The prenatal journey of human development begins with conception, but the term “fetus” applies to the final and longest stage of growth before birth. This period is characterized by the rapid growth, maturation, and functional refinement of all existing body systems. The developing organism transforms into an infant prepared for life outside the womb. This stage focuses on increasing size and complexity, allowing the organism to survive independently.
Defining the Stages of Development
The transition from embryo to fetus marks a fundamental shift in developmental focus. The embryonic period spans the first eight weeks following conception and is the time of organogenesis, where basic structures and major organ systems are established.
The fetal stage officially begins at the start of the ninth week post-conception. By this point, the organism has acquired a distinctly human form, and virtually all major structures are in place. The purpose of this subsequent stage is primarily one of growth, as the body lengthens and gains weight, and the internal systems mature functionally.
Key Developmental Milestones
The fetal period is a dynamic time marked by exponential increases in both physical size and systemic complexity, organized chronologically into three phases.
Second Trimester (Weeks 13-26)
During the second trimester, the fetus experiences significant lengthening, often growing from approximately three inches to over one foot in length. A notable physical development is skeletal ossification, where soft cartilage begins to be systematically replaced by hard bone tissue.
The nervous system begins to exert more coordinated control, leading to the first movements, known as quickening, often felt by the mother around weeks 18 to 20. The differentiation of external genitalia also becomes apparent during this phase, allowing for sex determination around weeks 16 to 20. By week 24, fine, downy hair called lanugo covers the body, and the skin remains thin due to the lack of subcutaneous fat.
Third Trimester (Weeks 27-40)
The third trimester is defined by rapid weight gain and the final maturation of systems necessary for postnatal survival. The fetus gains fat stores, which smooth the skin and serve as insulation after birth, increasing in weight to an average of six to nine pounds by full term. Brain development accelerates dramatically, with the cerebral cortex beginning to fold, increasing the surface area and complexity of the organ.
A key milestone is the maturation of the pulmonary system, specifically the production of surfactant, a lipoprotein substance. Surfactant lowers the surface tension in the alveoli to prevent lung collapse upon exhalation. While small amounts begin production around week 20, sufficient levels for independent breathing are typically not reached until week 35.
The Fetal Support System
Sustaining the fetus throughout this period of intense growth is a complex, temporary support network consisting of the placenta, the umbilical cord, and the amniotic fluid.
The Placenta
The placenta is an organ developed from both maternal and fetal tissue that acts as the primary interface for essential life functions. It facilitates the transfer of oxygen and nutrients from the mother’s bloodstream to the fetus, while simultaneously removing carbon dioxide and metabolic waste products.
The placenta maintains a remarkable separation between the two blood supplies, preventing the maternal and fetal blood from directly mixing. Instead, the exchange occurs across a thin membrane in the placental villi, allowing for diffusion and active transport of necessary materials. This organ also serves as an endocrine gland, producing hormones like human chorionic gonadotropin (hCG) and progesterone that are necessary to maintain the pregnancy.
Umbilical Cord and Amniotic Fluid
Connecting the fetus to the placenta is the umbilical cord, a flexible structure containing three vessels: one vein and two arteries. The single umbilical vein carries oxygenated and nutrient-rich blood from the placenta to the fetus, while the two umbilical arteries transport deoxygenated and waste-laden blood back to the placenta.
Amniotic fluid, contained within the amniotic sac, provides a buoyant, weightless environment that serves several important mechanical and developmental functions:
- It acts as a shock absorber, cushioning the fetus against external pressure or trauma.
- It maintains a consistent temperature.
- It allows the fetus the freedom to move, which is necessary for the proper development of muscles and skeletal structure.
- The fetus swallows and “breathes” the fluid, practicing the motions required for the digestive and respiratory systems to function after birth.
Sensory Perception and Interaction
As the neurological structures refine, the fetus begins to perceive and interact with its immediate environment through developing sensory modalities. Touch is the first sense to emerge, with sensory receptors developing around the face, particularly the lips and nose, as early as eight weeks gestation. The fetus engages in frequent tactile exploration, touching its own face, grasping the umbilical cord, and often sucking its thumb, a behavior observed by the mid-second trimester.
The auditory system matures significantly, with the cochlea becoming structurally functional between weeks 20 and 22. By the late second and early third trimester, the fetus regularly responds to sound, particularly low-frequency noises that easily pass through the maternal abdomen, such as the mother’s voice and heartbeat. This exposure leads to a form of prenatal learning, as newborns show a distinct preference for their mother’s voice and even stories or music heard repeatedly in the womb.
Taste and smell are also active senses, driven by the constant swallowing of amniotic fluid. The composition of this fluid is directly influenced by the mother’s diet, exposing the fetus to various flavors. Research indicates that frequent maternal consumption of certain flavors can result in the newborn showing less aversion to those tastes later in life.