Fetal growth is not perfectly even throughout pregnancy. A baby develops continuously from the earliest weeks, but there are periods when growth and development become especially noticeable. These periods are often described as fetal growth spurts.

Understanding fetal growth spurts can help you know what is happening as your pregnancy progresses, how your baby changes from week to week, and why doctors sometimes monitor fetal size with ultrasound.

For a complete week-by-week overview, see our guide to fetal growth by week.

What Are Fetal Growth Spurts?

A fetal growth spurt is a period during pregnancy when the fetus experiences rapid changes in size, weight, or development.

Growth does not happen at exactly the same rate every week. Early pregnancy focuses heavily on forming the embryo's major organs and body structures. Later pregnancy involves substantial increases in body weight, muscle, fat stores, and organ maturation.

Because every pregnancy is different, there is no single week when every baby experiences the same growth spurt.

When Do Fetal Growth Spurts Happen?

Fetal growth occurs throughout pregnancy, but different stages have different priorities.

Early pregnancy

During the first trimester, development is focused mainly on forming the basic structures of the body.

The embryo develops:

• The brain and spinal cord

• The heart and blood vessels

• The arms and legs

• The digestive system

• Other major organs and body structures

Although the fetus is still very small, development during these weeks is extremely rapid.

Second trimester

During the second trimester, the fetus continues developing while also increasing in length and weight.

You may notice major changes in:

• Body proportions

• Bone development

• Muscle development

• Movement

• Facial features

• Nervous system development

This is also the period when many pregnant people begin to feel fetal movement.

Third trimester

The third trimester is an important period for fetal weight gain and maturation.

During these weeks, the fetus generally gains significant weight while the brain, lungs, nervous system, and other organs continue to mature.

The baby also builds up fat stores that help with temperature regulation after birth.

Is There One Biggest Fetal Growth Spurt?

Not necessarily.

People sometimes talk about a single "biggest growth spurt," but fetal growth is more complicated than that. Different aspects of development accelerate at different times.

For example, early pregnancy involves extremely rapid structural development, while later pregnancy involves substantial increases in weight and continued organ maturation.

Your baby's growth should therefore be considered across the entire pregnancy rather than focusing on one specific week.

How Fast Does a Baby Grow During Pregnancy?

The rate of fetal growth changes considerably from one stage of pregnancy to another.

In the earliest weeks, even a small increase in size represents significant development. As pregnancy progresses, measurements such as crown-rump length, head circumference, abdominal circumference, and femur length can help healthcare professionals assess growth.

Later in pregnancy, estimated fetal weight becomes particularly useful when doctors are monitoring growth.

Keep in mind that ultrasound measurements are estimates rather than exact measurements.

How Doctors Monitor Fetal Growth

Healthcare professionals may monitor fetal growth during routine or medically indicated prenatal care.

Depending on the stage of pregnancy, assessment may include:

• Measuring the uterus

• Tracking pregnancy dates

• Ultrasound measurements

• Estimating fetal weight

• Checking fetal anatomy

• Monitoring fetal movement when appropriate

• Assessing placental and blood-flow findings when medically indicated

An ultrasound can provide useful information about fetal size and development.

If you want to understand this in more detail, read our guide to fetal growth by week.

What Is a Fetal Growth Ultrasound?

A fetal growth ultrasound is an ultrasound examination used to assess the baby's growth and development.

Measurements may include:

• Head circumference

• Abdominal circumference

• Femur length

• Other biometric measurements

• Estimated fetal weight

The measurements are compared with expected ranges for the baby's gestational age.

A doctor may recommend additional growth monitoring when there is a reason to follow fetal growth more closely.

What Does a Growth Spurt Mean on an Ultrasound?

If an ultrasound shows that your baby has grown significantly since a previous scan, this may simply reflect normal progression of pregnancy.

However, doctors do not usually evaluate growth based on one measurement alone.

