Fetal Development Week by Week: How Your Baby Grows

The journey from a single fertilized cell to a fully formed baby ready for birth represents one of nature's most extraordinary transformations. Over approximately 40 weeks, your baby develops from a microscopic cluster of cells into a complex human being with functioning organs, unique fingerprints, and the ability to recognize your voice. Understanding fetal development week by week not only satisfies natural curiosity but also helps you appreciate the critical importance of prenatal care, nutrition, and lifestyle choices at each stage. According to developmental biology research, every week brings specific milestones and changes, with certain periods being particularly crucial for organ formation and growth. This comprehensive guide takes you through each week of your baby's development, explaining what's happening at the cellular level, what structures are forming, and how your baby's size compares to familiar objects. Whether you're tracking your own pregnancy or simply fascinated by human development, this detailed timeline reveals the miraculous process occurring within the womb.

Conception to Week 4: The Beginning of Life

The journey begins before you even know you're pregnant. Understanding these earliest stages helps appreciate the complexity of human development from the very start.

Conception (Week 2-3 from LMP): Ovulation typically occurs around day 14 of your menstrual cycle. When sperm meets egg in the fallopian tube, fertilization creates a single cell (zygote) containing all genetic information needed to create a unique human being. Within hours, this cell begins dividing through mitosis. The gender is determined at fertilization by whether the sperm carries an X or Y chromosome. Week 3 - The Journey Begins: The fertilized egg, now called a morula (resembling a mulberry), continues dividing while traveling down the fallopian tube. By day 4-5 after fertilization, it becomes a blastocyst - a hollow ball of about 100 cells. The inner cell mass will become your baby, while the outer layer (trophoblast) will form the placenta. This is when identical twins may form if the inner cell mass splits. Week 4 - Implantation: Around 6-12 days after fertilization, the blastocyst burrows into the uterine lining (implantation), which may cause light spotting. The embryo is approximately 0.1-0.2 mm, smaller than a poppy seed. Crucial developments include: - Formation of the amniotic sac which will cushion your baby - Beginning of the yolk sac providing early nourishment - Start of placenta development - HCG production begins, making pregnancy tests positive - Three primary germ layers form: ectoderm (nervous system, skin), mesoderm (heart, muscles, bones), and endoderm (lungs, digestive system)

Weeks 5-8: The Embryonic Period

This critical period sees the formation of all major organ systems. The embryo is most vulnerable to harmful substances during these weeks.

Week 5 - Size of a Sesame Seed (2-3 mm):

- Neural tube forms, which becomes brain and spinal cord - Heart begins as a simple tube and starts beating around day 21-22 - Blood vessels begin forming - Arm and leg buds appear as tiny swellings - Facial features start developing with tiny dots where eyes will be

Week 6 - Size of a Lentil (4-6 mm):

- Heart divides into chambers and beats 110-160 times per minute - Neural tube closes completely (preventing spina bifida) - Eyes and ears begin forming - Tiny buds appear where arms and legs will grow - Digestive system begins developing - Kidneys start forming

Week 7 - Size of a Blueberry (10-13 mm):

- Brain develops rapidly, forming three distinct regions - Arms and legs elongate with paddle-like hands and feet - Facial features become more defined - Umbilical cord fully forms - Liver begins producing blood cells - Pancreas and appendix appear

Week 8 - Size of a Raspberry (14-20 mm):

- Officially becomes a fetus rather than embryo - Fingers and toes begin separating (still webbed) - Eyelids form and fuse shut - Ears, upper lip, and nose tip visible - All essential organs have begun forming - Tail disappears - Movement begins (too small to feel)

Weeks 9-13: Completing the First Trimester

Rapid growth and refinement of already-formed structures characterize this period.

Week 9 - Size of a Grape (23-30 mm, 2 grams):

- Reproductive organs develop internally (external genitals still identical) - Heart completes dividing into four chambers - Teeth buds form under gums - Fingers and toes fully separate - Major joints (elbows, knees) work - Facial features continue refining

Week 10 - Size of a Kumquat (31-42 mm, 4 grams):

- Vital organs fully formed and beginning to function - Brain produces 250,000 neurons per minute - Fingernails and toenails begin forming - Bones begin hardening from cartilage - Stomach produces digestive juices - Kidneys begin producing urine

Week 11 - Size of a Lime (44-60 mm, 7 grams):

- Diaphragm develops, allowing breathing movements - External genitalia begin differentiating - Hair follicles form - Nasal passages open - Tongue and palate develop - Baby can swallow and produces urine

Week 12 - Size of a Plum (54-74 mm, 14 grams):

- Reflexes develop - baby can open/close fingers, curl toes - Vocal cords form - Intestines move from umbilical cord into abdomen - Bone marrow begins making white blood cells - Pituitary gland begins producing hormones - Gender may be visible on ultrasound

Week 13 - Size of a Peach (67-82 mm, 25 grams):

- Fingerprints begin forming - Veins and organs visible through thin skin - Body begins growing faster than head - Vocal cords develop - Baby can put thumb in mouth - Pancreas begins producing insulin

Weeks 14-18: Early Second Trimester Development

The second trimester brings continued growth and increasing complexity of body systems.

