Fetal Growth and Development – A Comprehensive Guide to Fetus Size by Week

The growth and development of a fetus is a fascinating process that occurs week by week. Each week, the size of the fetus increases as it develops and matures. Tracking the size of the fetus during pregnancy is an important way for parents to monitor their baby’s health and progress.

Understanding the fetal size by week can help expectant parents appreciate the incredible journey their baby is on. From the early weeks of pregnancy, when the fetus is just a tiny cluster of cells, to the later weeks when it reaches a size that can be measured in inches, every stage of development is significant.

By keeping track of the fetal size by week, parents can gain insights into their baby’s growth and compare it to average measurements. This allows them to identify any potential concerns or irregularities that may need medical attention. It also provides a tangible way for parents to bond with their unborn child and marvel at the miracle of life.

Fetus Size by Week: A Comprehensive Guide to Tracking Your Baby’s Growth during Pregnancy

Tracking the fetal growth during pregnancy is an exciting and important part of the journey to motherhood. By monitoring the size and development of your baby from week to week, you can gain valuable insights into their health and well-being.

Understanding Fetal Growth

The fetus grows rapidly throughout pregnancy, undergoing incredible changes and transformations. The growth is measured based on the size of the fetus, which is typically done by taking measurements of specific body parts, such as the head circumference, abdominal circumference, and femur length.

By tracking the growth of these measurements week by week, healthcare providers can assess whether the baby is growing at a normal rate and identify any potential issues or concerns.

Week-by-Week Guide to Fetal Growth

During the first few weeks of pregnancy, the fertilized egg implants itself into the uterus and begins to develop. By around week 4, the embryo is about the size of a poppy seed. As the weeks progress, the fetus goes through various stages of development, with each week bringing new milestones.

For example, by week 12, the fetus is about the size of a plum and can suck its thumb. By week 20, it is about the size of a banana and starts to develop its own individual fingerprints.

By keeping track of these weekly milestones, you can truly appreciate the incredible journey of your baby’s growth and development.

Important note: It’s essential to remember that every pregnancy is unique, and fetal growth may vary from one pregnancy to another. If you have any concerns about your baby’s growth, always consult with your healthcare provider.

In conclusion, tracking your baby’s growth during pregnancy is an essential part of ensuring their health and well-being. By understanding the week-by-week fetal growth and regularly monitoring it, you can be proactive in addressing any potential issues and enjoy the incredible journey of your baby’s development.

Understanding Fetal Development

Understanding the development of the fetus is crucial during pregnancy. The size and growth of the fetus by week provides valuable insights into its development and helps monitor its progress.

Per week, the fetus goes through various stages of development. These stages include the formation of organs, limbs, and features. The size of the fetus also changes as it grows inside the womb.

During the early weeks of pregnancy, the fetus is just a cluster of cells. By the end of the fourth week, the fetus is about the size of a poppy seed. As the weeks progress, the fetal size increases rapidly.

By the eighth week, the fetus is approximately the size of a raspberry, and its vital organs, such as the heart, liver, and kidneys, have started to develop. The external features, like the eyes and ears, also begin to form.

By the twelfth week, the fetus reaches the size of a plum and is about 2.5 inches long. At this stage, the fetus has developed all its organs, and its genitalia can be identified during an ultrasound.

As the pregnancy continues, the fetal growth becomes more prominent. By the twentieth week, the fetus is the size of a banana and just over 10 inches long. The mother can feel the movements of the fetus, and the growth of hair and nails is also evident.

In the third trimester, the fetal size increases significantly. By the twenty-eighth week, the fetus is about the size of an eggplant, and its eyes can open and close. The lungs continue to develop, and the fetus gains weight rapidly.

By the thirty-sixth week, the fetus is the size of a honeydew melon and weighs around 6 pounds. With just a few weeks left for the due date, the fetus is almost fully developed and ready for birth.

Understanding fetal development provides expectant parents with knowledge about the growth and size of their baby. Monitoring the fetus by week helps ensure its healthy development and prepares parents for the upcoming arrival of their little one.

