Fetal Growth In Third Trimester | Baby’s Big Finish

The last stretch of pregnancy often feels long, yet inside the uterus a baby changes from a wiry little acrobat into a full-term newborn. The third trimester usually runs from week 28 to around week 40, when most births happen. During these weeks, growth shifts from building basic structures to adding size, strength, and fat reserves.

Health organizations such as the American College of Obstetricians and Gynecologists describe this period as the time when organ systems mature and weight gain speeds up so that the baby can cope with life after birth. Your body changes as well, with more frequent checkups and extra monitoring if any risk factors appear. Understanding what happens in late pregnancy helps many parents feel calmer about scan reports, fundal height measurements, and those strong kicks at night.

Fetal Growth In Third Trimester: Main Changes

The third trimester brings rapid size changes and a shift in how a baby looks and behaves. From around week 28 onward, weight gain rises and the body starts to look less skinny and more like the newborn you expect to meet.

Time Frame For Third Trimester Growth

Most clinical guides mark the start of the third trimester at 28 weeks of pregnancy and the end at 40 weeks, or whenever birth takes place near that point. A baby at 28 weeks often weighs close to 1.1–1.3 kilograms and measures around 37–38 centimeters from head to heel. By week 40, many babies reach about 3.2–3.6 kilograms and 50–52 centimeters, although plenty fall outside that range and still stay healthy.

Across these weeks the baby’s body proportions change. The head still takes up a large share of total length, yet the trunk and limbs catch up. Fat layers build under the skin, giving a rounder face, softer limbs, and smoother skin compared with earlier in pregnancy.

Weight Gain And Body Fat

Roughly half of a baby’s birthweight may arrive during the final 10 to 12 weeks of pregnancy. A typical pattern involves around 200 to 250 grams of gain per week toward the end, with some variation from baby to baby. This extra weight is not just bulk; it includes muscle, bone, and water, yet a large share is fat.

That fat plays a large role in temperature control and energy stores after birth. Brown fat, a special type of tissue that helps produce heat, increases during these weeks. This helps the newborn stay warm once no longer surrounded by amniotic fluid and maternal body heat.

Movement Patterns And Space

Babies stay active during late pregnancy, even as space becomes tight. Kicks may feel sharper as bones harden and muscles strengthen. Over time you might notice more rolling, stretching, and shifting rather than huge flips. Regular movement remains a key sign that the baby feels well inside the uterus.

Many care teams ask parents to pay attention to daily movement patterns and report any clear drop in activity. Simple kick count routines can help you spot changes, especially late in the third trimester when you know your baby’s usual rhythm.

Third Trimester Fetal Growth Milestones Week By Week

Week-by-week descriptions can make these final months feel more concrete. Medical sites such as the Mayo Clinic third trimester fetal development article outline a similar picture: steady length gains, rapid weight gain, and important organ finishing work.

Weeks 28 To 32: Growth Spurt Begins

During weeks 28 to 32, the baby’s length increases by a few centimeters and weight gains speed up. The brain grows fast, with more complex folds forming on the surface. The eyes open and close, sleep cycles become more regular, and many parents notice patterns of wakefulness tied to their own daily routine.

The skeleton continues to harden, yet bones such as those in the skull stay flexible. This flexibility helps the head adapt to the birth canal later on. At this stage the baby still has room to change position, so breech or sideways lies are common and often shift by themselves.

Weeks 33 To 36: Filling Out

By the mid-thirties in gestational weeks, many babies weigh between 2.1 and 2.7 kilograms. Arms and legs look rounder, cheeks puff out, and skin takes on a more opaque look as fat builds under the surface. Fine body hair (lanugo) begins to fade, while a waxy coating called vernix thickens to protect the skin.

The nervous system continues to mature, which improves temperature control and breathing rhythm after birth. Around this time many babies move into a head-down position in preparation for labor. Health systems such as the Cleveland Clinic third trimester guide note that parents often feel stronger Braxton Hicks contractions during these weeks as the uterus practices.

Weeks 37 To 40: Ready For Birth

From 37 weeks onward, a baby counts as term in many guidelines. Average weight may range from around 2.9 to 3.6 kilograms or more, with ongoing fat gain and lung finishing work. Fingernails reach the fingertips, hair grows thicker, and reflexes for sucking, swallowing, and gripping strengthen.

Even in these final weeks, extra days inside the uterus matter. Research shared by national institutes such as the NICHD pregnancy fact sheet shows that babies born before 37 weeks have higher risks of breathing, feeding, and temperature control problems, especially earlier preterm births.

Estimated Third Trimester Fetal Size By Week

The ranges in this table draw on fetal growth charts and public health data and give rough averages rather than targets. Individual babies often fall above or below these values and still stay healthy.

