre-pregnancy baseline. "Pregnancy glow" may appear (increased blood flow + oil production).

Symptoms: Increased energy; Growing appetite; Round ligament pain (stretching uterine ligaments); Nasal congestion and nosebleeds (hyperemia of nasal mucosa); Reduced breast tenderness

Tests: Quad screen (AFP, hCG, estriol, inhibin-A) offered 15-22 weeks if first-trimester screening not done; Amniocentesis available from 15 weeks if indicated (diagnostic accuracy >99%)

Warning signs: Fluid gush from vagina (premature rupture of membranes); Regular contractions before 37 weeks

Week 16-17: Quickening Approaches

Baby: Length ~120mm, weight ~100g. Bones continue hardening. Eyes can sense light (through closed lids). Meconium accumulates in intestines. Hearing begins developing — cochlea is forming. Patterned limb movements increase.

Mom: Uterus is between pubic bone and navel. Some women (especially multiparas) feel first fetal movements ("quickening") — described as fluttering, bubbles, or popcorn popping. Weight gain becomes more noticeable.

Symptoms: Possible quickening (first-time mothers may not feel until 20-22 weeks); Backache (postural changes from shifting center of gravity); Skin changes: melasma (mask of pregnancy), darkening nipples; Increased vaginal discharge; Mild edema in feet/ankles

Tests: AFP (alpha-fetoprotein) if not included in earlier screening — elevated AFP suggests neural tube defects; low AFP associated with chromosomal abnormalities

Warning signs: Absence of fetal movement by 24 weeks warrants evaluation; Severe headache with visual changes (early preeclampsia, though rare this early)

Week 18-20: Anatomy Scan

Baby: Length ~160mm, weight ~260g. Vernix caseosa (waxy protective coating) covers the skin. Myelin begins forming around nerves. The uterus is fully formed in female fetuses. Permanent teeth buds form behind baby teeth buds. Sleep-wake cycles emerge.

Mom: Fundal height approximately equals gestational age in centimeters. Belly is clearly visible. Cardiovascular system adapts — cardiac output increases 30-50% above pre-pregnancy levels. Blood pressure typically at its lowest point.

Symptoms: Obvious fetal movement for most women; Leg cramps (especially at night); Dizziness when standing quickly (orthostatic hypotension); Increased appetite and weight gain; Mild shortness of breath (progesterone stimulates deeper breathing)

Tests: Anatomy scan (detailed ultrasound) at 18-22 weeks: evaluates all major organ systems, measures fetal biometry, checks placental location, amniotic fluid volume, cervical length. Can determine sex with >99% accuracy.

Warning signs: Short cervix (<25mm) on anatomy scan — risk factor for preterm birth, may require cerclage or progesterone; Placenta previa identified (placenta covering cervical os) — most resolve by third trimester

Week 21-23: Approaching Viability

Baby: Length ~280mm, weight ~500g. Surfactant production begins in lungs (type II pneumocytes). Rapid brain growth — neurons forming at 250,000/minute. Taste buds functional (fetus swallows amniotic fluid). Fingerprints complete. Skin is translucent and wrinkled.

Mom: Braxton Hicks contractions may begin (irregular, painless uterine tightenings). Stretch marks may appear (striae gravidarum — affect 50-90% of pregnancies). Linea nigra darkens.

Symptoms: Braxton Hicks contractions (irregular, not progressively painful); Stretch marks on abdomen, breasts, thighs; Hemorrhoids (increased pelvic pressure + constipation); Swollen gums and possible bleeding (pregnancy gingivitis); Carpal tunnel symptoms (fluid retention compresses median nerve)

Tests: Follow-up ultrasound if anatomy scan showed any concerns

Warning signs: Regular contractions (<37 weeks) — preterm labor risk; Sudden swelling of face and hands (preeclampsia sign)

Week 24-25: Viability Milestone

Baby: Weight ~660g. Considered "viable" — survival rate at 24 weeks is ~60-70% with NICU care (increases ~3-4% per additional day). Lungs still immature but surfactant increasing. Eyes open for the first time. Startle reflex present. Fat deposition begins.

Mom: Glucose screening window opens. Weight gain typically 12-15 lbs total by now. Belly button may "pop" outward. Backache intensifies as the uterus grows. Sleep becomes more difficult.

