Pregnancy week 31 is an important third-trimester milestone. Major fetal structures are already formed, and development increasingly shifts toward rapid weight gain, organ maturation, fat storage, stronger breathing practice and preparation for birth.

ACOG specifically notes that by week 31 major development is essentially complete and the fetus begins gaining weight very quickly. MedlinePlus describes weeks 31–34 as a stage when the fetus grows rapidly, stores more fat, practices rhythmic breathing and begins storing important minerals such as iron, calcium and phosphorus.

For the pregnant person, week 31 can also be a practical turning point. Your healthcare professional may begin paying closer attention to uterine size, fetal position and whether the baby is head down, breech or still changing position.

Quick answer: At 31 weeks pregnant, the fetus is around 41 cm from head to heel, with major development largely complete and rapid weight gain becoming a major priority. The lungs are still maturing, rhythmic breathing movements occur, bones are fully formed but remain soft, and the body begins storing increasing amounts of iron, calcium and phosphorus.

Is 31 Weeks Pregnant in the Third Trimester?

Yes.

The third trimester begins at 28 weeks 0 days and continues until birth.

At week 31, pregnancy is increasingly focused on:

• fetal weight gain;
• brain maturation;
• lung maturation;
• fat accumulation;
• mineral storage;
• preparing the fetus for life outside the uterus.

ACOG describes the third trimester as the stage when fetal weight increases and organs mature so they can function after birth.

How Big Is the Baby at 31 Weeks?

The fetus is approximately 41 cm from head to heel at this stage.

Exact fetal size varies naturally.

Differences can be influenced by:

• genetics;
• fetal sex;
• exact gestational age;
• individual growth pattern;
• measurement method.

A single size estimate does not determine whether a fetus is growing normally.

Your healthcare provider assesses growth using clinical measurements and ultrasound when medically indicated.

What Is Developing at 31 Weeks?

Week 31 marks an important change in emphasis.

ACOG states that at week 31 major development is finished and the fetus is gaining weight very quickly.

MedlinePlus describes weeks 31–34 as a period when:

• the fetus grows quickly;
• substantial fat is added;
• rhythmic breathing occurs;
• lungs remain immature;
• bones are fully developed but still soft;
• the body begins storing iron, calcium and phosphorus.

This makes week 31 particularly important for growth and maturation, rather than formation of entirely new major organs.

Does “Major Development Is Finished” Mean the Baby Is Fully Mature?

No.

This wording means that the major body structures are already formed.

It does not mean every organ is fully mature.

Important development still continues in the:

• brain;
• lungs;
• nervous system;
• immune system;
• fat stores;
• bones.

Several more weeks inside the uterus provide important time for maturation.

Is the Baby Gaining Weight Quickly?

Yes.

Rapid weight gain is one of the defining features of week 31.

ACOG specifically states that at this stage the fetus begins gaining weight very quickly.

Much of this additional weight comes from:

• fat;
• muscle growth;
• organ growth;
• expanding tissues.

This helps the fetus prepare for temperature control and energy needs after birth.

Why Is Fetal Fat Important?

Fat contributes to:

• body-temperature regulation;
• energy storage;
• smoother skin;
• fuller body shape;
• protection after birth.

Earlier in pregnancy, the fetus had relatively little fat beneath the skin.

That changes rapidly during the third trimester.

Is the Baby Practicing Breathing at 31 Weeks?

Yes.

MedlinePlus states that rhythmic breathing occurs during weeks 31–34.

These are breathing-like fetal movements.

They help practice the muscles and structures involved in breathing.

The fetus still does not breathe air inside the uterus.

Are the Lungs Fully Mature?

No.

MedlinePlus specifically notes that although rhythmic breathing movements occur, the lungs are not yet fully mature.

Lung maturation continues throughout the remaining weeks.

The fetus still receives oxygen through the placenta and umbilical cord.

Is Surfactant Still Increasing?

Yes.

Surfactant production began earlier in the third trimester and continues to increase.

Surfactant helps keep the tiny lung air sacs open after birth.

