Baby development progresses through predictable stages each week, starting from conception and continuing through the first months of life. Understanding these weekly changes—from heartbeat formation in early pregnancy through rolling over and social smiling after birth—helps you recognize what's normal for your baby's age and know when to contact your healthcare provider.
This guide walks through the major developmental milestones week by week, covering pregnancy, birth, and the first six months postpartum. It explains what you'll typically see, what varies between babies, and which changes need medical attention. Use it to check in on your baby's development at each stage and to distinguish between common differences and potential concerns.
Table of Contents
- Pregnancy Weeks 1-4: Conception and Implantation
- Pregnancy Weeks 5-8: Organ Formation Begins
- Pregnancy Weeks 9-12: Transition to Fetus and First Trimester Screening
- Pregnancy Weeks 13-16: Feeling Movement and Second Trimester Begins
- Pregnancy Weeks 17-20: Detailed Anatomy Scan and Viability Milestone
- Pregnancy Weeks 21-28: Rapid Growth and Third Trimester Approaches
- Pregnancy Weeks 29-36: Final Development and Position
- Pregnancy Weeks 37-40: Full Term and Labor Preparation
- Birth and the First Week: Immediate Newborn Transitions
- Weeks 1-6 Postpartum: Early Newborn Development and When to Contact Your Provider
- Frequently Asked Questions
Pregnancy Weeks 1-4: Conception and Implantation
Development during the first four weeks happens before most people know they are pregnant. The fertilized egg divides rapidly as it travels down the fallopian tube toward the uterus. By week three, the embryo implants into the uterine wall, and specialized tissues begin forming the placenta and umbilical cord. Cells start organizing into three layers that will become all of your baby's organs and body systems.
During week four, the embryo is roughly the size of a poppy seed. Cell divisions accelerate, and the primitive neural tube—which becomes the brain and spinal cord—begins to form. The circulatory system starts developing. Most women miss their period around this time, which is the first sign to take a pregnancy test. Hormone levels rise sharply to support the pregnancy.
You may not feel any pregnancy symptoms yet, though some people notice breast tenderness or mild cramping. Others experience nausea, fatigue, or food aversions. There is wide variation in early symptoms, and minimal symptoms does not indicate a problem. At the end of week four, your healthcare provider can detect pregnancy through a blood test measuring human chorionic gonadotropin (hCG).
A standard urine pregnancy test becomes reliable a few days after a missed period. Prenatal care should begin as soon as pregnancy is confirmed so your provider can monitor early development and rule out complications.
Pregnancy Weeks 5-8: Organ Formation Begins
The embryo enters the period of organ formation, called organogenesis, when all major organ systems start developing. By week five, the heart begins beating—faintly at first, then more strongly. An early ultrasound can detect this heartbeat around week six, which reassures many parents-to-be. The brain grows rapidly, with neural connections forming at an astonishing rate.
Week six brings visible arm and leg buds emerging from the embryo's body. The eyes, ears, and nose start developing from external folds of tissue. The embryo is still smaller than a grain of rice, but the basic structures of a human are forming. The placenta continues thickening and will soon supply most of the oxygen and nutrients your baby needs.
By week seven, the embryo measures roughly half an inch from top to bottom. Fingers and toes are starting to separate from paddle-like appendages. The mouth begins forming, and primitive teeth buds appear in the gums. The spinal cord continues extending. Week eight brings more distinct human features. The tail-like structure that existed in earlier weeks disappears.
Eyelids form and will stay fused for several more weeks. Ears move from the neck toward their final position on the head. miscarriage risk drops significantly after week eight, though it remains possible throughout pregnancy.
Pregnancy Weeks 9-12: Transition to Fetus and First Trimester Screening
At nine weeks, the embryo becomes a fetus—the medical term for a baby from this point onward. Major organ systems are in place and will now focus on growing and refining. The fetus measures about an inch long. Brain development accelerates dramatically, with new neural connections forming constantly. The pituitary gland begins producing hormones that regulate growth.
Week ten brings webbing between fingers and toes beginning to recede. The fetus swallows amniotic fluid and can make small movements, though you will not feel them yet. Tooth buds are present for all twenty baby teeth. The intestines are moving from outside the body cavity into the abdomen, and the stomach begins producing digestive juices.
