Stages of Baby Development in Womb: A Guide

stages of baby development in womb

From conception to labor, your baby goes through various stages of development in the womb. Understanding these stages is essential for expectant parents to track the progress of their growing bundle of joy. Let’s explore the fascinating journey of fetal development and the incredible milestones that occur along the way.

Key Takeaways:

  • The stages of baby development in the womb include the formation of a blastocyst, embryo, and fetus.
  • By the 5th week, the baby’s heart cells start to flicker, and by 27 weeks, they have regular sleep and wake cycles.
  • At 39 weeks, the baby is fully physically developed and ready for birth.
  • Fertilization occurs during weeks 1-2, followed by implantation in the uterine lining, marking the beginning of pregnancy.
  • During weeks 3-4, the embryo develops crucial structures such as the face, heart, blood vessels, and stomach.

Week 1-2: Fertilization and Implantation

During these initial weeks of pregnancy, two essential processes take place: fertilization and implantation. These milestones mark the beginning of your remarkable journey towards parenthood.

Fertilization occurs when a sperm successfully penetrates the egg, resulting in the formation of a fertilized egg known as a blastocyst. This tiny cluster of cells contains all the genetic information necessary to develop a human being.

“Fertilization is the starting point of new life, where the union of egg and sperm sets in motion the miraculous process of pregnancy.” – Dr. Samantha Williams, obstetrician

After fertilization, the blastocyst travels down the fallopian tubes towards the uterus. It is during this journey that the blastocyst undergoes cell division, rapidly multiplying and preparing for its next crucial step.

Once the blastocyst reaches the uterus, it implants itself into the rich and nourishing lining of the uterine wall. This process, known as implantation, is essential for the blastocyst to receive the nutrients and support needed for further development. Implantation typically occurs around 6-12 days after fertilization.

The Blastocyst: The Beginning of Life

Let’s take a closer look at the blastocyst, the remarkable structure that forms after fertilization. Comprised of about 70-100 cells, the blastocyst consists of an outer layer called the trophectoderm and an inner cell mass.

While the trophectoderm eventually develops into essential structures for supporting the pregnancy, such as the placenta and umbilical cord, the inner cell mass transforms into the embryo itself.

Miracle in the Making: The Beginning of Pregnancy

With the successful implantation of the blastocyst, the journey of pregnancy officially begins. The blastocyst, now firmly nestled in the uterine lining, can continue to develop and grow over the coming weeks.

Stay tuned for our next section, where we’ll explore the fascinating development of the embryo during weeks 3-4.

Week 3-4: Embryo Development

In week 3 of pregnancy, the blastocyst transitions into an embryo, setting the stage for rapid growth and development. During this crucial period, the embryo begins forming the foundation of its body, including vital structures like the face, neck, heart, blood vessels, lungs, stomach, and liver.

At this stage, the embryo is about the size of a poppy seed, measuring approximately 0.1 inch (0.25 centimeters) in length. It may not be visible to the naked eye, but the intricate process of organogenesis is already underway. The embryo’s genetic blueprint guides the precise formation of these essential organs, ensuring that they develop in a coordinated and functional manner.

The progression of embryo development is not only a marvel of nature but also an exciting time for expectant parents. By the end of week 4, the embryo has grown significantly and reached a size of approximately 0.04 inches (1 millimeter).

During this week, the embryo is approximately the size of a poppy seed and is undergoing crucial processes that can be detected by a home pregnancy test. A positive result on a pregnancy test indicates the presence of the hormone human chorionic gonadotropin (hCG), which is produced by the developing embryo.

Week Embryo Development
Week 3 Transition from blastocyst to embryo
Week 4 Organ formation, including face, neck, heart, blood vessels, lungs, stomach, and liver

Embryo development during weeks 3-4 represents a crucial phase in the journey from conception to childbirth. While the embryo is still tiny, its growth and development are remarkable. Understanding these early stages of pregnancy is essential for expectant parents, allowing them to appreciate the intricate process of human life unfolding within the womb.

Week 5-9: Rapid Growth

During weeks 5 to 9 of pregnancy, your baby experiences rapid growth and significant developmental changes. This period is crucial for the formation of essential body systems and structures, setting the foundation for further growth and development.

One of the notable developments during this time is the formation of the circulatory system. Cells in the tiny “heart” of the embryo start to flicker by week 5, initiating the early stages of the circulatory system’s development. This vital system will be responsible for delivering oxygen and nutrients to the developing organs and tissues of the baby.

In addition to the circulatory system, limb development also begins to take shape during this period. The embryo starts developing tiny limb buds, which will eventually grow into arms and legs. This remarkable process lays the groundwork for your baby’s future mobility and dexterity.

Facial features, such as the nose and mouth, also start to form by week 6. These features gradually become more defined as your baby’s facial structures develop further. This stage marks the beginning of your baby’s unique appearance, although it may still be challenging to distinguish specific facial characteristics at this point.