Symptoms: Difficulty sleeping (positional discomfort, frequent urination); Itchy skin on stretching abdomen; Increased Braxton Hicks; Swollen feet and ankles (especially in warm weather); Tingling or numbness in hands

Tests: Glucose challenge test (GCT) at 24-28 weeks: 50g oral glucose load, 1-hour blood draw. Threshold: 130-140 mg/dL. If abnormal, proceed to 3-hour glucose tolerance test (GTT) with 100g load.; CBC repeat to check for anemia; Rh antibody screen repeat for Rh-negative mothers

Warning signs: Failed glucose screening (gestational diabetes affects 6-9% of pregnancies); Decreased fetal movement — begin kick counts (goal: 10 movements in 2 hours)

Week 26-27: End of Second Trimester

Baby: Weight ~900g, length ~36cm. Lungs begin practicing "breathing" movements (inhaling/exhaling amniotic fluid). Brain wave patterns show distinct sleep states (active/quiet sleep). Eyes can distinguish light from dark. Immune system developing — IgG antibodies transfer from mother.

Mom: RhoGAM injection given at 28 weeks for Rh-negative mothers. Third trimester discomforts begin emerging. Pelvic girdle pain increases. Many women begin prenatal education classes.

Symptoms: Pelvic pressure increasing; Shortness of breath (uterus pushes diaphragm up); Frequent urination returns (fetal head pressure on bladder); Vivid dreams; Restless leg syndrome (affects 15-30% of pregnancies)

Tests: RhoGAM (anti-D immunoglobulin) at 28 weeks for Rh-negative mothers (prevents hemolytic disease of the newborn); Tdap vaccine recommended 27-36 weeks (optimal: 27-30 weeks for maximum antibody transfer)

Warning signs: Leaking fluid (premature rupture of membranes — PPROM); Vision changes, severe headache, upper abdominal pain (preeclampsia); Decreased fetal movement from established baseline

Third Trimester (Weeks 28-40+)

The final stretch focuses on fetal growth, lung maturation, and birth preparation. The fetus gains approximately 200g per week. Regular monitoring increases, and birth planning begins in earnest.

Week 28-30: Rapid Growth Phase

Baby: Weight ~1.1kg. Brain surface develops gyri and sulci (folds that increase surface area). Fat stores accumulate rapidly — body fat increases from 3% to 15% by term. Pupils can dilate and constrict. Bone marrow takes over red blood cell production from liver.

Mom: Prenatal visits increase to every 2 weeks. Sleep becomes significantly disrupted. Heartburn intensifies (uterus pushes stomach upward). Blood volume peaks at 40-50% above pre-pregnancy levels.

Symptoms: Heartburn and acid reflux (prevalent in 40-80% of third trimester); Insomnia and sleep disruption; Shortness of breath (diaphragm displaced 4cm upward); Hemorrhoids (affects 35% of pregnancies); Rib pain from uterine pressure

Tests: Growth ultrasound if indicated (IUGR concerns, size discrepancy); Prenatal visits every 2 weeks through 36 weeks

Warning signs: Regular contractions before 37 weeks (preterm labor); Persistent headache not relieved by Tylenol; Sudden weight gain (>2 lbs/week) with edema — preeclampsia screen

Week 31-33: Lung Maturation Continues

Baby: Weight ~1.8kg. Surfactant production increases significantly. All five senses functional. Bones fully developed but skull remains soft (allows birth canal passage). Fetus assumes head-down (vertex) presentation in most cases. Rapid eye movement (REM) sleep present.

Mom: Braxton Hicks become more frequent and noticeable. "Nesting instinct" may emerge (urge to clean, organize, prepare). Increased pelvic pressure as baby grows. Colostrum may leak from breasts.

Symptoms: More intense Braxton Hicks contractions; Pelvic pressure and "lightning crotch" (sharp nerve pain); Leaking colostrum; Fatigue returns; Difficulty finding comfortable sleep position (left lateral recommended)

Tests: Fundal height measurement at each visit (should match gestational age +/- 2cm); Non-stress test (NST) may begin for high-risk pregnancies

Warning signs: Persistent contractions >4/hour before 37 weeks; Vaginal bleeding (placental abruption risk); Severe itching especially on palms/soles (cholestasis of pregnancy — bile acids test needed)

Week 34-35: Late Preterm Window

Baby: Weight ~2.3kg. Lungs approaching maturity — "late preterm" infants (34-36 weeks) have >95% survival but may need NICU for feeding/breathing support. Fat continues accumulating, giving a rounder appearance. Central nervous system maturing. Immune system receives maternal antibodies.