Week 1: Conception and Implantation

During the first week of pregnancy, conception and implantation of the fetus occur. Conception happens when a sperm fertilizes an egg, usually in the fallopian tube. The fertilized egg, known as a zygote, then begins its journey to the uterus for implantation.

At this early stage, the size of the fetus is extremely small and is not yet visible to the naked eye. It is estimated to be about the size of a poppy seed. Despite its tiny size, this is the beginning of an incredible journey of growth and development that will unfold over the coming weeks and months.

Development milestones during Week 1:

  • Conception: The sperm fertilizes the egg, forming a zygote.
  • Journey through the fallopian tube: The zygote starts its journey towards the uterus.
  • Implantation: The zygote attaches to the uterine wall and begins to receive nutrients and oxygen from the mother.

It is important to note that the size of the fetus by week is often measured from the first day of the woman’s last menstrual period (LMP), rather than from the actual day of conception. This is due to the difficulty in determining the exact day of conception.

Keep in mind that every pregnancy is unique, and the size and growth of the fetus can vary from woman to woman. The information provided here serves as a general guide to help track the growth and development of the fetus throughout pregnancy.

Week 2: Formation of the Embryo

In the second week of pregnancy, significant developmental changes occur as the embryo starts to form. This is a crucial period for fetal growth, as the foundation is laid for the baby’s future development.

Embryo Development

During week 2, the fertilized egg implants itself into the lining of the uterus. Rapid cell division takes place, leading to the formation of the embryonic disc, which will eventually develop into the baby.

Formation of the Amniotic Sac

At this stage, the amniotic sac starts to form around the embryo. The amniotic sac is a protective fluid-filled sac that will provide a cushion for the growing fetus.

Development of the Placenta

The placenta, a vital organ for pregnancy, begins to develop during week 2. The placenta serves as a lifeline, providing the fetus with essential nutrients and oxygen while removing waste products.

Fetal Size by Week 2

During week 2, the size of the fetus is tiny, measuring around 0.1 to 0.2 millimeters. It is difficult to visualize the embryo with the naked eye at this stage, but incredible growth and development are happening at a microscopic level.

It is important to stay healthy and take care of yourself during this critical period of embryonic development. Consult with your healthcare provider for personalized advice and guidance throughout your pregnancy journey.

Week 3: Development of Key Organs

During the fetal development, the size of the fetus grows rapidly, with significant changes occurring by week. In week 3, the development of key organs begins, setting the foundation for the baby’s growth.

By week 3, the fetus is approximately the size of a period at the end of a sentence. At this stage, the embryo is about 0.1 inches in length and is developing at a remarkable rate.

The key organs that start to develop during week 3 include the heart, brain, spinal cord, and blood vessels. The heart begins to form and beat, serving as the organ that pumps blood throughout the body. The brain and spinal cord develop from the neural tube, which will later become the central nervous system.

The blood vessels also start to form during this week, setting the stage for the circulatory system to develop. These blood vessels will play a crucial role in delivering oxygen and nutrients to the growing fetus.

Growth during week 3 is swift, as the fetus continues to multiply cells and develop new tissues. The embryo implants itself in the lining of the uterus and begins to receive nourishment from the mother through the placenta.

Developmental Milestones in Week 3:
Formation of key organs including the heart, brain, spinal cord, and blood vessels
Embryo implants in the lining of the uterus
Blood vessels start to form
Rapid multiplication of cells and development of new tissues

Overall, week 3 is a critical period in the fetal development journey. The formation of key organs and the rapid growth during this week set the stage for a healthy pregnancy and the future well-being of the baby.

Week 4: Growth of the Neural Tube

During week 4 of fetal development, the size of the neural tube is a key indicator of growth. The neural tube is a structure that eventually develops into the brain and spinal cord. It forms from the embryonic tissue and is essential for the proper functioning of the nervous system.