Greater surfactant production contributes to improved respiratory function as gestation advances.

What Does Rhythmic Breathing Look Like?

Fetal breathing movements involve regular motion of the:

• diaphragm;
• chest wall;
• respiratory muscles.

These movements can be observed during ultrasound.

They are developmental practice rather than actual breathing of air.

Are the Baby’s Bones Fully Developed?

MedlinePlus states that during weeks 31–34 the bones are fully developed but still soft.

This is important.

The fetal skeleton needs to be strong, but some flexibility remains necessary—particularly in the skull for birth.

Why Are the Bones Still Soft?

Fetal bones continue mineralizing.

Flexibility is especially important in the skull because the skull bones need to accommodate:

• brain growth;
• passage through the birth canal.

Bone development continues after birth.

Is the Baby Storing Calcium?

Yes.

MedlinePlus notes that the fetal body begins storing increasing amounts of calcium during weeks 31–34.

Calcium is important for:

• bones;
• muscles;
• nerves;
• heart function.

Is the Baby Storing Iron?

Yes.

Iron storage increases during this period.

MedlinePlus specifically lists iron among the minerals accumulated during weeks 31–34.

Iron is important for:

• hemoglobin;
• red blood cells;
• oxygen transport.

This is another reason maternal iron status remains important late in pregnancy.

What Is Phosphorus Used For?

Phosphorus works with calcium in:

• bone mineralization;
• energy metabolism;
• cellular function.

MedlinePlus notes that phosphorus storage also increases during weeks 31–34.

Is Bone Marrow Still Producing Red Blood Cells?

Yes.

ACOG notes that bone marrow produces red blood cells during the weeks 29–32 developmental stage.

These red blood cells carry oxygen supplied by the placenta throughout the fetal body.

Is the Brain Still Developing?

Yes.

Although major structural development is largely complete, brain maturation continues rapidly.

The third trimester remains an important period for:

• neural connections;
• sensory processing;
• movement coordination;
• sleep cycles;
• reflexes.

Brain development continues after birth as well.

Can the Baby See Light?

Yes.

ACOG states that during weeks 29–32 the eyes can sense changes in light.

However, fetal vision remains immature.

The uterus is dark and external light is heavily filtered through body tissues.

Does the Baby Have Hair?

Possibly.

ACOG notes that scalp hair may be present during weeks 29–32.

Hair amount varies considerably.

It cannot predict how much hair a newborn will eventually have.

Is Lanugo Disappearing?

Yes.

ACOG states that lanugo begins disappearing during the weeks 29–32 stage.

Some lanugo may remain at birth, especially in babies born earlier.

Is the Baby Moving Strongly at 31 Weeks?

Usually, yes.

At this stage you may feel:

• kicks;
• stretches;
• pushes;
• rolls;
• whole-body shifts;
• hiccups.

Because the fetus is larger, movement may feel more forceful.

At the same time, the type of movement may change as space becomes tighter.

Does the Baby Move Less Because There Is Less Space?

The character of movement can change, but movement should not simply stop.

You may notice fewer large flips and more:

• pushing;
• rolling;
• stretching;
• shifting.

A significant change from your baby’s usual pattern deserves medical assessment.

Should You Know Your Baby’s Movement Pattern?

Yes.

By week 31, many pregnant people have a clear sense of when their baby is normally active.

Each fetus has its own pattern.

The important point is not to compare with another pregnancy but to recognize what is usual for your baby.

What If Movement Becomes Reduced?

Contact your maternity provider if your baby’s movements become noticeably reduced or stop.

Do not rely on:

• a home Doppler;
• sugary drinks;
• cold water;
• loud music;

as substitutes for professional assessment.

Is the Baby Sleeping?

Yes.

Fetal sleep and waking cycles are well established by this stage.

The fetus may be active when you are resting and quieter at other times.

However, a concerning reduction in your baby’s established movement pattern should not automatically be explained as sleep.

What Position Is the Baby in at 31 Weeks?

The fetus can be:

• head down;
• breech;
• sideways;
• changing between positions.

