Insights into Early Fetal Development: about Pregnancy Week-by-Week

Discover the latest developments in ultrasound technology that offer fascinating 3D/4D images of the fetus, allowing for greater public and clinical insight into early fetal development and movement

Insights into early fetal development

Advances in ultrasound have ushered in an era of stunning, real-time, three-dimensional images of the fetus, giving both the public and medical professionals a deeper understanding of early fetal development and motility. Dr. Julie-Claire Becher explains our current understanding of the early stages of fetal development.

Crucial Aspects

Declaration of interests

Advances in ultrasound technology have culminated in real-time, three-dimensional (4D) fetal imaging that has captivated the public and medical professionals alike. These images not only provide precise delineation of congenital anomalies such as facial clefts, central nervous system anomalies, and heart defects, but also provide valuable insight for intrauterine procedures such as fetal surgery. In addition, obstetricians have noted the encouraging parental responses to visualizations of their unborn child.

Motor and sensory maturation

Published images of fetuses "walking" and "thumb-sucking" have stunned many, but these movements have long been documented in the scientific literature. To experts in obstetrics and ultrasound, the astonishing qualities of the images lie in their definition and real-time capabilities. Undoubtedly, the fetus exhibits complex movements, but their implications warrant further investigation. Fetuses that appear to take steps do not surprise pediatricians. Even premature neonates can be induced to "walk" when held upright on a flat surface shortly after birth. Many of the behaviors exhibited by newborns in the early postnatal period are due to primitive reflex activity that depends on the maturation of spinal reflex arcs that are not controlled by the brain and are completed as early as the eighth week of gestation. Reflexes are distinctly different from purposeful voluntary movements that develop during the first year of life. Voluntary movements depend on the maturation of the central nervous system, specifically myelination, which begins around the 18th week of gestation. In fact, all primitive reflexes must disappear before mastery of voluntary movement is achieved. Reflex activity usually disappears in healthy infants by about eight months of age, but may persist in those with neurological impairment. Knowledge of fetal behavior has accumulated primarily since the introduction of ultrasonography in the 1950s. The fetus displays a range of behaviors, beginning with slow flexion and extension of the spine and limbs at approximately 7.5 weeks of gestation. The range of movements rapidly expands over the next 3-4 weeks to include numerous patterns such as breathing, trunk rotation, limb flexion/extension, sucking, and yawning. As the fetus progresses toward term, the movements become more regular and coordinated as the nervous system matures. Fetal thumb sucking can be demonstrated as early as 12-14 weeks of gestation. Intrauterine preference for a particular thumb has been linked to neonatal head position preference and eventual right- or left-handedness. Handedness was once thought to depend on cerebral lateralization. However, fetuses show a preference for one thumb as early as 12 weeks, long before the brain gains control over movement. This nascent sucking behavior is likely controlled by reflexes. Brain stimulation is known to affect brain organization, leading to the hypothesis that this reflex activity may ultimately stimulate the development of "handedness" and subsequent lateralization of function. Fetal sensory development has been studied primarily through responses to auditory stimuli, with hearing detectable as early as 23 weeks of gestation. Fetuses respond to maternal speech with a slowed heart rate. There is evidence that fetuses can discriminate distinct speech sounds and show a preference for the mother's native language. Prenatal exposure to speech may initiate the process of postnatal language acquisition.

Fetal learning and memory

Fetal learning can be observed in studies of "habituation," the decrease in a specific behavioral response when a novel stimulus is repeatedly presented. The ability to ignore meaningless stimuli in a constant sensory environment is critical for efficient fetal functioning and survival. Although rudimentary, habituation is one of the most common methods of learning, and there is compelling evidence that it reflects a healthy nervous system. Human fetal habituation, primarily studied in response to auditory stimuli, has been documented as early as 23 weeks of gestation. Studies using other sensory modalities, such as taste and smell, which function at earlier stages, may reveal even earlier habituation events during pregnancy. Research has shown that fetal habituation predicts cognitive function in early childhood. Other aspects of fetal memory have been studied. Infants whose mothers consistently rested in front of a popular television program during pregnancy became alert, stopped moving, and exhibited a slowed heart rate a few days after birth when exposed to the program's theme song. This behavior was not exhibited by infants whose mothers had not watched the program during pregnancy. These findings suggest that fetuses can learn and remember familiar auditory stimuli and retain this information throughout the birth period.