At this stage, the neural tube is just beginning to form. It starts as a flat sheet of cells, which then fold and fuse together to create the tube. The process of neural tube closure occurs around day 28 after conception.

By the end of week 4, the neural tube has started to close at the top and bottom, but the middle section is still open. This closure eventually progresses throughout the entire length of the embryo, resulting in the formation of the brain and spinal cord.

Proper growth and development of the neural tube is crucial for the overall development of the fetus. Inadequate closure of the tube can lead to neural tube defects, such as spina bifida or anencephaly.

Key Points:

• Week 4 is marked by the growth and development of the neural tube.

• The neural tube forms from embryonic tissue and develops into the brain and spinal cord.

• By the end of week 4, the neural tube has started to close.

• Proper closure of the neural tube is vital for the normal development of the fetus.



[1] Kallen B. (2004). Confirmation of human fetal 4 mand 5 week landmarks using two- and three- 5 dimensional ultrasound. Birth Defects Research. Part A, Clinical and Molecular Teratology, 70(5), 389-393. [2] O’Rahilly R, Müller F. (2007). Human Embryology & Teratology, 3rd Edition. New York: Wiley.


Week 5: Formation of the Heart

During the fifth week of pregnancy, the fetus undergoes significant development. One of the most crucial milestones during this time is the formation of the heart.

The heart starts to take shape as a simple tube and begins to beat on its own. This vital organ is responsible for pumping blood throughout the body, ensuring that the fetus receives oxygen and nutrients necessary for growth and development.

Development of the Heart

At this stage, the fetal heart is approximately the size of a poppy seed, measuring about 1/17 of an inch (about 1.5 millimeters) in length. It consists of two tubes that will eventually merge to form the four chambers of the heart.

The heart’s development is a complex process involving the positioning and alignment of its chambers, valves, and blood vessels. Any disruption in this intricate process can result in congenital heart defects.

Growth and Size

The growth of the fetus during week 5 is rapid. It is estimated that the fetus grows at a rate of about 1 millimeter per day. By the end of this week, the fetus will be around 1/4 of an inch (about 6 millimeters) in length.

It is important to note that these measurements are approximate, and the size of the fetus may vary slightly for each individual pregnancy.

As the fetal development progresses, it is essential for the expectant mother to take proper care of her health and follow medical advice for a healthy pregnancy.

Week 6: Differentiation of Limbs

During the sixth week of fetal development, exciting changes are taking place in the size and growth of the fetus. At this stage, the embryo is rapidly developing and its major organs and systems are starting to form.

Size and Growth

By the sixth week of pregnancy, the fetus is approximately the size of a sweet pea. It measures about 0.25 inches (0.6 centimeters) in length. Although still very small, the fetus is growing at a rapid pace and its growth will continue throughout the pregnancy.

Differentiation of Limbs

One of the most remarkable developments during the sixth week is the differentiation of the limbs. The tiny buds that appeared in the previous weeks start to grow and take shape, developing into the arms and legs of the fetus. Although they are not yet fully formed, the limb buds can be seen as distinct structures at this stage. The fingers and toes will begin to form as well, although they may still be webbed.

The differentiation of limbs is a crucial step in the overall development of the fetus. It marks the beginning of a complex process that will eventually lead to the formation of fully functional arms and legs. As the pregnancy progresses, the limbs will continue to grow and mature, becoming more defined and capable of movement.

It is important to note that every fetus develops at its own pace, and there can be variations in the size and growth at each week. These estimates are based on averages and should be used as a general guideline. If you have any concerns about the development of your fetus, it is best to consult with your healthcare provider for personalized information and support.

Week 7: Rapid Brain Development

During week 7 of pregnancy, the fetus experiences rapid brain development. This is an exciting time as the size of the fetal brain increases significantly. The growth of the brain is essential for the future development and functioning of the baby.

By week 7, the fetus is about the size of a blueberry, measuring around 0.51 inches (1.3 centimeters) in length. Although still tiny, the brain is becoming more complex each day. The neural tube, which will later become the brain and spinal cord, is well-formed by this stage.