The NHS notes that some babies are already head down by week 31, but there is still time for babies in other positions to turn.

Does the Baby Need to Be Head Down at 31 Weeks?

No.

A head-down position is common and favorable for birth, but week 31 is still early enough for position to change.

The NHS specifically notes that if the fetus is head up, there is still time to move into the birth position.

What Is Breech Position?

A breech fetus is positioned with the:

• bottom;
• feet;

closer to the cervix instead of the head.

Breech position at 31 weeks does not automatically mean the fetus will remain breech at birth.

Can a Breech Baby Still Turn?

Yes.

There is still time at 31 weeks.

The uterus still allows fetal movement, and some babies turn head down later.

Your maternity team will monitor fetal position as pregnancy progresses.

Does Breech Position Mean You Need a Cesarean?

Not at week 31.

The NHS notes that persistent breech position later in pregnancy can influence birth options, but a fetus that is breech now may still turn.

Decisions about mode of birth are made later and depend on individual circumstances.

What Does “Engaged” Mean?

Engagement occurs when the baby’s head moves lower into the pelvis in preparation for birth.

The NHS notes that some babies do not engage until labor begins.

Therefore, not being engaged at week 31 is not automatically a problem.

Does the Bump Drop at 31 Weeks?

It can happen later, particularly when the fetal head moves into the pelvis.

Some people notice:

• the bump sitting lower;
• easier breathing;
• more pelvic pressure.

However, engagement timing varies considerably.

Can First Babies Engage Earlier?

Often, first babies may engage before labor, while later babies may not move into the pelvis until labor begins.

But there is substantial individual variation.

Your clinician can assess fetal position by examining the abdomen.

How Does the Midwife Check Baby Position?

The NHS notes that clinicians can gently feel the abdomen to determine which way the fetus is lying.

They may identify:

• the fetal head;
• back;
• bottom;
• limbs.

Ultrasound may be used if position is uncertain or clinically important.

What Is Fundal Height?

Fundal height is a measurement from the top of the pubic bone to the top of the uterus.

The NHS notes that around this stage, particularly in a first pregnancy, a midwife or doctor may measure the size of the uterus in this way.

It provides a simple method of following pregnancy growth.

Why Is Fundal Height Measured?

Fundal height can help identify whether uterine growth is broadly appropriate for gestational age.

If measurements are:

• unexpectedly small;
• unexpectedly large;
• changing unusually;

your healthcare professional may investigate further.

Fundal height alone does not diagnose fetal growth problems.

Does Fundal Height Equal Pregnancy Weeks?

Fundal height in centimeters may roughly correspond with gestational age during part of pregnancy, but it is not exact.

Factors affecting measurement include:

• maternal body shape;
• fetal position;
• multiple pregnancy;
• amniotic fluid;
• fibroids;
• examiner technique.

Your clinician interprets the measurement over time.

What Is Happening in Your Body at 31 Weeks?

The NHS notes that the fetus and bump continue growing and that another major growth spurt is approaching.

As the uterus enlarges, symptoms can include:

• breathlessness;
• heartburn;
• frequent urination;
• back discomfort;
• pelvic pressure;
• sleep problems;
• swelling;
• fatigue.

Why Might Breathlessness Continue?

The uterus occupies more abdominal space and can push upward toward the diaphragm.

Carrying additional weight also increases the effort required during activity.

Mild exertional breathlessness can occur.

Sudden or severe breathlessness requires medical assessment.

Is Heartburn Common at 31 Weeks?

Yes.

The growing uterus can increase pressure on the stomach.

Pregnancy hormones also relax digestive muscles.

Helpful measures may include:

• smaller meals;
• avoiding personal trigger foods;
• remaining upright after eating;
• avoiding large meals before sleep.

Is Frequent Urination Normal?

Yes.

As the fetus and uterus grow, pressure on the bladder can increase.

Frequent urination can be particularly noticeable at night.

However, urinary symptoms such as:

• burning;
• fever;
• blood in urine;
• back pain;

may suggest infection and deserve medical advice.