By week eleven, the fetus weighs roughly half an ounce. Fingernails and toenails start forming. The reproductive organs are developing but are not yet visible on ultrasound. The liver begins producing blood cells. Reflexes are developing—if you could touch the fetus, it would move away from the stimulus. Week twelve completes the first trimester. Many clinicians recommend first trimester screening tests between weeks eleven and fourteen, which combine an ultrasound measuring fluid behind the neck with blood tests.
These tests estimate the risk for chromosomal conditions like Down syndrome. This is when many parents decide to share their pregnancy news publicly, as miscarriage risk drops sharply after this point. The fetus weighs about half a pound and measures three to four inches.
Pregnancy Weeks 13-16: Feeling Movement and Second Trimester Begins
The second trimester begins at week thirteen, and many parents report feeling more energy and fewer symptoms. The fetus now weighs about one ounce and measures three and a half inches. Fingertips now have tiny fingerprints that are unique to this baby. The fetus can hear some sounds, though sounds are muffled by amniotic fluid and uterine tissues.
Week fourteen brings muscle development accelerating. The fetus begins practicing movements like sucking its thumb, which you might see on ultrasound. Hair starts growing on the scalp, though it is fine and unpigmented. Sex organs are now distinguishable, though an ultrasound may or may not be able to determine the baby's sex depending on position.
By week fifteen, eyebrows and eyelashes are visible. The fetus weighs about two ounces. Its limbs are more proportional to its body. Responses to light are developing—if a bright light shines on your abdomen, the fetus may move away. You may start feeling movement called "quickening," which feels like flutter, bubbles, or gentle rolling in your lower abdomen.
Week sixteen marks the moment when many pregnant people first feel fetal movement distinctly. Some describe it as butterflies, popcorn popping, or a goldfish swimming. First-time parents may detect movement later than experienced ones, and movement varies with fetus position and placenta placement. The fetus weighs about three ounces and measures five inches long. Lanugo—fine body hair covering the fetus—becomes visible.
Pregnancy Weeks 17-20: Detailed Anatomy Scan and Viability Milestone
Week seventeen brings the anatomy ultrasound offered by most practitioners, usually scheduled between weeks eighteen and twenty. This detailed scan checks organ formation, measures the fetus to confirm age, and checks amniotic fluid volume and placental position. Many parents learn the baby's sex during this scan, though it is not always clear. The fetus weighs about five ounces and measures five and a half inches.
By week eighteen, the fetus has developed unique sleep-wake cycles. It spends about sixteen hours sleeping and eight hours awake. The inner ear is now fully formed, so hearing continues improving. Brown fat—which keeps newborns warm—is beginning to accumulate. The fetus weighs about six ounces. Week nineteen marks the halfway point of pregnancy. The fetus now weighs about eight ounces and measures six inches.
Vernix caseosa, a waxy protective coating, begins covering the fetus's skin. This coating prevents skin from becoming waterlogged from constant amniotic fluid exposure and protects the skin during birth. By week twenty, the fetus weighs about ten ounces and measures about six and a half inches. From this point forward, the fetus is considered viable, meaning it could potentially survive outside the uterus with medical support.
Movement is now regular enough that you should feel it daily. Different babies have different activity patterns—some are very active while others are quieter, and both are normal.
Pregnancy Weeks 21-28: Rapid Growth and Third Trimester Approaches
The second half of pregnancy focuses on growth and refining existing systems. By week twenty-one, the fetus weighs about twelve ounces. The digestive system is more developed, and the fetus practices swallowing amniotic fluid. Fingernails reach the tips of fingers. Movement is visible from the outside—your partner or others may see your abdomen move when the baby kicks.
Week twenty-four is another medical milestone. At this gestational age, a baby born prematurely has a reasonable chance of survival with intensive medical care. Survival rates improve with each passing week. The fetus weighs about one and a quarter pounds and measures about eleven inches. The face is fully formed, with all features in place, though the eyes are still closed.
By week twenty-six, the fetus weighs about one and a half pounds. The eyes are now open, though still sealed with fused eyelids. Brain wave activity shows patterns similar to sleep and wakefulness. The fetus is swallowing amniotic fluid regularly and can cry, though no sound reaches outside the uterus. Hiccups are common and felt as rhythmic movements.