They may consider:

• Gestational age

• Previous ultrasound measurements

• Growth pattern over time

• Estimated fetal weight

• Amniotic fluid

• Placental findings

• Maternal health

• Other pregnancy factors

The overall growth pattern is often more useful than a single number.

Can You Feel a Fetal Growth Spurt?

You generally cannot directly feel your baby's growth occurring.

However, pregnancy changes associated with fetal development may become more noticeable as pregnancy progresses.

You may experience:

• Increasing abdominal size

• Changes in fetal movement

• Stretching sensations

• Changes in appetite

• Increasing physical discomfort later in pregnancy

These symptoms do not reliably indicate that your baby is experiencing a specific growth spurt.

Your healthcare provider is better able to assess fetal growth through appropriate prenatal examinations and ultrasound when needed.

Do Babies Have Growth Spurts in the Womb?

Yes, fetal growth is dynamic, and the rate of growth changes during pregnancy.

However, fetal growth should not be thought of exactly like a newborn baby's growth spurts.

A fetus is undergoing several processes simultaneously, including:

• Growth in length

• Weight gain

• Organ development

• Brain development

• Bone development

• Muscle development

• Development of body fat

Different systems mature at different rates.

What If the Baby Is Measuring Small?

A baby measuring smaller than expected does not automatically mean there is a problem.

Ultrasound measurements have a margin of error, and babies naturally vary in size.

Doctors may look at the baby's growth over time and consider other findings before deciding whether additional monitoring is necessary.

If your baby is measuring small or there is concern about fetal growth, follow your healthcare provider's recommendations for repeat scans or other testing.

What If the Baby Is Measuring Large?

Similarly, measuring larger than expected does not automatically mean something is wrong.

Differences can occur because of:

• Normal biological variation

• Pregnancy dating differences

• Measurement variation

• Maternal factors

• Fetal factors

Your healthcare provider can interpret the measurement in the context of your pregnancy.

Fetal Growth by Week

Fetal development changes significantly from the first weeks through the third trimester.

A week-by-week pregnancy guide can help you understand changes in:

• Fetal size

• Weight

• Organ development

• Movement

• Brain development

• Physical features

• Pregnancy milestones

For a detailed overview, visit our fetal growth by week guide.

You can also use our pregnancy calculator to estimate your pregnancy week and due date.

Frequently Asked Questions

What are fetal growth spurts?

Fetal growth spurts are periods when fetal growth or development progresses more rapidly. Growth continues throughout pregnancy, but the rate and type of development change from one stage to another.

When does the fetus grow the most?

There is not one universal week when every fetus grows the most. Early pregnancy has extremely rapid structural development, while later pregnancy includes substantial weight gain and organ maturation.

Can fetal growth be seen on ultrasound?

Yes. Ultrasound can measure different parts of the fetus and provide an estimate of fetal size and growth.

Does a growth spurt mean the baby will move more?

Not necessarily. Fetal movement changes as pregnancy progresses, but movement should not be used as a direct measurement of fetal growth.

Is it normal for fetal growth to vary?

Yes. Babies naturally vary in size. Healthcare professionals look at gestational age, growth patterns, ultrasound measurements, and other pregnancy factors when assessing fetal growth.

What should I do if my baby is measuring behind?

Talk with your obstetrician or healthcare provider. A measurement that is behind expected size may have several explanations, and your provider can determine whether repeat ultrasound or additional monitoring is appropriate.

Final Takeaway

Fetal growth is a continuous process that changes throughout pregnancy. Early pregnancy is dominated by rapid formation of major body structures, while the second and third trimesters bring continued growth, increasing weight, and maturation of the baby's organs and nervous system.

Rather than focusing on one supposed growth-spurt week, it is more useful to look at your baby's overall growth pattern.

For a detailed pregnancy timeline, explore our fetal growth by week guide and use the pregnancy calculator to estimate your current pregnancy week.