Is Backache Normal?

Yes.

Back pain is common because:

• abdominal weight increases;
• posture changes;
• the center of gravity shifts;
• pregnancy hormones affect ligaments.

Helpful measures may include:

• supportive footwear;
• careful lifting;
• gentle appropriate exercise;
• avoiding prolonged awkward positions.

Is Pelvic Pressure Normal?

Some increased pelvic pressure may occur as pregnancy advances.

However, significant pressure associated with:

• regular contractions;
• fluid leakage;
• vaginal bleeding;
• persistent cramping;

can be concerning for preterm labor and requires assessment.

Can Braxton Hicks Happen at 31 Weeks?

Yes.

Irregular uterine tightening is common in the third trimester.

Braxton Hicks contractions are usually:

• irregular;
• brief;
• not progressively stronger;
• not progressively closer together.

Regular painful contractions at 31 weeks need prompt assessment.

What Are Possible Signs of Preterm Labor?

Seek maternity advice urgently for symptoms such as:

• regular painful contractions;
• persistent pelvic pressure;
• new persistent lower back pain;
• vaginal bleeding;
• watery fluid leakage;
• repeated abdominal cramping.

Do not try to diagnose preterm labor using an online timing rule.

Is Swelling Normal?

Mild swelling of the:

• feet;
• ankles;
• hands;

can occur.

Sudden or significant swelling is more concerning, especially when accompanied by:

• severe headache;
• vision changes;
• pain below the ribs;
• feeling very unwell.

These symptoms can occur with pre-eclampsia.

Is Vaginal Discharge Normal?

White or milky discharge may remain normal.

Seek medical advice if it becomes:

• foul smelling;
• green or yellow;
• painful;
• itchy;
• associated with fever.

A gush or continuing watery leakage requires prompt assessment.

What About Bleeding?

Any bleeding during the third trimester should be medically assessed.

Seek urgent care for:

• heavy bleeding;
• bleeding with contractions;
• bleeding with severe pain;
• bleeding with fainting;
• bleeding with fluid leakage.

Is Gestational Diabetes Still Relevant?

Yes.

Routine screening is usually completed earlier, around weeks 24–28.

If gestational diabetes has been diagnosed, ongoing care may involve:

• blood-glucose monitoring;
• nutrition changes;
• physical activity;
• medication when needed;
• additional fetal monitoring in some pregnancies.

Follow the plan provided by your healthcare team.

Is Iron Still Important at 31 Weeks?

Yes.

Iron needs remain high in the third trimester.

This is particularly relevant because MedlinePlus notes that the fetus is now storing iron and bone marrow is actively producing blood cells.

Useful iron sources can include:

• meat;
• beans;
• lentils;
• fortified cereals;
• leafy vegetables.

Additional supplements should follow clinical advice.

Is Calcium Especially Important Now?

Yes.

The fetus is storing calcium rapidly during this stage.

Calcium supports:

• fetal bone mineralization;
• muscle function;
• nerve function;
• maternal bone health.

Sources can include:

• dairy foods;
• fortified alternatives;
• calcium-rich plant foods.

Is Phosphorus Important?

Yes.

Phosphorus works with calcium in fetal bone development.

Balanced diets usually provide phosphorus through foods such as:

• dairy;
• meat;
• legumes;
• whole grains.

Routine high-dose phosphorus supplements are not generally something to start without medical advice.

Do You Need Extra Calories?

Energy needs are modestly higher during the third trimester.

This does not mean doubling your food intake.

Focus on nutrient-dense foods containing:

• protein;
• iron;
• calcium;
• whole grains;
• fruit;
• vegetables;
• healthy fats.

Is Protein Important?

Yes.

Protein supports:

• fetal growth;
• placenta;
• maternal tissues.

Choose varied food sources rather than relying on one supplement or “superfood.”

Is Vitamin D Important?

Yes.

Vitamin D supports bone and muscle health.

Supplement recommendations vary by country, diet, season and individual risk.

Follow local pregnancy guidance.