Mom: Prenatal visits increase to weekly starting at 36 weeks. Baby may "drop" (engage in pelvis) — lightening. Increased vaginal discharge. Cervix may begin ripening (softening, effacing).

Symptoms: Lightening — baby drops lower, easier breathing but more pelvic pressure; Increased vaginal discharge (mucus plug may begin loosening); Waddling gait (pelvic ligament laxity); Difficulty with stairs and bending; Frequent urination intensifies

Tests: Group B Streptococcus (GBS) culture at 36-37 weeks — vaginal/rectal swab. Positive in 25% of women. If positive, IV antibiotics during labor (penicillin or ampicillin); Weekly prenatal visits begin at 36 weeks; Cervical exam may begin (dilation, effacement, station)

Warning signs: Heavy vaginal bleeding (placental abruption or previa); Gush of fluid (premature rupture of membranes); Decreased fetal movement (fewer than 10 kicks in 2 hours)

Week 36-37: Early Term

Baby: Weight ~2.7kg. Lungs are mature. Lanugo shedding. Fat pads fill out cheeks (important for feeding). Liver stores iron. The fetus descends into the pelvic inlet (engagement). Head circumference ~33cm.

Mom: Cervical ripening accelerates. Mucus plug may pass ("bloody show"). Braxton Hicks intensify. Nesting urges peak. Birth plan should be finalized. Bag packed for hospital.

Symptoms: Increased pelvic pressure and hip pain; Difficulty sleeping; Frequent urination (nearly every hour for some); Mood swings and anxiety about labor; Mucus plug passage (clear, pink, or blood-tinged)

Tests: Weekly prenatal visits: BP, urine protein, fundal height, fetal heart tones, fetal position; GBS results returned; Bishop score assessment (cervical readiness for labor)

Warning signs: Blood pressure >140/90 on two readings (preeclampsia); Severe headache + visual changes + upper abdominal pain (HELLP syndrome); No fetal movement for extended period

Week 38-39: Full Term

Baby: Weight ~3.1-3.2kg. All organ systems mature. Meconium fills intestines. The brain weighs ~400g (will triple by age 1). Vernix mostly absorbed. The fetus is considered "full term" at 39 weeks (ACOG definition).

Mom: Cervical dilation may begin (0-3cm without active labor is common). Increased Braxton Hicks. Mucus plug passage. "Bloody show" (blood-tinged mucus) indicates cervical changes. Nesting behaviors peak.

Symptoms: Cervical dilation beginning; Increased Braxton Hicks (may be confused with early labor); Loose stools (prostaglandin effect — natural bowel preparation); Lower back pain; Emotional readiness/anxiety for labor

Tests: Weekly visits continue; Non-stress test (NST) may be performed weekly; Biophysical profile (BPP) if indicated: combines NST with ultrasound assessment of amniotic fluid, fetal breathing, movement, and tone

Warning signs: Regular contractions 5 minutes apart, lasting 1 minute, for 1 hour (5-1-1 rule — go to hospital); Water breaking (note color: clear=normal, green/brown=meconium, red=bleeding); Decreased fetal movement

Week 40-42: Due Date & Beyond

Baby: Average birth weight 3.3-3.5kg, length 50-51cm. Skull bones are flexible with fontanelles. All reflexes present (rooting, sucking, grasping, Moro). Ready for extrauterine life.

Mom: Only 5% of babies are born on their actual due date. Post-dates pregnancy (>41 weeks) requires increased surveillance. Risk of stillbirth increases after 42 weeks, prompting induction discussions at 41 weeks.

Symptoms: Intense pelvic pressure; Braxton Hicks vs real contractions (real contractions are regular, progressive, and do not stop with position change); Possible membrane sweeping offered; Anxiety and impatience; Continued cervical changes

Tests: NST twice weekly after 41 weeks; Amniotic fluid index (AFI) — oligohydramnios (<5cm) is indication for delivery; Bishop score for induction readiness

Warning signs: Post-term pregnancy (>42 weeks) — induction strongly recommended by 41-42 weeks due to increased stillbirth risk; Meconium-stained fluid; Absent fetal movement