Your Baby’s Growth Month by Month: Amazing Changes Inside the Womb
From a single cell smaller than a grain of sand to a fully formed human being — here is everything that happens in those extraordinary nine months.
There is nothing quite like pregnancy to make you aware of how miraculous the human body really is. Inside you, without any conscious effort on your part, a breathtakingly complex series of events is unfolding. Organs are forming. A heart is beginning to beat. A face is taking shape. Tiny fingers are developing fingerprints that are entirely unique to this one person who has never existed before and will never exist again.
Most pregnancy content focuses on what’s happening to you — the symptoms, the discomforts, the appointments. This article is about what’s happening to your baby. Month by month, from conception through birth, here is the full story of fetal development told in a way that is accurate, detailed, and genuinely wondrous.
Because when you understand what’s actually happening in there, even the difficult days of pregnancy take on a different meaning.
A note on timing: Pregnancy is typically measured in weeks from the first day of your last menstrual period (LMP), which means the first two weeks technically occur before conception. We’ll organize this guide by calendar month (months 1–9) while referencing the corresponding weeks so you can follow along with your own journey.
Month 1: The Beginning of Everything (Weeks 1–4)
What’s Happening
In the first week, there is technically no baby yet. Your body is preparing for ovulation, and the clock starts counting from the first day of your last period. Conception itself happens around week 2, when a single sperm — out of the 200 to 300 million that set out on the journey — successfully penetrates an egg in the fallopian tube.
In that moment, something extraordinary happens. Two half-cells merge into one complete cell called a zygote, containing all 46 chromosomes that will define this human being — their sex, their blood type, the color of their eyes, the shape of their nose, and thousands of other inherited traits, all determined in an instant.
Within 12–24 hours, the zygote begins dividing. Two cells become four. Four become eight. By the time this tiny cluster of cells — now called a blastocyst — reaches the uterus around days 5–6, it contains about 100 cells and is still smaller than a grain of sand.
Around day 6–10, the blastocyst burrows into the lining of the uterus in a process called implantation. This is often the moment that causes light spotting that some women mistake for an early period. Once implanted, the blastocyst begins producing human chorionic gonadotropin (hCG) — the hormone that pregnancy tests detect.
By the end of week 4, what was a single cell has transformed into an embryo with three distinct cell layers — the ectoderm, mesoderm, and endoderm — each of which will give rise to specific organs and tissues. The neural tube, which will become the brain and spinal cord, is already beginning to form.
Size This Month
About the size of a poppy seed by the end of week 4.
Did You Know?
The sex of your baby was determined at the exact moment of fertilization, but the physical differences between male and female embryos won’t appear until around week 7. For the first several weeks, all embryos follow identical development pathways.
Month 2: The Embryo Takes Shape (Weeks 5–8)
What’s Happening
Month two is one of the most intense periods of development in the entire pregnancy. An enormous amount happens in just four weeks — so much that doctors consider this the most critical window for fetal development, when the embryo is most vulnerable to environmental factors like alcohol, certain medications, and infections.
Week 5 brings the first heartbeat — a moment that, if you have an early ultrasound, is genuinely spine-tingling to see and hear. It’s not yet a fully formed four-chambered heart; it’s a primitive cardiac tube that has begun beating at roughly 100–160 beats per minute. The embryo is also developing the foundations of the brain, spinal cord, and digestive system.
Week 6 sees the beginnings of facial features. Dark spots appear where the eyes will be. Small depressions mark where the ears will form. The nose, jaw, and mouth are beginning to shape themselves. Arm buds appear — tiny paddle-like protrusions that will become your baby’s arms.
Week 7 brings leg buds. The brain is developing rapidly, forming distinct regions that will eventually govern movement, sensation, emotion, and thought. The embryo begins making spontaneous, jerky movements — though you won’t feel any of this for months.
Week 8 is a milestone. The embryo is now officially called a fetus (meaning “offspring” in Latin), marking the transition from the period of organogenesis — organ formation — to the period of growth and refinement. All the major organ systems are present in some form. The fetus has recognizable fingers and toes, though they are still webbed. Bones are beginning to harden. The liver is producing blood cells. The kidneys are starting to function.