Should You Continue Pelvic Floor Exercises?

Yes.

The growing uterus places increasing pressure on pelvic-floor muscles.

Regular exercises may help support:

• bladder control;
• pelvic organs;
• postpartum recovery.

A pelvic-health physiotherapist can help if you are unsure of technique.

Can You Exercise at 31 Weeks?

Many people with uncomplicated pregnancies can continue appropriate physical activity.

Possible choices include:

• walking;
• swimming;
• prenatal exercise;
• appropriate strength training.

Modify activity for:

• balance;
• fatigue;
• pelvic discomfort;
• breathlessness.

Avoid activities with high fall or abdominal-trauma risk.

Should You Keep Sleeping on Your Side?

Yes.

Side sleeping remains the preferred position later in pregnancy.

If you wake on your back, simply return to your side.

Pillows can help support:

• abdomen;
• knees;
• back.

Is It Time to Prepare the Hospital Bag?

Yes.

Week 31 is a reasonable time to have at least the basics organized.

Consider:

• identification;
• pregnancy notes;
• comfortable clothing;
• toiletries;
• newborn clothing;
• phone charger;
• items recommended by your birth facility.

Should You Finish a Birth Plan?

You can continue refining one.

Consider preferences about:

• labor support;
• pain relief;
• movement during labor;
• cesarean birth if needed;
• skin-to-skin contact;
• feeding.

Remember that a birth plan records preferences rather than guaranteeing exactly how labor will unfold.

Is It Time to Learn About Breech Birth?

You can learn about fetal position, but there is no need to panic if the fetus is not head down at 31 weeks.

There is still time for turning.

Position becomes more clinically important later in pregnancy.

Should You Attend Antenatal Classes?

Yes.

This is a useful time to attend classes covering:

• labor;
• breathing and coping techniques;
• pain relief;
• newborn feeding;
• newborn care;
• cesarean birth;
• partner support.

Should You Learn About Breastfeeding?

Yes.

Learning beforehand can help you understand:

• early feeding cues;
• colostrum;
• positioning;
• latch;
• where to get help.

If you plan to formula feed or combination feed, prenatal education can also help you prepare safely.

Is It Time to Prepare for Newborn Care?

Yes.

You can begin learning about:

• safe sleep;
• feeding;
• diapering;
• newborn screening;
• cord care;
• warning signs of illness.

Use guidance from reputable healthcare sources rather than social-media myths.

Can You Travel at 31 Weeks?

Travel may still be possible for some uncomplicated pregnancies, but restrictions become increasingly important.

Check:

• airline rules;
• medical documentation requirements;
• travel insurance;
• maternity care at the destination;
• your own risk of preterm labor.

Discuss major travel with your maternity provider.

Is Sex Safe at 31 Weeks?

For most uncomplicated pregnancies, sexual activity may remain safe.

Seek individualized advice if you have:

• vaginal bleeding;
• fluid leakage;
• placenta-related complications;
• cervical concerns;
• regular painful contractions;
• medical advice to avoid intercourse.

When Should You Seek Medical Help at 31 Weeks?

Seek urgent professional assessment for:

• heavy vaginal bleeding;
• regular painful contractions;
• significant or ongoing fluid leakage;
• severe abdominal or pelvic pain;
• meaningful reduction or stopping of fetal movement;
• severe headache;
• visual changes;
• sudden major swelling;
• severe pain below the ribs;
• fainting;
• chest pain;
• sudden or severe breathlessness;
• high fever;
• severe one-sided leg swelling or pain.

At 31 weeks, signs of preterm labor, pre-eclampsia, reduced fetal movement or ruptured membranes require professional assessment rather than online self-diagnosis.

Pregnancy Week 31 FAQs

How big is the baby at 31 weeks?

The fetus is around 41 cm from head to heel on average.

What is developing at 31 weeks?

Major structural development is largely complete, while rapid weight gain, fat storage, lung maturation, mineral storage and brain development continue. ACOG specifically notes that major development is finished and weight gain accelerates at week 31.

Are the lungs mature at 31 weeks?