The rapid brain development in week 7 involves the formation of billions of neurons, which are nerve cells responsible for transmitting information. These neurons start to connect with one another, forming the foundation of the baby’s future nervous system.

Additionally, at this stage, the fetus’s facial features begin to form, including the eyes, ears, and nose. This marks the beginning of the baby’s ability to detect external stimuli.

As the brain continues to develop rapidly in week 7, the fetus’s central nervous system is also developing. This intricate system is responsible for coordinating various bodily functions, such as breathing, heart rate, and movement.

It is important to note that every fetus develops at its own pace. The provided measurements and stages are general guidelines based on average growth. Always consult with a healthcare provider for personalized information about the growth and development of your fetus.

Week 8: Facial Features and Hearing

During the eighth week of pregnancy, the fetal growth is taking place rapidly. By this week, the size of the fetus is approximately the size of a raspberry. While still tiny, the facial features are beginning to form, including eyes, nose, and mouth. The ears are also developing, allowing the fetus to start detecting sound waves.

Facial features are essential for the growth and development of the fetus. As the facial structures take shape, the fetus becomes more recognizable. The eyes, nose, and mouth will continue to develop and refine throughout the remainder of the pregnancy.

Additionally, the hearing ability of the fetus begins to develop around this time. While it may not yet be able to hear external sounds, the internal structures of the ears are forming, including the cochlea, which is responsible for translating sound waves into signals that the brain can understand.

At this stage, it is crucial for expectant parents to maintain a healthy lifestyle, as the fetus is now going through significant changes. This includes maintaining a balanced diet, getting regular exercise, and attending prenatal care appointments to ensure the continued growth and development of the fetus.

Week 9: Development of Fingers and Toes

By week 9 of pregnancy, the fetus continues to grow and develop at a rapid pace. During this week, the development of fingers and toes becomes more apparent as the baby’s limbs continue to elongate and take shape.

Growth and Size

At this stage, the fetus is approximately the size of a grape, measuring around 0.9 inch (2.3 centimeters) from crown to rump. Although still tiny, the baby is growing at a remarkable rate, with each passing week bringing new changes and developments.

Development of Fingers and Toes

By week 9, the fingers and toes of the fetus begin to form. What started as small buds now appear as distinct digits, gradually lengthening and separating from one another. The hand and footplates also become more defined, contributing to the overall development of the baby’s limbs.

During this period, the cells in the hands and feet undergo programmed cell death, or apoptosis, which helps to shape the fingers and toes. This process eliminates the tissue between the digits, allowing the individual fingers and toes to separate and become more defined.

As the fetus continues to develop, the fingers and toes will continue to grow and mature, eventually becoming fully formed and functional.

Week 10: Growth of the Skeletal System

During week 10 of pregnancy, the fetus is continuing to grow in size and develop. At this stage, the skeletal system begins to form and undergo significant growth.

The fetus is about the size of a prune, measuring approximately 1.2 inches in length. It’s hard to believe that such a tiny being already has the foundation for a complete skeleton!

Per week 10, the bones are still in the early stages of development, with cartilage forming the framework. Over time, this cartilage will continue to harden and transform into bone, allowing for more support and structure.

The development of the skeletal system is essential for the fetus as it enables mobility, protection of internal organs, and the ability to maintain posture. As the baby continues to grow, so will the bones, gradually developing into a more mature and intricate skeletal structure.

It’s important to note that during this stage of fetal development, the bones are still quite flexible to accommodate growth and movement. However, as the pregnancy progresses, the bones will become stronger and more rigid.

Week 10 marks an exciting time in the growth of the fetus as the skeletal system begins to take shape. It’s awe-inspiring to think about the incredible journey of development that the fetus is going through, laying the foundation for a healthy and functioning body.

Week 11: Development of the Digestive System

During week 11 of fetal development, the size of the baby continues to grow. By this time, the fetus is approximately 1.6 inches long, about the size of a key lime.

