A fetus is the term for a developing human from the ninth week of pregnancy until birth. Before that point, the developing organism is called an embryo. The shift in terminology marks a real biological transition: by week nine, all major organs and body structures have started forming, and the remaining months are devoted to growth, maturation, and fine-tuning.
From Embryo to Fetus
Prenatal development moves through three distinct stages. It begins as a fertilized egg (called a zygote), which implants in the uterus and becomes an embryo around the third week of pregnancy. The embryonic stage is when the basic body plan takes shape: the heart, brain, spinal cord, and limbs all begin forming during this intense six-week window. By the end of the eighth week, every major organ system has at least started to develop.
At that point, the embryo becomes a fetus. The distinction isn’t arbitrary. It reflects a shift from building new structures to refining and growing the ones already in place. A nine-week fetus is roughly the size of a grape, with recognizable features like fingers, toes, and eyelids just beginning to form. Over the next 30-plus weeks, it will grow from about an inch long to a full-term baby.
How the Fetus Gets Oxygen and Nutrients
A fetus doesn’t breathe or eat. Everything it needs comes through the placenta, a temporary organ that connects the mother’s bloodstream to the fetus via the umbilical cord. Oxygen and nutrients pass from the mother’s blood through the placental membrane and into the fetal bloodstream. Carbon dioxide and other waste products travel the opposite direction, back to the mother’s circulation for disposal.
The fetal circulatory system works differently than it will after birth. Oxygenated blood from the placenta enters through the umbilical vein with an oxygen saturation of about 70 to 80%. Most of this blood bypasses the liver entirely, flowing directly into the main circulation. After delivering oxygen to the body, the now-deoxygenated blood (around 40% saturation) returns to the placenta through two umbilical arteries to pick up fresh oxygen. This loop runs continuously throughout pregnancy, and the mother’s and fetus’s blood never actually mix directly.
Major Organ Development
Although the fetal stage is primarily about growth, some of the most critical maturation happens during this period.
The heart is one of the earliest organs to function. It develops four chambers by around week four and can be detected on ultrasound by about week six. By the time the fetal stage begins, the heart is already pumping blood through a working circulatory system, though structural refinements continue.
The brain undergoes enormous change throughout the entire pregnancy. The neural tube, which will become the brain and spinal cord, closes around week four. During weeks five through eight, the brain divides into distinct regions that will eventually become structures responsible for movement, sensation, memory, and basic body regulation like temperature and hunger. Neurons destined for the outer brain layer start forming around week seven, and this process of building and connecting brain cells continues well beyond birth.
The lungs are among the last organs to fully mature. Lung buds appear early in embryonic development, but the air sacs needed for breathing don’t start producing surfactant, a slippery substance that keeps them from collapsing, until around week 24. Even then, adequate surfactant levels for reliable breathing aren’t reached until about week 32. This is one of the main reasons premature babies born before 32 weeks often need breathing support.
Bone and Skeletal Growth
A fetus starts out with a skeleton made almost entirely of cartilage. The process of replacing that cartilage with actual bone begins between weeks six and seven and continues long after birth, not finishing until the mid-twenties. By birth, most of the cartilage has been replaced, but the skull bones remain slightly flexible and separated by soft spots (fontanelles) to allow the head to pass through the birth canal. These soft spots close gradually during infancy.
Movement and Sensory Development
Fetuses begin moving early, but the movements are too small to feel at first. Most pregnant people notice fetal movement, called quickening, between 16 and 20 weeks. Those who have been pregnant before tend to recognize it sooner, around 16 weeks, while first-time mothers often don’t feel it until closer to 20 weeks. Early movements feel like flutters or bubbles. By the third trimester, they become unmistakable kicks, punches, and somersaults.
Hearing develops in a gradual, frequency-by-frequency pattern. Research published in the Archives of Disease in Childhood found that fetuses respond to low-pitched sounds as early as 19 weeks. By 27 weeks, nearly all fetuses respond to low and mid-range tones, but higher-pitched sounds don’t get a reaction until 33 to 35 weeks. This is why a fetus may seem to respond to a parent’s voice (which is relatively low-pitched) well before it reacts to other sounds.
Viability Outside the Womb
Viability refers to the point at which a fetus could potentially survive outside the uterus with medical support. The American College of Obstetricians and Gynecologists defines the “periviable period” as weeks 20 through 25. Survival rates during this window increase dramatically with each additional week of development.
Births before 23 weeks carry a survival rate of only 5 to 6%, and nearly all survivors face serious long-term health complications. At 23 weeks, survival rises to roughly 23 to 27%. By 24 weeks, it reaches 42 to 59%, and by 25 weeks, 67 to 76%. These numbers reflect modern neonatal intensive care, including the ability to deliver artificial surfactant to help underdeveloped lungs function. After about 34 weeks, the vast majority of babies do well with minimal or no intensive support.
The leap in survival between 23 and 25 weeks ties directly to lung maturation. Those few weeks make a significant difference in how well the lungs can handle air, which is often the single biggest factor in whether a premature baby survives.