Not fully. Rhythmic breathing movements occur, but MedlinePlus states that the lungs remain immature.

Is the baby gaining fat quickly?

Yes. MedlinePlus describes weeks 31–34 as a stage of rapid growth and significant fat gain.

Are the bones fully developed?

Yes structurally, but they remain soft and continue mineralizing.

Is the baby storing iron and calcium?

Yes. MedlinePlus notes increasing storage of iron, calcium and phosphorus during weeks 31–34.

Should the baby be head down at 31 weeks?

Not necessarily. Some babies are already head down, but there is still time for a breech or transverse baby to turn.

What does breech mean?

Breech means the baby’s bottom or feet are closer to the cervix instead of the head.

Can a breech baby still turn after 31 weeks?

Yes. There is still time for position to change.

What is fundal height?

Fundal height is the distance from the top of the pubic bone to the top of the uterus. It may be used to monitor pregnancy growth.

Should you feel movement every day?

Many people know their baby’s usual movement pattern by this stage. A meaningful reduction should prompt contact with maternity care.

Are Braxton Hicks normal at 31 weeks?

Irregular tightening can occur, but regular painful contractions require prompt assessment for preterm labor.

Is pelvic pressure normal?

Some pressure may occur as pregnancy advances, but significant pressure with contractions, bleeding or fluid leakage requires medical assessment.

Is week 31 in the third trimester?

Yes. Week 31 is firmly within the third trimester.

How many weeks are left at 31 weeks pregnant?

About 9 weeks remain until a 40-week estimated due date, although actual birth timing varies.

What Comes Next?

During Pregnancy Week 32, the fetus continues the weeks 29–32 stage of:

• strong kicking and stretching;
• grasping motions;
• light sensing;
• red-blood-cell production;
• scalp-hair development;
• gradual loss of lanugo.

The next stage, weeks 33–36, will increasingly focus on:

• harder bones while the skull remains flexible;
• fingernails reaching the fingertips;
• limbs becoming chubbier;
• the fetus potentially moving into a head-down birth position.

Previous: Pregnancy Week 30
Next: Pregnancy Week 32
Tool: Pregnancy Due Date Calculator

Medical References

This article is based on current educational information from established medical and public-health organizations, including:

• NHS — 31 Weeks Pregnant
• American College of Obstetricians and Gynecologists (ACOG) — How Your Fetus Grows During Pregnancy
• ACOG — Changes During Pregnancy
• MedlinePlus — Fetal Development

Editorial & Medical Information

Last content review: October 2026

This article provides general educational pregnancy information only. It does not diagnose preterm labor, pre-eclampsia, fetal growth problems, breech presentation complications, reduced fetal movement or any other pregnancy condition; prescribe treatment; or replace individualized prenatal care.

Fetal size, position, movement and symptoms vary between pregnancies. Contact a qualified maternity healthcare professional for personal concerns about movement, contractions, bleeding, fluid leakage, fetal position, severe swelling, headaches, breathlessness or other pregnancy symptoms.

Conclusion

Pregnancy week 31 marks an important shift from major structural development toward rapid growth and maturation. ACOG specifically notes that at week 31 major development is finished and the fetus begins gaining weight very quickly.

MedlinePlus adds several distinctive developments for weeks 31–34: rapid growth, increasing fat stores, rhythmic breathing movements, fully developed but still-soft bones, and increasing storage of iron, calcium and phosphorus.

For prenatal care, the NHS highlights another important transition: clinicians may begin measuring uterine size and checking fetal position more deliberately. Some babies are already head down, while others remain breech or change position—and at week 31 there is still time for that position to change.

This makes Week 31 less about entirely new anatomy and more about growth, mineral storage, lung practice, birth positioning and preparing for the final weeks of pregnancy.

Continue regular prenatal care, appropriate nutrition, physical activity and awareness of your baby’s usual movement pattern. Heavy bleeding, painful regular contractions, fluid leakage, severe headache with visual changes, sudden major swelling or a meaningful reduction in fetal movement requires prompt professional assessment.