Size This Month
From about 1.5 mm at the start of week 5 to roughly 1.6 cm (about the size of a raspberry) by the end of week 8.
Did You Know?
Your baby’s heart will beat approximately 54 million times before birth. In week 6, that tiny cardiac tube beats faster than an adult heart at rest — about 110 beats per minute — and will accelerate further as development progresses.
Month 3: Looking More Human Every Day (Weeks 9–12)
What’s Happening
By month three, the fetus is unmistakably human in shape, even though it would fit in the palm of your hand. This month marks the end of the first trimester — a significant milestone because after week 12, the risk of miscarriage drops dramatically.
The fingers and toes are now fully separated, no longer webbed. Fingernails are beginning to form. The ears are moving from the neck toward the sides of the head where they belong. Eyelids have formed and sealed shut — they won’t open again until around week 26–28.
The brain is developing at an extraordinary pace. Neurons are forming at a rate of roughly 250,000 per minute during peak development periods. The cerebral cortex — the part of the brain responsible for thought, language, and consciousness — is beginning to differentiate.
The fetus begins making more purposeful-looking movements this month. It can flex its fingers, open and close its mouth, and even suck its thumb — a reflex that will be essential for feeding after birth. If you could look inside, you might see your baby yawning, stretching, and moving its head.
The external genitalia are beginning to differentiate this month, though a skilled sonographer may or may not be able to identify sex on an ultrasound at this stage. By the end of week 12, the genitalia are more distinct.
The placenta — that remarkable organ that is essentially the baby’s life support system — is now fully functional. It is delivering oxygen and nutrients while removing carbon dioxide and waste products, all without the baby’s blood and the mother’s blood ever directly mixing.
Size This Month
From about 2.5 cm at the start (roughly the size of a grape) to about 7.5 cm (about the size of a lime) by the end of week 12. Weight is approximately 14 grams.
Did You Know?
By the end of month three, your baby has developed its own unique set of fingerprints — ridges on the fingertips that formed as the skin grew faster than the tissue underneath, creating those distinctive patterns. No two people in history have ever shared identical fingerprints.
Month 4: Movement, Senses, and Growing Fast (Weeks 13–16)
What’s Happening
Welcome to the second trimester — widely considered the most comfortable period of pregnancy for most women, and a period of remarkable acceleration in fetal development.
The fetus is growing rapidly this month, and its movements are becoming more coordinated and intentional. The skeleton, which has until now been made of soft cartilage, is beginning to be replaced by real bone through a process called ossification. You can see this clearly on an ultrasound — the bones are showing up bright white.
The nervous system is maturing quickly. Nerve connections are forming throughout the brain and body, allowing for increasingly complex movements and responses. The fetus can now make facial expressions — grimacing, squinting, and what looks remarkably like smiling, though this is reflexive rather than emotional at this stage.
One of the most exciting developments of month four is that many women begin to feel fetal movement for the first time. This is called quickening, and it’s often described as feeling like bubbles, flutters, or the gentle brush of a butterfly’s wing from the inside. First-time mothers often notice it later than those who have been pregnant before, because they know what to look for.
The fetus can now hear — not clearly, but primitive sound processing is underway. Low-frequency sounds travel through the amniotic fluid, and the auditory system is beginning to respond. This is why many parents begin talking, reading, and playing music to their baby during the second trimester.
The thyroid gland is now functioning, producing hormones essential for metabolism and brain development. The fetus is also beginning to practice breathing movements — inhaling and exhaling amniotic fluid, which helps develop the lungs for the moment of birth.
Size This Month
From about 8 cm at the start to roughly 12–14 cm (about the size of an avocado) by the end of week 16. Weight is approximately 100 grams.
Did You Know?
The lanugo — a fine, downy hair that covers the entire body of the fetus — begins growing this month. It helps regulate body temperature in the womb and is usually shed before birth, though some babies are born with patches of it still visible, particularly on their shoulders and back.
