Prenatal development transforms a single fertilized cell into a human infant. This journey is divided into three main periods: the germinal, the embryonic, and the fetal stages. The fetal period is the longest portion of gestation, beginning in the ninth week and lasting until birth. This stage focuses on the maturation of all organs and structures established earlier, preparing the developing human for life outside the uterus.
Defining the Fetal Stage and Its Predecessor
The fetal stage begins at the start of the ninth week after conception, marking a fundamental shift in developmental priorities. The preceding embryonic stage (weeks three through eight) is when the basic body plan and major organ systems are first formed, a process known as organogenesis. During this period, the developing organism is most vulnerable to external factors because its foundational structures are rapidly differentiating.
Once the fetal period commences, the organism is called a fetus, signifying that all fundamental structures are present in rudimentary form. The focus transitions from forming these systems to increasing their size, refining their function, and making the body proportional. The fetal stage is characterized by a dramatic increase in weight and length, culminating in an organism ready for independent existence.
The Three Defining Milestones of Fetal Development
Rapid Growth and Structural Refinement
The initial milestone is characterized by explosive physical growth and structural refinement. Early in this stage, the fetus develops distinct facial features, limbs, organs, and muscles. A significant proportional change occurs as the head, which initially accounts for about half of the body’s total length, becomes more proportionate to the rest of the body.
Internally, ossification (the process of bone hardening) begins as the cartilaginous skeleton is progressively replaced by bone. Fingers and toes become fully formed. The digestive and urogenital systems become active, with the fetus beginning to excrete urine into the amniotic fluid. This period of rapid growth is also when the mother may first detect fetal movement, often referred to as quickening, typically felt around weeks 17 through 20.
Sensory and Movement Development
The second milestone centers on the development of the nervous system, allowing for complex sensory perception and coordinated movement. The fetal cochlea, the organ of hearing, becomes functional by about the 20th to 22nd week of gestation, allowing the fetus to respond to sounds, including the mother’s voice and heartbeat. The brain continues its rapid development, nearly doubling in size between weeks 16 and 28 and forming new neural connections.
Fetal movements evolve from simple twitches to deliberate, complex actions, demonstrating early sensorimotor behavior. Fetuses exhibit reflexive actions like grasping the umbilical cord and are observed touching their face and mouth, which contributes to the development of motor control. During this time, the fetus develops distinct sleep and wakefulness cycles, reflecting the maturation of its central nervous system.
Achieving Viability and Birth Preparation
The final milestone focuses on the maturation of systems necessary for survival outside the uterus. This period includes achieving the age of viability, generally considered to be around 24 weeks of gestation, though survival is possible earlier with intensive care. A primary factor in viability is the maturation of the lungs, specifically the production of surfactant, a substance that prevents the air sacs from collapsing upon breathing.
The fetus accumulates a layer of subcutaneous fat during this late stage, which is necessary for regulating body temperature after birth. The brain undergoes its most rapid period of growth, preparing the nervous system to coordinate all bodily functions independently. By the end of this milestone, all major organ systems are prepared for the transition to the external world, and the fetus often shifts into the head-down position for delivery.
The Role of the Placenta in Fetal Support
Throughout the fetal stage, the placenta acts as the temporary organ responsible for sustaining the growing fetus. This unique structure serves as the interface between the maternal and fetal blood supplies, facilitating necessary exchanges without the bloodstreams mixing. Its primary function is to deliver oxygen and nutrients, such as glucose and amino acids, from the mother’s circulation directly to the fetus.
The placenta is equally responsible for removing metabolic waste products, including carbon dioxide, urea, and uric acid, transporting them back into the maternal blood for excretion. The placenta is also an endocrine organ, producing hormones like human chorionic gonadotropin (hCG), progesterone, and estrogen. These hormones are necessary for maintaining the uterine lining and supporting the pregnancy until birth.