By week 9, the embryo has developed its basic anatomy, and its tail disappears as the formation of more recognizable body structures takes place. At this stage, your baby is no longer referred to as an embryo but is now called a fetus, signifying the transition into the next phase of development.

Week Development
5 The cells in the tiny “heart” begin to flicker, initiating the development of the circulatory system.
6 Limb development begins, and early facial features start to take shape.
9 The embryo has developed basic anatomy, and its tail disappears.

Week 10-13: Fetal Development

fetal development

During weeks 10 to 13 of pregnancy, the embryo undergoes significant changes and is now officially called a fetus. At this stage, the fetus experiences remarkable growth and development, with the size increasing rapidly. Let’s explore the key highlights of fetal development during this period:

Fetal Weight and Size

By week 10, the fetus weighs about 0.14 ounces (4 grams) and measures approximately 1.22 inches (3.1 centimeters) long. As the weeks progress, the fetus continues to grow rapidly, and by the end of week 13, it weighs about 1 ounce (28 grams) and measures around 3 inches (7.6 centimeters) long.

Sex Determination

Between weeks 10 and 13, the fetus develops external genitalia, eventually allowing for sex determination. While it may not be visible on an ultrasound until around week 12, the fetus’s sex is determined during this period, marking an exciting milestone for expectant parents.

Facial Features, Teeth, and External Ears

Weeks 11 to 13 represent a crucial time for the development of facial features. The fetus begins to form its unique facial characteristics, including the nose, mouth, and eyes. Additionally, teeth buds start to develop, laying the foundation for future oral health. External ears also begin to take shape, enhancing the fetus’s appearance.

Overall, during these weeks, the fetus goes through tremendous changes, both in size and appearance. It’s an exciting phase of pregnancy where expectant parents can marvel at the remarkable progress their baby is making.

Week Fetal Weight (ounces) Fetal Length (inches)
10 0.14 1.22
11 0.35 1.61
12 0.70 2.13
13 1.00 3.03

Week 14-18: Advanced Fetal Development

As pregnancy enters the second trimester, an exciting phase of advanced fetal development begins. During this period, the fetus experiences significant growth and important milestones in its journey towards birth.

One of the key aspects of advanced fetal development during weeks 14-18 is organ development. The organs, such as the heart, lungs, liver, and kidneys, continue to mature and refine their functions. This crucial process lays the foundation for the baby’s overall health and wellbeing.

Furthermore, this stage is marked by the exciting emergence of baby movements. The fetus gains the ability to move its tiny arms and legs, bringing an endearing vibrancy to its developing presence. The sensation of these early movements can be a source of joy and connection for expectant parents.

By the time week 18 arrives, the fetus undergoes a remarkable transformation. It develops a protective coating around its nerves, known as myelin, which facilitates more coordinated movements. This coating enhances the fetus’s motor skills and further establishes its growing awareness of its surrounding environment.

The advanced fetal development taking place during weeks 14-18 illustrates the remarkable journey of life unfolding in the womb. As the organs continue to flourish and the baby’s movements become more pronounced, expectant parents can witness the extraordinary process of their little one’s development.

Week 19-23: Sensory Development

sensory development ultrasound

During weeks 19 to 23 of pregnancy, your baby’s sensory development takes significant strides. The senses of smell, vision, touch, taste, and hearing all begin to develop and play a crucial role in the baby’s overall growth and interaction with the environment.

At this stage, your baby’s olfactory system, responsible for the sense of smell, starts to form. The development of the olfactory receptor cells paves the way for the baby to detect and respond to different scents around them.

Vision also begins to develop as the baby’s retinas and eye structures continue to mature. Although their vision is still limited, they may perceive changes in light and experience a sense of darkness and brightness.

The sense of touch becomes more pronounced as the baby’s skin becomes more sensitive. They can feel gentle nudges, movements, and even respond to external stimuli. This development contributes to the baby’s overall awareness of its surroundings.

Taste buds also start forming during these weeks, laying the foundation for your baby’s future sense of taste. The baby can taste the flavors of the amniotic fluid, which can be influenced by the mother’s dietary choices.

Ultrasound at 20 Weeks

One of the pivotal moments during this stage is the ultrasound at around 20 weeks of pregnancy. This ultrasound not only gives parents the opportunity to catch a glimpse of their baby but also provides valuable insights into the baby’s development.

During the ultrasound, the technician can determine the baby’s sex, marking a significant milestone in the pregnancy journey. This revelation often brings excitement and anticipation for expectant parents as they prepare for the arrival of their little one.

The Baby’s Developing Ears

By the time you reach week 23, your baby’s ears become more refined and sensitive. They are better equipped to pick up sounds from their surroundings. Your baby may respond to external noise and vibrations, which can be felt through the amniotic fluid.