Month 5: The Baby You’ll Recognize (Weeks 17–20)
What’s Happening
Month five is when many parents have their anatomy scan — the detailed ultrasound performed around weeks 18–20 that checks all major organs and structures, and where the sex of the baby is often revealed. For many families, this is one of the most emotional and memorable moments of the pregnancy.
And there is a lot to see. By now, the fetus looks remarkably like a newborn baby, just much smaller. The face has fully developed features — a nose, lips, eyelids, eyebrows. Hair may be visible on the scalp. The proportions are becoming more balanced, with the head no longer as disproportionately large as it was in earlier months.
The baby is now covered in a waxy, white substance called vernix caseosa — literally “cheesy varnish” — which protects the delicate skin from the constant exposure to amniotic fluid. You’ll likely see traces of it on your baby at birth, particularly in the skin folds.
Sensory development is accelerating. The baby can now detect light and dark — if you shine a bright light on your belly, the baby may turn away from it. Sound perception is becoming much more sophisticated. Research suggests babies begin forming memories of voices and sounds they hear regularly in the womb. Babies born to mothers who read the same story aloud repeatedly during pregnancy have shown recognition of that story’s rhythm and cadence after birth.
Taste buds are now functioning. The amniotic fluid carries flavors from the foods you eat, and research shows that babies swallow more when the fluid is sweet and less when it’s bitter. The foods you eat during pregnancy may actually influence your baby’s food preferences after birth.
Perhaps most remarkably, if you’re having a girl, her ovaries already contain all the eggs she will ever have — approximately 6–7 million of them, which will reduce to about 1–2 million by birth. If those eggs are ever fertilized, they carry half the genetic material of your potential grandchildren. At five months pregnant with a daughter, you are already, in a sense, carrying the next generation.
Size This Month
From about 14 cm to roughly 25 cm (about the size of a banana) by the end of week 20. Weight is approximately 300 grams.
Did You Know?
The anatomy scan at 18–20 weeks can detect a remarkable range of conditions and characteristics — from heart defects and cleft palate to the position of the placenta and the amount of amniotic fluid. It is one of the most information-rich moments of the entire pregnancy.
Month 6: Eyes Open, Brain Blooming (Weeks 21–24)
What’s Happening
Month six is defined by two major milestones: the baby’s eyes open for the first time, and the lungs begin a critical phase of development that will determine viability outside the womb.
Around week 26–28, the fused eyelids separate and the baby opens its eyes for the first time. The irises are still forming — most babies, regardless of their eventual eye color, have blue or grey eyes at birth, as melanin production hasn’t fully developed yet. The retina is beginning to detect light and send signals to the brain.
The brain is experiencing one of its most dramatic growth phases. The surface of the cerebral cortex, which was smooth in earlier months, is now beginning to develop the characteristic folds and grooves (called gyri and sulci) that give the adult brain its wrinkled appearance. These folds are necessary to pack the enormous surface area of the cortex into the limited space of the skull. The more complex the brain, the more folding it requires.
The baby is increasingly active and moves with more strength and coordination. Many parents can now see kicks and rolls from the outside — watching your belly move with the baby’s movements is one of the truly magical experiences of late pregnancy. The baby also responds to external stimuli with increasing predictability: to sound, to light, to touch on the belly, and to changes in the mother’s position.
The lungs are beginning to produce surfactant — a substance that coats the air sacs and prevents them from collapsing when the baby exhales after birth. Without adequate surfactant, breathing is not possible. Babies born before 24 weeks have almost no surfactant, which is why 24 weeks is considered the threshold of viability — the point at which survival outside the womb becomes possible with intensive medical support.
The baby is also beginning to develop a sleep-wake cycle, though it doesn’t align with yours. Many pregnant women notice that their baby is most active at night — often when the mother lies still and the rocking motion that soothes the baby during the day stops.
Size This Month
From about 27 cm to roughly 30 cm (about the size of an ear of corn) by the end of week 24. Weight is approximately 600 grams.
Did You Know?
By week 24, the baby’s grip is strong enough that if it grabbed your finger, you would feel it. The grasping reflex is well developed long before birth — babies have been observed on ultrasound grasping the umbilical cord.
