Fetal Development In The Third Trimester | Ready For Birth

Third Trimester Basics

By the start of week 28, pregnancy moves into the last stretch. Health organizations describe this phase as running from about week 28 through week 40, or until birth. During this span the fetus often grows from around two and a half pounds to somewhere between six and nine pounds.

Growth does not follow a smooth line. Some weeks bring clear jumps in weight or new skills. The uterus stretches, your center of gravity shifts, and daily movement starts to feel different.

This stage matters because many organs reach a point where life outside the womb becomes realistic. Brain tissue builds fast, lungs move toward readiness for breathing air, and the digestive system and liver continue to mature.

What Changes Most In Late Pregnancy

Size And Weight

In the third trimester, baby weight often triples. Fat stores build under the skin, giving a plumper look and helping with temperature control after birth. Muscles strengthen as the fetus pushes and stretches against the uterine wall.

Length also increases. By the end of this phase many babies measure around 19 to 21 inches from head to heel. That extra size explains why movements tend to shift from sharp kicks to rolling or stretching motions as space tightens.

Brain And Nervous System Growth

The brain grows at a fast pace during this time. Folds deepen, connections between nerve cells multiply, and the surface expands. This growth supports more complex movement, basic temperature control, and early sleep cycles.

The nervous system also gains better control over body functions. Reflexes such as sucking, swallowing, and blinking become more coordinated. These skills prepare your newborn for feeding and breathing after delivery.

Lung Maturation

Lungs remain a work in progress through much of the third trimester. Air sacs continue to form, and cells produce more surfactant, a substance that keeps the sacs from collapsing. This preparation gives the newborn a stronger chance of steady breathing at birth.

Guidance from the Mayo Clinic fetal development overview notes that babies born closer to term usually breathe on their own, while those who arrive earlier may need help from machines and medication.

Senses, Sleep, And Movement Patterns

By late pregnancy, many fetuses can sense changes in light through the uterine wall and react to loud sounds. Hearing develops enough for familiar voices and music to stand out. Some parents notice that babies move more when certain songs play or when they speak near the bump.

Patterns of motion shift as well. There may be times of strong activity and stretches of quiet rest. The fetus practices breathing movements, hiccups often appear, and stretches, rolls, and jabs mark the day.

Fetal Development In The Third Trimester Week By Week Timeline

Every pregnancy has its own rhythm, yet looking at this span in two week blocks gives a useful view. Ultrasound reports and prenatal visits often use this type of breakdown to track growth.

According to the American College of Obstetricians and Gynecologists, the third trimester begins at 28 weeks and runs until birth. Other medical groups describe similar timing and growth patterns.

Week Range	Approximate Size And Weight	What Baby Is Working On
28–30 weeks	About 15 inches long, near 2½–3 pounds	Turns more often, may start to move head down, gains muscle tone and responds to sound.
30–32 weeks	Roughly 16½–17 inches, 3–4 pounds	Brain growth speeds up, body fat builds, movement patterns feel stronger and more regular.
32–34 weeks	Around 17–18 inches, 4–5 pounds	Bones are almost fully formed yet still flexible, eyes open and close, lungs keep maturing.
34–36 weeks	Close to 18–19 inches, near 5½–6 pounds	Rapid weight gain, clearer sleep and wake cycles, many babies move into a head down position.
36–37 weeks	About 19 inches, over 6 pounds	Organs refine function, the nervous system coordinates breathing and feeding reflexes.
37–38 weeks	19–20 inches, 6½–7½ pounds	Body fat smooths features, grasp becomes stronger, lungs approach readiness for birth.
38–40 weeks	19–21 inches, 6–9 pounds	Full term for many pregnancies, baby practices breathing and swallowing and settles into birth position.

These figures come from large groups of pregnancies, so they describe ranges, not strict targets. A fetus that measures a little above or below these points can still be growing in a healthy way.

How Growth Affects Your Body

As the fetus grows, the uterus presses upward on the diaphragm and outward on the rib cage. Shortness of breath becomes common, especially during activity. Rest breaks and upright posture may bring some relief.

More weight around the front of the body changes balance. Back muscles work harder, which can lead to aches. Supportive shoes, gentle stretching, and rest often help with these changes.

The placenta, which supplies oxygen and nutrients, also works harder. Blood volume rises, and the heart pumps more with each beat. Prenatal visits during this phase often become more frequent so the care team can track blood pressure, fetal growth, and signs of labor.

How Your Baby Prepares For Birth

Building Fat And Energy Stores

One major task in late pregnancy is building fat and glycogen reserves. These give the newborn fuel during the first days while feeding routines take shape. Brown fat, a type of tissue that helps with heat production, grows around the shoulders, chest, and back.

This extra padding also smooths the skin. Earlier in pregnancy the skin can look thin and wrinkled on ultrasound images, while near term it appears thicker and more opaque.