As your baby’s sensory development continues, they will become more attuned to the world around them, setting the stage for a rich and stimulating experience after birth.

Week 24-28: Third Trimester Begins

As the journey through pregnancy continues, the third trimester marks another significant phase of development for the growing fetus. Between weeks 24 and 28, several key developments take place, including lung development and increasing fetal movements.

By week 24, the fetus’s skin begins to fill out with baby fat, providing insulation and protection. This fat layer also helps the baby maintain a stable body temperature after birth. It’s a notable milestone that contributes to the overall development and well-being of the baby.

Lung development also progresses during this period. The fetal lungs continue to mature, preparing for the crucial moment when the baby takes its first breath outside the womb. To practice for this event, the baby starts inhaling and exhaling amniotic fluid, which aids in the growth and strengthening of the respiratory system.

Additionally, by week 28, the third trimester officially begins, bringing with it increased brain activity in the baby. The neural connections in the brain become more complex, allowing for heightened cognitive development. At this stage, the baby’s movements become more pronounced and noticeable, as it has more space to explore and demonstrate its developing motor skills.

The third trimester represents a period of remarkable growth and preparation as the baby approaches the final stages of development in the womb. With lung development underway and increasingly prominent fetus movements, the countdown to meeting your little one is well and truly underway.


Pregnancy is an extraordinary journey that spans several weeks, with each stage showcasing the marvels of prenatal development. From the initial fertilization and implantation to the growth and formation of organs, the baby undergoes a remarkable transformation in the womb. The stages of baby development in the womb provide a timeline that allows expectant parents to appreciate the intricacies of this incredible journey.

Starting from the blastocyst stage, where the fertilized egg implants itself in the uterine lining, to the development of basic anatomy and facial features during the embryonic period, these early stages lay the foundation for the baby’s growth. As the fetus develops, organ systems form, and senses begin to emerge, offering a glimpse of the future individual.

Understanding the various prenatal development stages can help expectant parents track the progress of their baby and foster a deeper connection with the growing life within them. From the flickering of the baby’s heart at week 5 to the coordinated movements of the limbs by week 18, each milestone is a testament to the intricate process of life formation.

As the baby continues to develop, reaching the third trimester at week 28, the lungs mature, and the baby starts to practice breathing by inhaling and exhaling amniotic fluid. This period also marks an increase in brain activity and more noticeable movements, bringing the pregnancy journey closer to the anticipated labor and delivery.


What are the stages of baby development in the womb?

The stages of baby development in the womb include fertilization and implantation, embryo development, rapid growth, fetal development, advanced fetal development, sensory development, and the beginning of the third trimester.

What happens during week 1-2 of baby development?

During week 1-2, fertilization occurs when a sperm penetrates the egg. The fertilized egg, known as a blastocyst, then travels down the fallopian tubes and implants itself in the uterine lining.

What occurs during week 3-4 of baby development?

Week 3-4 marks the development of the embryo. Structures that will form the face, neck, heart, blood vessels, lungs, stomach, and liver start to develop during this period. By the end of week 4, the embryo is about the size of a poppy seed and a home pregnancy test may show a positive result.

How does the baby grow during week 5-9 of pregnancy?

During week 5-9, the embryo experiences rapid growth. The circulatory system starts to form, and cells in the tiny “heart” begin to flicker by week 5. Limb development begins, and facial features, such as the nose and mouth, start to take shape by week 6. By week 9, the embryo has developed basic anatomy, and its tail disappears.

What happens during week 10-13 of fetal development?

During week 10-13, the embryo is considered a fetus. Its size increases, and it develops teeth buds and external genitalia, eventually determining its sex. By the end of week 13, the fetus weighs about 1 ounce and measures around 3 inches long.

What developments occur during week 14-18 of pregnancy?

Week 14-18 marks the beginning of the second trimester. The fetus’s organs continue to develop, and it can move its arms and legs. By week 18, the fetus has developed a protective coating around its nerves and can make more coordinated movements.

What occurs during week 19-23 of baby development?

Week 19-23 is when the baby’s senses, including smell, vision, touch, taste, and hearing, start to develop. An ultrasound at 20 weeks can reveal the baby’s sex. By week 23, the baby’s ears are better at picking up sounds.

What happens during week 24-28 of pregnancy?

During week 24-28, the fetus’s skin starts to fill out with baby fat. Lung development continues, and the baby now inhales and exhales amniotic fluid to practice for its first breath. By week 28, the third trimester begins, and the baby’s brain activity increases. Its movements become more noticeable.

What is the timeline for prenatal development?

Prenatal development begins with fertilization and implantation, followed by the formation of the blastocyst and embryo. As the weeks progress, the embryo develops into a fetus and goes through various stages of growth and organ development. The baby is fully physically developed by 39 weeks, marking the end of pregnancy.

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