Gestational Week	Approximate Length (Head To Heel)	Approximate Weight
28	About 37–38 cm	About 1.1–1.3 kg
30	About 40–41 cm	About 1.5–1.7 kg
32	About 42–43 cm	About 1.9–2.1 kg
34	About 45–46 cm	About 2.3–2.5 kg
36	About 47–48 cm	About 2.7–2.9 kg
38	About 49–50 cm	About 3.0–3.3 kg
40	About 50–52 cm	About 3.2–3.6 kg

Growth charts group these measurements into percentiles. A baby near the middle lines of a chart lies close to the average, while values above or below place the baby in higher or lower percentiles. Care teams use these patterns to decide whether extra tests or follow-up scans are needed.

Average Size And Weight Near Birth

For many pregnancies, birth around 39 or 40 weeks brings a baby in the 3.2–3.6 kilogram range and around 50 centimeters long. Some babies weigh less, some more. Genetics, maternal height, number of previous births, and medical conditions all influence final size.

Guides from groups such as the ACOG fetal growth overview explain that size ranges matter less than steady growth over time. A smaller baby who keeps climbing along the same percentile line may draw less concern than one who drops sharply between scans.

Doctors classify babies born before 37 weeks as preterm and those between 34 and 36 weeks as late preterm. Many late preterm babies do well, yet they still face higher chances of breathing and feeding problems than babies born at 39–40 weeks. This is one reason elective early delivery without a medical reason is discouraged.

Organs And Systems That Mature Before Birth

While the basic body plan stands before the third trimester, the final months bring finishing touches that make life outside the uterus safer. The Mayo Clinic fetal development overview notes that lungs, brain, and liver all gain function rapidly at this stage.

Brain And Nervous System

During late pregnancy the brain grows quickly, forming grooves and folds that increase surface area. Nerve pathways that coordinate breathing, sucking, swallowing, and temperature control become more organized. This coordination helps full-term babies latch to the breast or bottle, breathe steadily, and stay warm after birth.

Sleep patterns also become more structured. You may notice cycles of kicks and stretches followed by quiet periods. These cycles reflect different sleep states inside the uterus and often continue during newborn life.

Lung Maturation

Lung tissue remains a central focus in the third trimester. Air sacs multiply, and cells produce more surfactant, a substance that helps the sacs stay open after the first breaths. Babies born earlier in the third trimester sometimes need help with breathing because this process is still under way.

Each extra week close to 39–40 weeks gives lungs more time to mature. That is why many professional bodies and hospital policies avoid scheduling non-medical deliveries before 39 weeks.

Digestive System, Liver, And Immune Readiness

The digestive tract already swallows amniotic fluid, yet in late pregnancy it practices more for feeding. The liver stores glycogen, a form of sugar reserve, and helps with blood clotting. The placenta passes antibodies from the mother, giving the newborn some early protection against infections during the first months of life.

These processes depend on steady blood flow through the placenta and umbilical cord. Conditions that limit blood flow can affect how well these systems mature, which is why growth monitoring and blood pressure checks matter so much in late pregnancy.

How Clinicians Track Third Trimester Growth

Care teams use a mix of physical measurements and imaging to watch fetal growth in the third trimester. Guidelines such as the Cleveland Clinic third trimester overview page describe common steps parents can expect during prenatal visits at this stage.

Fundal Height Measurements

At routine appointments, a clinician often measures fundal height from the pubic bone to the top of the uterus using a tape measure. From roughly 24 to 36 weeks, the number in centimeters usually tracks close to the gestational week. Large gaps above or below this range may trigger an ultrasound to review growth more closely.

Ultrasound Growth Scans

Ultrasound estimates size by measuring head circumference, abdominal circumference, and femur length. Software combines these measurements to estimate fetal weight and percentile. One scan offers a snapshot; repeat scans show the growth trend over several weeks.

Extra Monitoring When Needed

For pregnancies with high blood pressure, diabetes, growth concerns, or other risk factors, extra tests may appear in the calendar. These can include nonstress tests, biophysical profiles, or Doppler studies of blood flow to and from the placenta. The goal is to spot any early signs of stress so that timing of delivery can balance ongoing growth with safety.

Common Third Trimester Growth Checks

The table below lists frequent tests used in late pregnancy when growth needs closer observation.

Test Or Check	What It Looks At	Typical Use In Third Trimester
Fundal Height	Uterus size measured in centimeters	Screening at routine visits from around 24–36 weeks
Standard Ultrasound	Basic size, position, fluid level, placenta location	Re-checks growth when belly size seems off track
Formal Growth Scan	Head, abdomen, and femur measurements for weight estimate	Tracks babies small or large for gestational age
Nonstress Test (NST)	Fetal heart rate patterns and response to movement	Monitors well-being in higher risk pregnancies
Biophysical Profile	NST plus ultrasound review of movement and fluid	Checks for signs of stress when growth restriction is suspected
Doppler Ultrasound	Blood flow in cord or certain fetal vessels	Evaluates placenta function when growth slows
Serial Weight And Blood Pressure	Maternal health markers that affect blood flow	Spots patterns linked with hypertension or preeclampsia

Factors That Shape Growth In Late Pregnancy

Many elements influence how a baby grows during the third trimester. Some relate to the placenta and uterus, some to parental health, and some to lifestyle habits.