One of the significant developments during this week is the formation and growth of the digestive system. The digestive system plays a crucial role in the absorption and processing of nutrients essential for the baby’s growth and development.

At this stage, the digestive system is starting to take shape. The baby’s esophagus, stomach, and intestines are developing and beginning to differentiate. The cells of the digestive organs are multiplying rapidly to form the different sections of the system.

The small and large intestines begin to form, which will eventually be responsible for absorbing nutrients from the amniotic fluid that surrounds the fetus. These nutrients are essential for the baby’s growth and development.

By week 11, the digestive system is still immature, and the baby is not yet capable of digesting food. However, the organs are slowly developing and preparing for their vital role after birth.

It is fascinating to see the progress of the baby’s growth and development during week 11. The digestive system is a crucial part of the overall development, and it is amazing how the tiny organs are forming and getting ready to function per baby’s needs.

Week Fetal Size
Week 11 About 1.6 inches (size of a key lime)

Week 12: Sex Organs and Urinary System

During week 12 of fetal development, the sex organs of the fetus continue to develop. These sex organs, which include the ovaries and testes, will determine the baby’s biological sex. At this stage, it may be possible to determine the sex of the fetus through an ultrasound examination.

The urinary system of the fetus is also developing during week 12. The kidneys, which play a crucial role in filtering waste from the blood and producing urine, are becoming more functional. The fetal bladder is also developing and will eventually store and release urine. The amniotic fluid, which surrounds and protects the fetus, is largely made up of the baby’s urine.

Key Developments at Week 12:

  • Continued development of sex organs
  • Functional kidneys
  • Development of the fetal bladder

As the fetus grows, its sex organs and urinary system will continue to develop further. It is important to note that every pregnancy is unique, and the rate of development may vary from fetus to fetus. Regular prenatal check-ups with a healthcare provider can provide more information about the growth and development of the fetus.

Week 13: Formation of Teeth

During fetal development, many important milestones occur week by week. In week 13, one of the notable developments is the formation of teeth in the fetus.

The growth of teeth begins early in pregnancy, but by week 13, the process becomes more pronounced. The cells responsible for tooth formation, called odontoblasts, are now actively functioning. These cells are essential in the development of dentin, which is the hard tissue that makes up the bulk of the tooth structure.

By this stage, the fetus has all the primary teeth buds formed. These teeth buds will eventually develop into the primary teeth that will erupt after birth. The primary teeth, also known as baby teeth, are necessary for proper speech development, nutrition, and the alignment of the jaw.

It’s important for expecting mothers to have a balanced diet during this stage, as the growth of teeth heavily relies on proper nutrition. Consuming sufficient amounts of calcium, phosphorus, and Vitamin D is crucial for the development of healthy teeth in the fetus.

The Role of Calcium

Calcium plays a significant role in the formation of teeth and bones. It helps in the mineralization process, which is vital for the hardening of tooth tissues. Expectant mothers are advised to include dairy products, leafy greens, and fortified foods in their diet to ensure an adequate calcium intake.

Importance of Oral Hygiene

Even though the baby’s primary teeth haven’t erupted yet, it’s essential to establish good oral hygiene practices early on. As the formation of teeth progresses during pregnancy, maintaining oral health can help ensure the health of both the mother and the baby. Regular brushing, flossing, and visits to the dentist are crucial for maintaining optimal oral health.

By week 13, the fetal growth and development are well underway, and the formation of teeth is one of the remarkable milestones during this period. It’s important for expecting mothers to prioritize their own health and dental care as they prepare for the arrival of their little one.

Week 14: Maturing of the Reproductive System

One of the significant developments during this week is the maturing of the reproductive system. The fetus’s genitals, which had been developing since early in the first trimester, continue to develop and take on male or female characteristics. At this point, it may be possible to determine the baby’s gender through ultrasound imaging.

Additionally, the fetus’s liver starts to produce bile, while the spleen begins to produce red blood cells. The kidneys are now functioning and producing urine, which is released into the amniotic fluid. This fluid helps protect the fetus and allows for movement and growth.