Month 7: Fat, Flavor, and Final Systems (Weeks 25–28)
What’s Happening
Month seven marks the beginning of the third trimester — the home stretch. The baby is now considered viable outside the womb, and from this point forward, each additional week of gestation significantly improves outcomes if early delivery becomes necessary.
The most important development this month is the rapid accumulation of body fat. Until now, the fetus has been relatively thin, with its organs and bones visible through translucent skin. Now, fat is depositing under the skin, filling out the baby’s silhouette and giving it the rounded, chubby appearance we associate with newborns. This fat is critical — it will insulate the baby and regulate body temperature after birth, when it suddenly has to manage its own thermoregulation instead of relying on yours.
The brain continues its extraordinary growth. The cerebellum — the region responsible for coordination and balance — is growing particularly rapidly. The nervous system is mature enough that the baby now experiences pain. This is an important development from an ethical and medical standpoint, and it informs how procedures on fetuses at this gestational age are approached.
The immune system is beginning to receive a crucial gift: antibodies from you, passed across the placenta. These maternal antibodies will provide the newborn with temporary protection against many diseases while its own immune system matures after birth. This passive immunity lasts for roughly the first 3–6 months of life.
Lung development continues. The amount of surfactant is increasing, and babies born at 28 weeks have significantly better respiratory outcomes than those born at 24 weeks. The lungs will continue maturing until the final weeks of pregnancy.
The baby’s senses are now highly developed. It responds to music and voices with movement. It can distinguish the sound of the mother’s voice from other voices — research shows that the fetal heart rate changes in response to the mother speaking. Some studies suggest babies can even recognize and respond to the emotional tone of the mother’s voice.
Size This Month
From about 34 cm to roughly 37 cm (about the size of a head of cauliflower) by the end of week 28. Weight is approximately 1 kilogram.
Did You Know?
Babies begin dreaming in the womb. REM sleep — the stage associated with dreaming — has been detected in fetuses as early as 23 weeks. By 28 weeks, the sleep-wake cycling is quite structured. What a fetus dreams about remains, of course, one of the great mysteries.
Month 8: Big, Bold, and Getting Ready (Weeks 29–32)
What’s Happening
By month eight, the baby has essentially all the structures it will have at birth. The remaining weeks are primarily about growth, fat accumulation, and the final maturation of systems — particularly the lungs, brain, and immune system.
The baby is running out of room. Where once it could somersault freely in the amniotic fluid, it is now increasingly cramped. Most babies settle into their final position this month — ideally head-down (vertex position) in preparation for birth. The kicks and rolls you’ve been feeling since month five are now replaced by slower, more deliberate movements — you might feel an elbow rolling across your belly or a foot pressing firmly against your ribs.
The bones are continuing to harden, though the skull bones remain soft and slightly separated at birth — intentionally so, to allow the baby’s head to compress slightly during passage through the birth canal, and to accommodate the continued rapid brain growth in the first year of life.
The pupils are now able to dilate and constrict in response to light. Brain wave activity is sophisticated enough that researchers can detect patterns of alertness and sleep. The taste buds are fully formed and functional. The baby swallows amniotic fluid regularly — up to a liter a day by this point — which is processed by the kidneys and returned to the fluid. This cycling of amniotic fluid is essential for healthy lung and kidney development.
The toenails and fingernails have grown to the tips of the digits. Many babies are born needing their nails trimmed almost immediately. The skin, once translucent and then wrinkled, is now smoother and more opaque as fat fills in underneath.
For parents, month eight often brings the nesting instinct into full force — the irresistible urge to prepare, organize, and make everything ready. Your baby is doing the same thing in its own way, consolidating systems, building reserves, and preparing for one of the most dramatic transitions any human being ever makes.
Size This Month
From about 38 cm to roughly 42 cm (about the size of a large butternut squash) by the end of week 32. Weight is approximately 1.7–2 kilograms.
Did You Know?
The baby is now practicing all the reflexes it will need at birth — sucking, swallowing, grasping, and breathing movements. These aren’t random; they are organized rehearsals for life outside the womb, driven by a developing nervous system that is preparing itself for its dramatic debut.
