The fetal stage is the longest phase of prenatal development, spanning from the ninth week of gestation until birth. It begins once the embryo has laid down the basic blueprint for every organ system and officially becomes a fetus. From that point forward, the work shifts from building new structures to growing, refining, and maturing the ones already in place.
How the Fetal Stage Differs From the Embryonic Stage
During weeks three through eight of gestation, a developing human is called an embryo. This is the period of organogenesis, when all major organ systems take shape from three foundational cell layers. By the end of week eight, rudimentary versions of every organ exist, but most are either nonfunctional or only partially working. The heart beats, and a basic circulatory loop is running, but the lungs, kidneys, and digestive tract are far from ready.
The transition at week nine isn’t a single dramatic event. It’s a recognition that the construction phase is essentially complete and the maturation phase has begun. The fetus already has limbs, fingers, toes, facial features, and internal organs. What it needs now is time to grow, wire its nervous system, and bring those organs up to working order.
What Happens in the First Trimester (Weeks 9–12)
The earliest weeks of the fetal stage bring rapid visible change. By the end of week 12, the fetus has identifiable sex characteristics, and all limbs, bones, and muscles are present. The circulatory, digestive, and urinary systems are functioning at a basic level, and the liver is already producing bile. The fetus is only about three inches long at this point, but structurally it looks unmistakably human.
Touch is the first sense to come online, with receptor development starting around week eight on the lips and nose. By week 12, touch receptors have spread to the palms and soles of the feet. Swallowing begins as early as week 11, giving the fetus its first opportunity to practice a reflex it will need immediately after birth.
Brain Growth and Nervous System Wiring
The brain undergoes extraordinary growth throughout the fetal stage. The cells that will become neurons multiply most rapidly between months two and four of gestation. Starting around weeks six to eight, these newly formed neurons begin migrating outward to their permanent positions in the brain. This migration continues through the eighth month and is responsible for the brain’s characteristic folded surface, with its ridges and grooves. That folded architecture dramatically increases the brain’s surface area, packing far more processing power into the skull.
Around month six, the brain enters a phase called organization, in which neurons begin sprouting branches and forming connections with neighboring cells. These connections, called synapses, are the basis of every signal the brain will ever send or receive. Organization continues well past birth, which is one reason the brain remains so vulnerable to disruption during late pregnancy and early infancy.
Organ Maturation Through the Second Trimester
The second trimester (weeks 13 through 27) is when most organ systems move from rudimentary function toward genuine capability. Touch receptors reach the abdomen by week 17, meaning the fetus can sense contact across much of its body. By week 24, the lungs have developed their basic structure, though they are not yet mature enough to function outside the uterus. The musculoskeletal system finishes its core development during this period, and the circulatory system reaches completion.
Hearing also develops during this window. Research first confirmed in the 1980s that babies can hear while still in the womb and are able to distinguish voice patterns before birth. By the third trimester, fetuses have been shown to respond to patterns of light shone through the uterine wall, suggesting that rudimentary visual processing is underway even in the dark environment of the womb.
The Placenta’s Role During Fetal Growth
The placenta is the fetus’s lifeline for the entire stage, handling oxygen delivery, nutrient transfer, and waste removal. It functions as both a conduit and an active organ: it delivers oxygen to the fetal bloodstream while simultaneously consuming a significant share of that oxygen to fuel its own work, including hormone production and nutrient transport. In very early pregnancy, before the placenta is fully established, the developing embryo relies on secretions from the uterine lining and maternal plasma. Once the placenta matures, it takes over entirely, and fetal growth depends on how efficiently it performs.
The Third Trimester: Final Preparation
From week 28 onward, the focus is weight gain and the final maturation of organs that take the longest to finish: the lungs, brain, and nervous system. By week 32, the fetus’s skin is no longer translucent, and most organs other than the lungs and brain are well formed and functionally ready. During weeks 33 through 36, the lungs approach full maturity, and the fetus continues adding body fat that will help regulate temperature after birth.
By full term, a healthy fetus swallows roughly 400 milliliters of amniotic fluid per day. This constant swallowing and the practice breathing movements that accompany it serve as rehearsal for the coordinated sucking, swallowing, and breathing the newborn will need to feed.
When Survival Outside the Womb Becomes Possible
Viability, the point at which a fetus can potentially survive outside the uterus, falls within what clinicians call the periviable period: weeks 20 through 25. The odds shift dramatically week by week. Births before 23 weeks carry only a 5 to 6 percent survival rate, and nearly all survivors experience serious complications. At 23 weeks, survival ranges from 23 to 27 percent. By 24 weeks, it rises to 42 to 59 percent, and at 25 weeks, 67 to 76 percent of neonates survive to hospital discharge. The lungs are the primary bottleneck. They are one of the last organs to mature, and without sufficient development, a premature infant cannot exchange oxygen on its own.
How the Fetal Stage Is Monitored
The most detailed routine check during the fetal stage is the anatomy scan, an ultrasound typically performed between weeks 18 and 22. A sonographer measures and photographs the heart, brain, spine, kidneys, limbs, hands, feet, and facial structures. They also record the fetal heart rate, check blood flow through the umbilical cord, assess the position of the placenta, and measure the volume of amniotic fluid.
This scan can detect a range of structural conditions, including congenital heart abnormalities, spina bifida, cleft lip, and certain chromosomal conditions like Down syndrome, trisomy 18, and trisomy 13. For many parents, it’s the first time they get a detailed look at their developing baby and the first opportunity to learn whether organs are forming as expected. Additional ultrasounds or monitoring may follow if the anatomy scan raises any concerns, or in the third trimester as the due date approaches.