Week twenty-eight marks the start of the third trimester. The fetus weighs two to two and a half pounds and measures about fifteen inches. Fat layers are accumulating under the skin, which will help the newborn regulate temperature after birth. Movement is strong and regular. Many pregnant people experience Braxton-Hicks contractions—practice contractions that feel like the uterus tightening—though these are not true labor.
Pregnancy Weeks 29-36: Final Development and Position
During the third trimester, the fetus focuses primarily on growth and gaining fat reserves. By week twenty-nine, the fetus weighs about two and a half to three pounds. The pupils of the eyes can now dilate and constrict in response to light. The fetus is developing its own sleep-wake cycle that may not match yours—some babies are night owls even before birth, staying most active when you are trying to sleep.
Week thirty-two brings the fetus to about three and a half to four pounds. Most babies turn head-down into the vertex position during this period, preparing for head-first birth. Some babies take longer to turn or never turn—this requires discussion with your healthcare provider, as it affects birth options. The fetus's lungs are nearly developed, though they won't be fully ready until closer to birth.
By week thirty-four, the fetus weighs about four and a half to five pounds. Most maternal antibodies have transferred across the placenta, giving the newborn some immunity to diseases the mother is immune to. The vernix coating thickens. Movement remains regular, and you should continue feeling the baby daily. Sudden decreases in movement warrant a call to your provider.
Week thirty-six marks the point at which the baby is considered "early term." The fetus weighs about five and a half pounds and measures about eighteen and a half inches. The skin is less wrinkled as fat deposits underneath plump it out. The lanugo is mostly gone except on the shoulders and upper back. The cervix may begin softening and dilating in preparation for labor, though this can happen weeks before active labor begins.
Pregnancy Weeks 37-40: Full Term and Labor Preparation
At thirty-seven weeks, the baby is considered full term. The fetus weighs about six to six and a half pounds and measures about nineteen inches. The lungs are fully mature. All major organ systems are ready for life outside the uterus, though the brain will continue developing for years. Many babies drop lower into the pelvis around this time, a process called lightening that makes breathing easier but increases bladder pressure.
Week thirty-eight brings the fetus to about six and a half to seven pounds. The amniotic fluid volume peaks and will gradually decrease. Most of the lanugo is gone. Your provider may check your cervix at this point to see if early labor signs are present, though cervical changes do not predict when labor will start.
Some women with significant dilation go weeks before labor begins; others with minimal dilation go into labor within days. By thirty-nine weeks, the fetus is nearly ready and weighs about seven to seven and a half pounds. Most babies are head-down and settled into the pelvis. Meconium—the baby's first stool, made of materials ingested in utero—fills the intestines and is typically passed after birth.
Some babies pass meconium before birth, staining the amniotic fluid greenish, which sometimes requires monitoring. Week forty marks the due date for many pregnancies, though only five percent of births occur on the exact due date. Pregnancies are considered overdue after forty-two weeks, at which point most providers recommend induction to reduce risks. The fetus weighs about seven and a half pounds on average, though healthy newborns range from five and a half to nine pounds. The baby is ready for birth.
Birth and the First Week: Immediate Newborn Transitions
The first moments after birth bring dramatic changes. Newborns cry to clear their lungs of fluid and begin breathing air. The umbilical cord stops pulsing and is clamped and cut. The baby's skin may be blue at first due to the blood still in the umbilical cord and the transition to breathing air; this color should pink up within minutes as oxygen reaches the skin.
Newborns are covered in vernix, blood, and sometimes meconium, which is entirely normal. During the first hour of life, your baby will likely be alert and may attempt to breastfeed. This is called the golden hour and is an optimal time for skin-to-skin contact and initial bonding. Newborns have a startle reflex and may jump at sudden movements or sounds.
They prefer high-pitched sounds and will turn toward their parent's voice almost immediately. Over the first day, newborns are screened for critical conditions including critical congenital heart disease, sickle cell disease, and other metabolic disorders. Hearing is screened using a device placed gently in the ear canal. Blood tests are drawn from the baby's heel to screen for rare but serious treatable conditions.
Vision is checked. These screenings prevent serious illness by catching problems early. Feeding begins within the first hours. Breastfeeding newborns may latch and nurse, though milk production does not fully begin until day three or four. Newborns born to mothers planning to bottle-feed receive formula. Newborns lose five to ten percent of their birth weight in the first few days—this is normal and not concerning. They should regain this weight by two weeks old.