Month 9: The Final Countdown (Weeks 33–40+)
What’s Happening
The final month of pregnancy — which technically spans from week 33 to week 40 or beyond — is about completion. The baby is essentially ready. What remains is fine-tuning.
Weeks 33–36 are focused on fat and lung maturation. The baby gains roughly half a pound per week during this period. The lungs are producing increasing amounts of surfactant. A baby born at 34 weeks has a very high chance of survival without major complications; at 36 weeks (late preterm), outcomes are nearly as good as full term in most cases.
Weeks 37–40 are full term. By week 37, all systems are considered mature. The brain, however, continues growing and developing rapidly — in fact, the last few weeks of pregnancy see some of the most significant brain growth of the entire journey. The brain at 37 weeks is approximately 60% larger than it was at 35 weeks. This is one of the reasons the final weeks of pregnancy matter, even when everything seems ready.
The baby has settled deeply into the pelvis (called engagement or “dropping”), which you may notice as a change in the shape of your belly and a sudden ability to breathe more easily (as the baby moves away from your diaphragm). The cervix is beginning the process of effacement (thinning) and eventual dilation in preparation for labor.
The lanugo is almost entirely gone, shed into the amniotic fluid and swallowed by the baby — it becomes part of the meconium, the dark, sticky first stool your baby will pass after birth. The vernix caseosa is still present, though thinning.
By 40 weeks, the average baby is approximately 50 cm long and weighs about 3.4 kilograms (7.5 pounds), though there is enormous natural variation. The baby has developed a fully functional cardiovascular, respiratory, immune, digestive, and neurological system. It has a personality — patterns of movement, sleep, and response to stimuli that parents often recognize as consistent with the child they come to know after birth.
And then, triggered by a cascade of hormonal signals that scientists still don’t fully understand, labor begins. The baby descends. The world changes. And everything that has been building in the quiet dark of the womb meets the light for the very first time.
Size This Month
From about 43 cm at 33 weeks to approximately 50 cm (about the size of a small pumpkin) at 40 weeks. Weight approximately 3.4 kilograms (7.5 pounds) at term.
Did You Know?
The exact signal that triggers the onset of labor is still not completely understood by science. It is believed to involve a complex interplay of signals from the baby, the placenta, and the mother’s body — a coordinated biological conversation that ends nine months of preparation and begins one of the most transformative experiences in human life.
A Quick Reference: Baby’s Development Month by Month
| Month | Weeks | Key Milestone | Approximate Size |
|---|---|---|---|
| 1 | 1–4 | Fertilization, implantation, neural tube forms | Poppy seed |
| 2 | 5–8 | Heartbeat begins, organ systems form, embryo becomes fetus | Raspberry |
| 3 | 9–12 | Fingerprints form, thumb sucking begins, placenta functional | Lime |
| 4 | 13–16 | Bones harden, quickening begins, hearing develops | Avocado |
| 5 | 17–20 | Eyes open, vernix forms, anatomy scan, sex often visible | Banana |
| 6 | 21–24 | Eyelids open, brain folds form, viability threshold reached | Ear of corn |
| 7 | 25–28 | Fat accumulates, pain sensation develops, REM sleep begins | Cauliflower |
| 8 | 29–32 | Baby drops into pelvis position, bones continue hardening | Butternut squash |
| 9 | 33–40 | Full term, brain growth surge, labor preparation begins | Small pumpkin |
What This Journey Means
Nine months. Forty weeks. Roughly 280 days from that first cell to a fully formed human being who breathes, cries, sees, hears, feels, and reaches for you.
No matter how many times you’ve heard the word “miracle” applied to pregnancy, knowing the actual science of what happens — the specific, precise, breathtaking sequence of events — makes it feel even more extraordinary, not less. This is not magic. It is biology. And it is perhaps the most impressive thing biology does.
Every time you felt uncomfortable in the first trimester, a heart was beginning to beat. Every time you couldn’t sleep in the third, a brain was learning to dream. Every kick was a nervous system rehearsing. Every movement was preparation.
Your body knew exactly what to do. And it did it beautifully.