Week 14 - Size of a Lemon (80-93 mm, 43 grams):

- Facial expressions possible due to muscle development - Body covered with fine hair (lanugo) - Thyroid gland begins producing hormones - Roof of mouth (palate) forms - Prostate gland develops in boys - Ovaries move into pelvis in girls

Week 15 - Size of an Apple (95-103 mm, 70 grams):

- Bones continue hardening - Scalp hair pattern forms - Baby can sense light through closed eyelids - Legs now longer than arms - All joints move freely - Taste buds develop

Week 16 - Size of an Avocado (108-116 mm, 100 grams):

- Eyes move closer together - Baby can make sucking motions - Nervous system allows limb coordination - External ears nearly in final position - Circulatory system fully functional - Many mothers feel first movements

Week 17 - Size of a Turnip (110-120 mm, 140 grams):

- Fat deposits begin forming under skin - Hearing begins developing - Fingerprints and footprints unique patterns set - Umbilical cord grows stronger and thicker - Skeleton changes from cartilage to bone - Sweat glands develop

Week 18 - Size of a Bell Pepper (125-140 mm, 190 grams):

- Myelin begins coating nerves - Chest moves up and down mimicking breathing - If female, uterus and fallopian tubes formed - If male, genitals may be visible on ultrasound - Blood vessels visible through thin skin - Meconium begins forming in intestines

Weeks 19-24: Mid-Pregnancy Milestones

This period includes the important anatomy scan and viability milestone.

Week 19 - Size of a Mango (140-153 mm, 240 grams):

- Vernix caseosa (waxy coating) protects skin - Brain develops specialized areas for senses - Permanent teeth buds form behind baby teeth - If female, eggs develop in ovaries - Hair may be visible on scalp - Sleep-wake cycles establish

Week 20 - Size of a Banana (145-160 mm head to rump, 300 grams):

- Halfway point of pregnancy - Detailed anatomy scan typically performed - Iron stores begin building - Digestive system produces meconium - If female, vaginal canal develops - Skin thickens and develops layers

Week 21 - Size of a Carrot (250 mm, 360 grams):

- Bone marrow produces blood cells - Small amounts of sugar absorbed from amniotic fluid - Eyelids and eyebrows fully developed - Rapid eye movements begin - Vagina hollows out in females - Testes begin descent in males

Week 22 - Size of a Papaya (270 mm, 430 grams):

- Pancreas develops steadily - Lips become more distinct - Tooth buds for permanent teeth appear - Inner ear fully developed - Brain waves similar to newborn - Extreme preemies may survive with intensive care

Week 23 - Size of a Grapefruit (280 mm, 500 grams):

- Rapid weight gain begins - Skin appears red and wrinkled - Blood vessels in lungs develop - Loud noises may startle baby - Pancreas produces insulin - Fat deposits turn skin less transparent

Week 24 - Size of an Ear of Corn (300 mm, 600 grams):

- Viability milestone - survival possible with NICU care - Lungs produce surfactant - Brain waves show rapid eye movement (REM) - Nostrils begin opening - Unique footprints and fingerprints complete - Inner ear controls balance

Weeks 25-30: Rapid Growth and Development

The third trimester begins with accelerated growth and organ maturation.

Week 25 - Size of a Rutabaga (320 mm, 660 grams):

- Capillaries form under skin - Hands develop dexterity - Spine structures solidify - Nostrils open allowing practice breathing - Memory development begins - Responds to familiar voices

Week 26 - Size of a Zucchini (355 mm, 760 grams):

- Eyes open and close - All eye components developed - Immune system strengthening - Males: testicles begin descending - Females: labia developing - May respond to touch on mother's belly

Week 27 - Size of a Cauliflower (365 mm, 875 grams):

- Sleep and wake cycles regular - Brain tissue increases rapidly - Retinas form allowing light perception - Hiccupping common - Taste buds fully developed - Dreams may occur during REM sleep

Week 28 - Size of an Eggplant (375 mm, 1005 grams):

- Eyes can open and blink - Eyelashes fully grown - Muscle tone improves - Central nervous system can control body temperature - Billions of neurons developing - Practices breathing movements regularly

Week 29 - Size of a Butternut Squash (380 mm, 1153 grams):

- Bones hardening but skull remains soft - Brain can control primitive breathing - Sensitivity to light, sound, taste, and smell - Fat deposits making skin smoother - Adrenal glands produce hormones - Very active with less room to move

Week 30 - Size of a Cabbage (400 mm, 1319 grams):

- Bone marrow takes over blood cell production - Brain growth causes skull expansion - Lanugo begins disappearing - Can follow light source with eyes - Grip strength increasing - Digestive tract nearly mature

Weeks 31-35: Final Preparations

Major organs complete development while baby gains weight rapidly.