Placenta And Umbilical Cord

The placenta brings oxygen and nutrients from the maternal bloodstream and carries away waste. If this organ develops poorly, or if the cord has flow problems, babies may not gain weight as hoped. Ultrasound and Doppler studies help detect such issues, and care teams adjust monitoring or delivery plans based on those findings.

Parental Health Conditions

Conditions such as chronic hypertension, preeclampsia, kidney disease, or autoimmune disorders can restrict blood flow to the placenta. On the other side, gestational diabetes can push birthweight higher, especially around the shoulders and trunk.

Medication choices, meal planning, and visit schedules often change when these conditions appear. Close follow-up with an obstetric team keeps both parent and baby under steady watch.

Lifestyle Habits

Smoking, heavy alcohol use, and certain recreational drugs reduce oxygen delivery to the baby and raise the risk of growth restriction or preterm birth. Stopping these exposures at any point in pregnancy brings benefits. If quitting feels hard, ask your midwife or doctor about resources and local programs.

Balanced meals with enough protein, iron, calcium, and healthy fats give the baby raw material for tissue building. Regular activity approved by your care team and adequate sleep also support circulation and hormonal balance during this stage.

When Growth Seems Too Slow Or Too Fast

Scan reports that show a baby smaller or larger than average can stir strong emotions, yet they do not always signal a serious problem. Care teams usually place estimated weight into context by reviewing growth over time, maternal height, family patterns, and other factors.

Fetal Growth Restriction

When estimated weight or abdominal size falls below a certain percentile, especially below the tenth percentile with flow changes on Doppler, doctors may use the term fetal growth restriction. This phrase means that the baby is not reaching expected size for gestational age, not that something is wrong with the baby’s worth or future abilities.

Management often includes more frequent ultrasounds, NSTs, and careful timing of delivery. If blood flow patterns or testing suggest growing risk inside the uterus, a planned birth around 37 weeks or sometimes earlier may lead to better outcomes than waiting for spontaneous labor.

Large For Gestational Age

Babies estimated above the ninetieth percentile for weight may be labeled large for gestational age. Gestational diabetes, previous large babies, and parental body size all play roles. Concerns here center on birth complications such as shoulder dystocia and recovery for the mother.

Management might include tighter blood sugar control, more detailed birth planning, and careful assessment closer to term. Some babies labeled large on ultrasound arrive smaller than predicted, since weight estimates can be off by several hundred grams.

Practical Ways To Nurture Third Trimester Growth

Parents cannot control every detail of fetal growth in the third trimester, yet daily habits still matter. The goal is not perfection but steady choices that protect the placenta, keep blood flow healthy, and give the baby enough nutrients and time.

Keep Up With Prenatal Visits

Prenatal appointments often shift to every two weeks, then weekly during the last month. These visits allow your team to track fundal height, blood pressure, urine tests, and any new symptoms. Extra checks may feel tiring, yet they give early warning if growth or well-being drifts off course.

Eat Regular, Balanced Meals

Focus on steady intake of whole grains, fruits, vegetables, lean protein, dairy or alternatives, and healthy fats. Iron-rich foods, such as beans, lentils, and leafy greens, help prevent anemia, which can affect oxygen delivery. If nausea or heartburn limits meal size, smaller, more frequent meals can still provide enough calories and nutrients.

Rest, Move, And Hydrate

Late pregnancy brings fatigue, back ache, and swelling for many people. Gentle walking, stretching, and prenatal yoga approved by your doctor can lift mood, improve circulation, and ease discomfort. Lying on your side with pillows between the knees can improve blood flow to the uterus and help with sleep.

Drinking enough water helps keep amniotic fluid at healthy levels and supports kidney function. Many guides suggest sipping water through the day rather than waiting for thirst, especially in hot weather.

Watch Baby’s Movements

Daily awareness of kicks, rolls, and stretches strengthens the bond with your baby and gives a simple safety check. Choose a quiet time each day, often after a meal, and notice how long it takes to feel a set number of movements. If activity drops sharply or feels different from the usual pattern, call your care team promptly rather than waiting for the next appointment.

Third Trimester Growth Snapshot

During the third trimester, a baby grows from a lean, active fetus into a plump newborn ready for feeding, breathing, and life outside the uterus. Weight rises quickly, organs finish key tasks, and the brain and lungs gain function right up to birth. At the same time, your body works hard to carry the extra load, and your care team steps up monitoring to keep both of you safe.

This article offers general information and does not replace personal advice from your own doctor or midwife. If you have concerns about fetal growth, movement patterns, or scan results, bring your questions to your prenatal appointments so that your team can review your specific situation.