Fine Tuning Organs

Many organs reach major milestones during the third trimester. The liver works on processing waste and storing nutrients such as iron. The kidneys filter blood more efficiently and produce urine that adds to the amniotic fluid.

The digestive system practices movements that later move milk through the gut. Around this time the first stool, called meconium, starts to form in the intestines. Hormone systems also adjust, helping the baby handle stress at birth.

Positioning For Delivery

During late pregnancy, most babies settle into a head down position with the back of the head toward the front of the pelvis. This posture usually makes birth smoother. Some fetuses stay breech, with buttocks or feet down, or take other positions that may lead to special delivery planning.

Amniotic fluid volume shifts as well. Early in the third trimester there is more space to float and move. Near term the fluid level often drops a bit while baby size increases, so moves may feel slower yet still strong.

Third Trimester Fetal Development And Daily Life

Changes in fetal development and changes in your day often go hand in hand. Sleep patterns, meal size, and activity choices may all shift in response to how your body feels.

Many parents notice stronger movement at night when the rest of the body feels still. Others feel firm kicks after meals or sweet drinks. Paying attention to these patterns helps you know what feels usual for your own pregnancy.

Health professionals often recommend counting movements once a day during this phase. A common approach is to note how long it takes to feel ten distinct motions. A sudden drop in movement pattern deserves prompt contact with your care team.

Simple daily habits can help you connect with your growing baby during this phase. Many people enjoy rubbing the bump in slow circles, talking or singing during active spells, or playing the same short song at bedtime. Repeated cues like these may help your baby settle into familiar rhythms and give you a sense of routine.

Comfort at night often needs a few adjustments. Many providers suggest sleeping on one side, especially the left, with pillows between the knees and under the belly. Small snacks and steady fluid intake through the day, with a lighter drink in the hour before bed, can ease heartburn and midnight bathroom trips for you.

When Growth Does Not Match The Calendar

Some pregnancies progress at a slower or faster pace than the calendar suggests. Ultrasound measurements may label a fetus as small or large for dates. Reasons can range from parental height and body build to factors such as placental function or blood sugar levels.

Growth scans, Doppler studies, and non stress tests help teams track how the fetus handles these conditions. The goal is steady growth, healthy oxygen levels, and timing of birth that balances risks of staying inside against risks of arriving early.

Preterm Birth And Development

Babies born before 37 weeks miss some of the growth that usually happens in late pregnancy. That includes brain volume, lung maturation, and fat storage. Many of these infants begin life in a neonatal intensive care unit where machines assist breathing, feeding, and temperature control.

Even when scans show no structural brain injury, research shows that brain growth outside the womb can differ from growth inside. Findings reported in pediatric studies of preterm infants highlight the need for careful follow up and early therapy when concerns arise.

The March of Dimes week-by-week pregnancy guide explains that infants born between 34 and 36 weeks are often called late preterm and may still face feeding or breathing challenges, even though they look close to full term.

Aspect	What Baby Is Doing	What You May Notice
Movement pattern	More defined periods of rest and activity, stretches instead of sharp kicks.	Rolling motions, pressure under the ribs, jabs that feel lower in the pelvis.
Position	Head often turns downward and settles into the pelvis.	More pressure on the bladder, less pressure on the diaphragm, a lower bump shape.
Lung practice	Rhythmic breathing motions and frequent hiccups.	Regular tiny jumps in the bump that last a few minutes at a time.
Brain and sleep cycles	Stronger link between light, sound, and sleep or wake states.	Clearer times of day when the baby tends to be awake or quiet.
Growth and fat gain	Rapid increase in weight and rounder limbs.	Steady weight gain on the scale, tighter waistbands, more stretch marks.
Preparation for birth	Head moves lower, pressure on the cervix increases.	Stronger pelvic pressure, more practice contractions that come and go.

Questions To Bring To Prenatal Visits

Understanding fetal development in the third trimester can make prenatal visits feel more practical. Specific questions give your care team a clear view of your concerns and help shape each appointment.

You might ask how your baby’s measurements compare with the expected range for your week. You can also ask which movements should prompt a call, and what pattern of contractions suggests labor instead of practice tightening.

Other useful topics include birth preferences, pain relief options, feeding plans, and what to expect in the first hours after delivery. Writing questions in a small notebook or phone note before each visit keeps them from slipping your mind during the appointment.

Guidance from the Eunice Kennedy Shriver National Institute of Child Health and Human Development stresses the value of regular prenatal care for tracking growth and spotting concerns early.

Getting Ready To Meet Your Baby

The final weeks of pregnancy can feel long. Physical discomfort may rise at the same time that excitement about meeting the baby grows. Knowing the work going on inside the uterus can bring a sense of purpose to each kick, stretch, and hiccup.

Each day in this stage gives your baby more reserves and better organ function. While every pregnancy takes its own path, knowledge about third trimester fetal development can help you track progress, notice changes early, and work with your care team for a safe birth. Always talk with your own doctor or midwife about questions or worries, since this overview cannot replace personal medical advice.