Clinical implications

Fetal behavior can be argued to represent the functioning and integrity of the nervous system. Establishing a standard of "normal behavior" for the fetus allows assessment of fetal well-being. Altered fetal behavior has been observed in pregnancies complicated by maternal smoking or recreational drug use, fetal anomalies (such as Down syndrome), and pregnancies that later ended in spontaneous abortion. The etiology of cerebral palsy has led many researchers to conclude that the majority of brain damage occurs before birth. A recent study from Edinburgh found that most infants with neurological symptoms who died during the neonatal period had prenatal brain damage, with no discernible cause identified at the time of pregnancy screening. Routine ultrasound imaging is now widespread throughout the developed world. The advent of 4-dimensional ultrasound technology may eventually provide detailed information about fetal behavioral normalcy. Assessing the quantity and quality of fetal movement could help determine the severity and extent of any neurological damage present. The more severe the nervous system injury, the greater the behavioral deviation from a fetus with an intact nervous system. Identification of fetuses with such behavioral patterns could provide insight into the pathophysiology of intrauterine brain injury and facilitate optimal management of these pregnancies.


This article was commissioned and accepted by Professor Neil McIntosh FRCP Edin, Consultant Pediatrician, Department of Child Life & Health, Edinburgh.

Introduction to Early Fetal Development

The Importance of Early Development

Fetal development is a fascinating and complex process that has captivated scientists and parents alike for centuries. The early stages of development, from conception to birth, are crucial for ensuring the health and well-being of the baby. In these delicate stages, the baby's organs, tissues, and systems begin to form, setting the foundation for future growth and development.

The Basics of Fetal Growth

Fetal growth can be broken down into three trimesters, with each trimester representing a unique stage of development. The first trimester, which includes weeks 1 through 12, is when the majority of the baby's organs and systems begin to take shape. This article will focus on the early weeks of fetal development, providing a week-by-week guide of the changes taking place during this critical period.

Week 1: The Journey Begins


The process of early fetal development begins with fertilization, when the sperm meets the egg in the fallopian tube. At this moment, the genetic material from the sperm and egg combine, creating a single cell called a zygote. This zygote contains all of the genetic information needed for the baby's development.

The First Cell Division

Once fertilization occurs, the zygote begins to divide rapidly, doubling in size every few hours. These cell divisions are essential for the formation of the baby's tissues, organs, and systems, and will continue throughout the pregnancy.

Week 2: Rapid Cellular Growth

The Morula Stage

Around the third or fourth day after fertilization, the rapidly dividing cells form a solid ball called the morula. The morula continues to divide and grow as it makes its way down the fallopian tube toward the uterus.

The Blastocyst Stage

By day five or six, the morula has transformed into a blastocyst, a hollow ball of cells that will eventually implant itself into the uterine wall. The outer layer of cells, called the trophoblast, will give rise to the placenta, while the inner cell mass will develop into the embryo.

Week 3: Organogenesis Begins

The Gastrula Stage

As the blastocyst implants itself into the uterine wall during week 3, it undergoes a transformation known as gastrulation. During this process, the inner cell mass differentiates into three distinct layers: the ectoderm, mesoderm, and endoderm. These layers will give rise to all the various tissues and organs within the developing baby.

Early Tissue Formation

Each of the three germ layers begins to form specific tissues and structures within the embryo. The ectoderm forms the nervous system, skin, and hair; the mesoderm gives rise to the muscles, bones, and circulatory system; and the endoderm develops into the digestive and respiratory systems, as well as other internal organs.

Week 4: Heartbeat and Limb Buds

The First Heartbeat

One of the most significant milestones in early fetal development is the formation of the heart. By the end of week 4, the baby's heart starts to beat, pumping blood throughout the tiny body. This heartbeat can often be detected using ultrasound technology, providing expectant parents with a thrilling first glimpse of their baby's life.

The Development of Limb Buds

Another remarkable development during week 4 is the formation of limb buds. These small, paddle-like structures will eventually give rise to the baby's arms and legs. Over the coming weeks, the limb buds will continue to grow and develop, taking on a more recognizable shape.

Week 5: The Start of Facial Features

The Formation of Eyes and Ears

As the embryo continues to grow during week 5, the facial features begin to take shape. Small indentations appear on the sides of the head, marking the future locations of the eyes and ears. These structures will continue to develop and differentiate over the next few weeks, eventually forming fully functional organs.

The Growth of the Nose and Mouth

Simultaneously, the embryo's nose and mouth start to form as small depressions on the front of the head. As the facial features develop, the baby's unique appearance begins to emerge.  

The first five weeks of fetal development are a crucial and remarkable time, setting the stage for the baby's continued growth and maturation. From the initial fertilization to the formation of facial features, these early weeks are filled with rapid changes and critical milestones. Understanding the processes that occur during early fetal development can provide valuable insight for expectant parents and healthcare professionals alike, helping to ensure the health and well-being of the developing baby.


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