Weeks 3-4: Gastrulation and Neurulation: Forming the Foundation

Early fetal development is a complex process that marks the beginning of human life, unfolding in a meticulously orchestrated series of events starting from conception. It is during this critical period, from the 3rd to the 11th week, that the embryo undergoes an extraordinary transformation. Through rapid cell division, differentiation, and the establishment of foundational structures, the embryo begins the journey toward forming a viable fetus. Each stage of development is critical, as mistakes during this period can lead to congenital anomalies or pregnancy loss.

By the 3rd week of pregnancy, a remarkable process known as gastrulation occurs, transforming the simple blastocyst into a sophisticated structure composed of three distinct germ layers: ectoderm, mesoderm, and endoderm (Figure 1). Each of these layers plays a vital role in future development, giving rise to various tissues and organs. The ectoderm, for instance, is the precursor to the central nervous system and skin. During this transformative week, another critical event begins – the formation of the neural plate from the ectoderm, setting the groundwork for the future nervous system⁷.

As the embryo progresses into the 4th week, neurulation commences, marking the initial phase of neural tube development. During this stage, the neural plate undergoes a fascinating transformation, folding to create the neural tube. This structure is paramount as it eventually develops into the brain and spinal cord, serving as the central command center for the developing fetus⁷. The precise closure of the neural tube is crucial; any failure in this process can result in conditions such as spina bifida⁵. As these foundational structures take shape, the intricate choreography of cellular processes continues to lay down the building blocks of human life.

Figure 1

Ectoderm

Nervous System

Epidermis

Lenses of the eye

Hair

Nails

Neural tube

(Brain & Spinal Cord)

Mesoderm

Circulatory System

Muscle

Bone

Connective tissue

Axial Skeleton

Internal sex organs

Endoderm

Epithelial lining (Digestive & Respiratory Systems)

Liver/Bladder

Pancreas/Colon

Gallbladder

Lungs

Intestines

Weeks 5-6: Neural Tube Closure and Cardiac Development

Figure 2

At 5 weeks of embryonic development, the embryo reaches a critical milestone with the beginning of neural tube formation, a structure that will eventually develop into the brain and spinal cord. This process is fundamental for the establishment of the central nervous system. During this stage, the heart also begins to function, marking the onset of a basic circulatory system. Initially, the heart forms from two separate endocardial tubes derived from the mesoderm layer¹. These tubes gradually fuse, producing a single primitive heart tube that starts to pulsate (Figure 2), indicating the first heartbeat. This early cardiac activity is essential as it facilitates the circulation of nutrients and oxygen, which are essential for the embryo's rapid growth. By the 6th week, the cephalic pole becomes apparent, albeit detailed brain structures remain indistinct. A key developmental process occurring during this time is the closure of the neural tube, which starts at the midregion and progresses towards both cranial and caudal ends. This closure is mandatory and helps to prevent neural tube defects, such as spina bifida, which can lead to serious complications. The culmination of these processes highlights the intricate and coordinated nature of early embryonic development, laying the groundwork for further growth and differentiation⁵.

Weeks 7-8: Completion of Organogenesis

Figure 3, Rhombencephalon, the precursor to the fourth ventricle, is particularly noticeable in a sagittal view.

Between the 7th and 8th weeks of embryonic development, a pivotal transformation occurs as essential anatomical structures begin to form. During this period, the brain undergoes rapid expansion, with the major divisions of the central nervous system—the forebrain, midbrain, and hindbrain (Figure 4) —starting to clearly delineate. Notably, the rhombencephalon (Figure 3), which will develop into the future fourth ventricle, emerges as the largest cerebral vesicle located at the superior aspect of the cephalic pole. This vesicle is characterized by its "inverted pyramid" shape, with an average length of 3.7 mm and a volume ranging from 8.5 to 20 mm⁴. Concurrently, limb buds appear, signaling the early stages of appendage formation, with small protuberances eventually giving rise to the arms and legs. Significant advancements also occur in the cardiovascular system; the heart, initially a simple tubular structure, begins to partition into four distinct chambers: two atria and two ventricles. Functioning valves develop, ensuring unidirectional blood flow, and the establishment of initial blood circulation supports the growing needs of the embryonic tissues. This period represents a critical phase of organogenesis, as these foundational structures are crucial for the subsequent growth and functional maturation of the fetus⁷.

Forebrain	Contains the cerebrum, thalamus, hypothalamus, pineal gland
Midbrain	Superior portion of the brainstem, which connects the brain to the spinal cord
Hindbrain	Contains cerebellum, pons, and medulla
4^th Ventricle	Intracranial translucency, screening marker for neural tube defect (spina bifida)

Figure 4

Weeks 9-11: Embryonic Transformations and Placental Advancements

As the embryo transitions into the fetal stage, the groundwork laid during weeks 9 to 11 is critical for ensuring healthy development in the coming months.

9 Weeks: Choroid plexuses appear as echogenic areas surrounded by brain tissue during this developmental phase, where the axial cephalic diameter measures between 9 and 12 mm. These plexuses in the lateral ventricles are visible, with their volume ranging from 40 to 120 mm⁴, while the ventricles themselves have a volume of approximately 70 to 200 mm⁴. At this point in development, choroid plexuses assume a "club shape," and the cerebral hemispheres display a "C-shape," with the cortex appearing as a hypoechoic mass. The trunk continues to lengthen and becomes straight at the abdomen. Another normal developmental feature at this stage is the "physiologic herniation" of the mid-gut, which manifests as a rounded isoechoic bulge within the peritoneal membrane at the base of the umbilical cord (Figure 5). This occurrence results from the primitive intestine growing faster than the abdominal wall. The embryo is nearing the transition to the next growth phase to eventually become a fetus⁴.

Figure 5

10-11 Weeks: As the first trimester advances, the ventricular region expands at a quicker pace than the choroid plexus, resulting in altered proportions between the two. At this stage, the size of the choroid plexuses ranges from 2 to 3 mm¹. By the final stage of embryonic development, the calvarium has commenced its mineralization, with the frontal bone appearing as a hyperechoic outline visible in the sagittal plane. At this point, the choroid plexuses (CPs) of the lateral ventricles (LVs) are the most developed intracranial structures visible, encased by the anechoic presence of cerebrospinal fluid (CSF) (Figure 6). During this week, the physiological herniation of the mid-gut, characterized by its distinctive protrusion (Figure 5), typically resolves as the small intestine retracts back into the abdominal cavity⁴.

Figure 6

11 Weeks: The process of skull ossification initiates with the occipital bone, and the facial bones (Figure 7) are now fully formed⁵. At this stage, the eyelids remain shut and will not open for several months. The ear buds, located on the sides of the head, are evolving to resemble ears more closely. The head constitutes approximately one-third of the fetus's total body length. The body is undergoing rapid growth, becoming more elongated and with fingers and toes continuing to separate. Fingernails have already developed during this phase. The fetus exhibits preliminary breathing movements and has started to open its mouth and swallow. Concurrently, the placenta is rapidly developing and is expected to be completely formed by around the 12th week of gestation.

Figure 7

This period of rapid change sets the stage for further growth, as the fetus's systems become increasingly complex and capable of supporting life outside the womb. The remarkable transformations in fetal anatomy and the placenta during this time underscore the intricate coordination required between mother and baby. These developments not only reflect the embryo's progress toward becoming a fully formed human but also prepare both the fetus and mother for the demands of birth and beyond.

Advanced Fetal Ultrasound: Innovations & Insights for Early First Trimester Assessment

Chapter 1: Understanding Early Fetal Development