The respiratory system also continues to mature during week 14. While the fetus does not yet breathe air, the lungs continue to develop and prepare for breathing after birth. The tiny air sacs, called alveoli, are forming, and blood vessels are developing to support the exchange of oxygen and carbon dioxide.

As the fetus’s various systems and organs continue to grow and mature, it becomes increasingly vital to maintain a healthy lifestyle and follow prenatal care guidelines. This includes a balanced diet, regular exercise (with your healthcare provider’s approval), and regular prenatal check-ups to monitor the progress of both you and your baby.

Week 15: Active Fetal Movement

The development of the fetal period is an exciting time for expectant parents. By week 15, your baby has grown to the size of a pear, measuring about 4 inches long and weighing around 2.5 ounces. This week marks an important milestone in your baby’s growth, as they are becoming more active and their movements can now be felt.

During this stage of pregnancy, your baby’s muscles and bones are becoming stronger, enabling them to move more freely. You may start to feel tiny flutters or gentle taps as your baby kicks, punches, or rolls around in the womb. These movements, known as quickening, are often described as feelings similar to butterfly wings or popping popcorn.

What does active fetal movement feel like?

Every woman’s experience of feeling fetal movement can be different, and it may vary from pregnancy to pregnancy. At week 15, you might start noticing these movements when you are in a quiet, relaxed state, such as when lying down or sitting still. Some women describe the sensation as a subtle fluttering or a light tapping, while others may feel more pronounced kicks or rolls.

It’s important to remember that if this is your first pregnancy, it may take a bit longer for you to notice fetal movement. Furthermore, factors such as the position of the placenta and the amount of amniotic fluid can also affect how distinctly you feel your baby’s movements.

Why is fetal movement important?

Feeling your baby move is not only an exciting milestone, but it is also an important indication of their well-being. Active fetal movement is a sign that your baby’s nervous system is developing properly and that they are getting the nutrients and oxygen they need. It is also a reassuring sign that your baby is growing and thriving in the womb.

If you are ever concerned about the frequency or intensity of your baby’s movements, it’s always a good idea to reach out to your healthcare provider. They can provide you with additional guidance and reassurance.

As you progress through your pregnancy, you will continue to feel your baby’s movements become stronger and more frequent. By keeping track of your baby’s growth and movement during each week of pregnancy, you can stay connected to their development and experience this miraculous journey together.

Week 16: Developing Sensory Capabilities

In week 16 of fetal development, the fetus continues to grow in size. By this week, the fetus is typically about the size of an avocado. This growth is an exciting milestone for parents-to-be as they can begin to see and feel the baby’s movements more clearly.

During this week, the fetus’s sensory capabilities also start to develop. The nerve cells responsible for the senses, such as hearing, sight, and touch, are forming and connecting in the brain. While the fetus’s senses are still developing, they are becoming more refined.

One of the primary sensory capabilities that begins to develop during week 16 is hearing. The fetus’s ears have fully formed, and they are now able to detect sounds from the outside world. This is an important milestone as it allows the fetus to start experiencing the sounds of their parents’ voices or other external noises.

Another developing sensory capability is sight. Although the fetus’s eyelids are still fused shut, the eyes are continuing to develop. The retinas are forming, and the optic nerve is becoming more complex. These developments lay the foundation for the fetus to begin seeing once the eyelids open later in the pregnancy.

As for the sense of touch, the fetus’s skin continues to become more sensitive. The nerve endings in the skin are maturing, enabling the fetus to react to touch stimuli. This development allows the fetus to explore their surroundings and respond to gentle caresses from the mother or external pressure on the mother’s belly.

In summary, week 16 of pregnancy is an exciting time for both fetal growth and sensory development. The fetus is rapidly increasing in size, and their sensory capabilities, including hearing, sight, and touch, are starting to develop. It is a crucial period for bonding with the baby as parents can begin to communicate with them through gentle touches and talking to them.