Week 31 - Size of a Coconut (410 mm, 1502 grams):

- Brain connections rapidly forming - Can turn head side to side - Irises react to light - Fat layers regulating body temperature - Processing information from all five senses - Toenails fully grown

Week 32 - Size of a Napa Cabbage (425 mm, 1702 grams):

- Bones hardening except skull - Skin becomes opaque - Sleep 90-95% of the day - Breathing practice continues - If born now, excellent survival rate - Hair may be visible on ultrasound

Week 33 - Size of a Pineapple (440 mm, 1918 grams):

- Pupils can dilate and constrict - Brain waves show rapid development - Immune system developing antibodies - Less vernix as fat develops - May settle into head-down position - Skull bones remain flexible for birth

Week 34 - Size of a Cantaloupe (450 mm, 2146 grams):

- Central nervous system maturing - Breathing more rhythmic - Body temperature regulation improving - Fingernails reach fingertips - Most babies head-down by now - Sleep cycles more distinct

Week 35 - Size of a Honeydew Melon (460 mm, 2383 grams):

- Kidneys fully developed - Liver can process waste - Most physical development complete - Rapid weight gain continues - Lungs nearly mature - Hearing fully developed

Weeks 36-40: Ready for Birth

Final weeks focus on weight gain and lung maturation.

Week 36 - Size of a Romaine Lettuce (470 mm, 2622 grams):

- Considered "late preterm" if born - Digestive system ready for milk - Circulation and immune systems ready - Shedding most vernix and lanugo - Head may "drop" into pelvis - Grasping reflexes strong

Week 37 - Size of a Winter Melon (485 mm, 2859 grams):

- Full term pregnancy begins - All organs capable of function - Practicing breathing movements - Firm grasp reflex - Head circumference same as abdomen - May have full head of hair

Week 38 - Size of a Leek (490 mm, 3083 grams):

- Brain and nervous system ready for life outside - Lungs produce more surfactant - All organ systems go for birth - Swallowing amniotic fluid - Producing hormones for birth - Limited movement due to size

Week 39 - Size of a Watermelon (505 mm, 3288 grams):

- Official due date approaching - Chest prominent feature - Placenta provides antibodies - Tear ducts not yet open - Skin smooth and plump - Ready for birth any day

Week 40 - Size of a Pumpkin (510 mm, 3462 grams):

- Average birth weight 7.5 pounds - 95% of babies born within two weeks of due date - Skull bones not yet fused - Continues gaining antibodies - Reflexes coordinated - Ready to meet the world

Beyond 40 Weeks: Post-Term Considerations

Week 41-42 - Continued Growth:

- Baby continues gaining weight - Vernix decreases, may have dry skin - Placenta may show signs of aging - Monitoring increases for safety - Most providers recommend induction by 42 weeks - Average weight may exceed 8-9 pounds

Understanding Size Comparisons and Measurements

Throughout pregnancy, baby's size is communicated through various measurements:

Crown-Rump Length (CRL): Used until about 20 weeks, measuring from head to bottom Biparietal Diameter (BPD): Width of baby's head Femur Length (FL): Thighbone measurement Abdominal Circumference (AC): Around baby's belly Estimated Fetal Weight (EFW): Calculated from various measurements

Size comparisons to fruits and vegetables help visualize growth, though individual babies vary significantly. These are averages - healthy babies can be smaller or larger.

Critical Periods of Development

Understanding when specific structures develop emphasizes the importance of prenatal care timing:

Weeks 3-8: Most critical for organ formation. Exposure to harmful substances can cause major birth defects Weeks 9-12: Continued organ development and early function Weeks 13-27: Growth and maturation of formed structures Weeks 28-40: Final maturation, particularly lungs and brain

Factors Affecting Fetal Development

Multiple factors influence how your baby grows:

Maternal Nutrition: Adequate calories, proteins, vitamins, and minerals essential Maternal Health: Chronic conditions require careful management Environmental Factors: Toxins, medications, infections can impact development Genetics: Influence size, growth rate, and some developmental timing Placental Function: Efficiency affects nutrient and oxygen delivery

Monitoring Fetal Development

Healthcare providers track development through:

Ultrasounds: Visualize structures and measure growth Fundal Height: Uterus size corresponds to gestational age Fetal Movement: Indicates wellbeing after 28 weeks Non-Stress Tests: Monitor heart rate patterns Biophysical Profiles: Comprehensive wellbeing assessment

Supporting Optimal Development

You can support your baby's development by:

- Taking prenatal vitamins including folic acid - Eating balanced, nutritious meals - Staying hydrated - Avoiding harmful substances - Managing stress - Attending prenatal appointments - Getting adequate rest - Staying physically active as approved

Celebrating Developmental Milestones

Each week brings amazing progress worth celebrating:

- First heartbeat (week 6) - All organs formed (week 10) - Gender visible (week 14-16) - First movements felt (week 16-20) - Viability reached (week 24) - Eyes open (week 26) - Full term achieved (week 37)

Understanding fetal development week by week deepens the connection with your growing baby and emphasizes the importance of prenatal care. While every baby develops at their own pace, this timeline provides a framework for appreciating the miraculous transformation occurring within. From those first cell divisions to the moment of birth, each week represents crucial progress in your baby's journey toward life outside the womb. Cherish each milestone, knowing that your body is orchestrating one of nature's most complex and beautiful processes.