Weeks 1-6 Postpartum: Early Newborn Development and When to Contact Your Provider
In the first week after birth, your baby's primary job is sleeping, eating, and eliminating waste. Newborns sleep sixteen to eighteen hours daily but not in consolidated blocks—they wake every two to four hours to eat. This is exhausting for parents but developmentally necessary, as newborn stomachs are tiny and milk or formula passes through quickly.
Expect to feed every two to three hours around the clock. By day three or four, your baby's color darkens as the skin sheds dead cells. The umbilical cord stump turns from wet to dry over several days and falls off around one to three weeks. In the meantime, keep it dry and clean. Jaundice—yellowing of the skin and eyes—often appears around day three and is usually harmless and resolves on its own, though severe jaundice requires treatment.
Weeks two and three bring the first social smiles—a magical milestone. Your baby will look at your face, make eye contact, and smile in response to your voice or presence. This confirms that the newborn recognizes you and is engaging socially. Your baby is also becoming alert for slightly longer periods, though the majority of time is still sleep.
By week four, your baby weighs back to birth weight or more. The gut is maturing, and some babies develop a more regular feeding and elimination pattern, while others remain unpredictable. Colic—extended crying without clear cause—may appear around three weeks and can be distressing for parents. It typically peaks at six weeks and resolves by three months.
Contact your provider if your baby has a fever, refuses to feed, vomits forcefully, seems lethargic, or has fewer wet or dirty diapers than expected. By weeks five and six, your baby is more alert and awake during the day. Reflexes like rooting (turning the head when the cheek is touched) and sucking are strong.
Your baby may begin to follow objects with their eyes more deliberately. Some babies lift their head briefly when on their tummy. Growth is rapid—babies typically gain five to seven ounces per week in the first month. Contact your provider if your baby has signs of infection, feeding problems, or developmental concerns, but remember that development in the first weeks varies widely and catches up over months.
Frequently Asked Questions
When can I feel my baby move during pregnancy?
Most first-time parents feel movement between sixteen and twenty-five weeks, though it can happen slightly earlier in subsequent pregnancies. Early movements feel like fluttering, bubbles, or gentle rolling. Movement may feel stronger and happen more frequently as your pregnancy progresses and the baby gets bigger. If you're not sure whether what you're feeling is fetal movement, mention it at your next prenatal visit.
What does it mean if my baby isn't meeting milestones on schedule?
Babies develop at different rates, especially in the first months of life. A baby who rolls over at five months instead of four months is usually developing normally. Prematurity also affects the timeline—a baby born at thirty-two weeks should be compared developmentally to a baby three months younger. Mention concerns to your healthcare provider, who can assess whether your baby needs evaluation or whether variation is normal.
Is it normal for newborns to sleep so much?
Yes. Newborns sleep sixteen to eighteen hours daily because growth and brain development happen during sleep, and frequent feeding is exhausting. Your newborn will wake every two to four hours to eat. As your baby gets older, longer sleep stretches will gradually develop, though this can take months. Sleep patterns vary widely—some babies are naturally better sleepers than others.
What are Braxton-Hicks contractions, and are they labor?
Braxton-Hicks are practice contractions your uterus performs as it prepares for labor. They usually start around twenty-eight weeks and feel like the uterus tightening and then relaxing. Unlike true labor contractions, Braxton-Hicks are typically painless, irregular, and stop with movement or rest. If contractions become regular, painful, and continue despite movement or position changes, contact your provider to confirm whether labor has begun.
When should I be concerned about my baby's development?
Contact your provider if your newborn has a fever, refuses to feed, vomits forcefully, seems unusually sleepy or unresponsive, has fewer wet or dirty diapers than expected, or if you notice any other concerning change. In later infancy, mention delays in rolling over, reaching for objects, responding to sounds, or social interaction. Your provider can assess whether your baby needs evaluation or reassure you that development is on track.
How much weight should my baby gain each month?
Newborns typically gain five to seven ounces per week in the first month and slow slightly to about four to five ounces per week by months two through four. Growth rate varies widely based on genetics, feeding method, and individual metabolism. As long as your baby is feeding well, gaining steadily, and developing normally, the exact amount is less important than the trend. Your provider will track growth at checkups.
