In newborns, blood cells are produced almost entirely in the bone marrow, which becomes the dominant site of blood cell production (hematopoiesis) well before birth. By around 10 to 12 weeks of gestation, blood-forming stem cells have already migrated to the bone marrow, and by the time a baby is born, the marrow has taken over as the primary factory for red blood cells, white blood cells, and platelets.
Why Bone Marrow Takes Over Before Birth
During early fetal development, blood cells are produced in several places outside the bone marrow. The yolk sac handles the job first, followed by the liver and spleen. These organs serve as temporary production sites while the skeleton is still forming. Once fetal bones develop enough to house marrow, blood-forming stem cells migrate there and set up permanent residence. This migration happens around 10 to 12 weeks of pregnancy in humans.
By the time a full-term baby is born, the liver and spleen have largely stopped making blood cells. This outside-the-marrow production, called extramedullary hematopoiesis, normally ceases before birth. If it shows up in a newborn, it typically signals that something is wrong with the bone marrow’s ability to keep up with demand.
An Infant’s Entire Skeleton Makes Blood
One major difference between infants and adults is how much of the skeleton is involved. At birth, virtually the entire skeleton is filled with active red bone marrow. Every bone, from the skull to the tiniest bones in the fingers and toes, is churning out blood cells. In adults, active marrow retreats to just a handful of locations: the pelvis, spine, ribs, sternum, and the ends of the large limb bones.
The distribution isn’t perfectly even, though. In newborns, about 28% of all active marrow sits in the skull, and roughly 21% is found in the lower limbs. This is strikingly different from adults, where the skull and limbs contribute far less. Over the course of childhood and into young adulthood, red marrow gradually converts to yellow marrow, which is mostly fat and doesn’t produce blood cells. This conversion starts at the tips of the limb bones and works its way inward toward the trunk, following a predictable pattern that isn’t fully complete until around age 25.
The Transition From Fetal to Adult Blood Cells
Even though the bone marrow is already the main production site at birth, the blood cells themselves are still in transition. A newborn’s blood contains a mixture of fetal-type and adult-type cells, because both fetal and adult stem cells coexist in the marrow during the first weeks of life. Fetal blood cells gradually disappear as the infant matures. In animal models, fetal-type blood production is almost completely replaced by adult-type production within three to four weeks after birth. During this same window, blood-forming stem cells shift from actively dividing to a quieter, more stable state that characterizes adult marrow.
This transition period is one reason newborns have some unique blood characteristics. For instance, newborn red blood cells have a shorter lifespan than adult red blood cells. In healthy adults, a red blood cell survives about 120 days. Neonatal red blood cells last only about 54 days on average. This shorter lifespan means an infant’s bone marrow has to work harder, proportionally, to maintain an adequate red blood cell count during those early weeks.
How the Hormone Signal Shifts After Birth
Blood cell production doesn’t happen on its own. It’s driven by a hormone called erythropoietin, which signals the bone marrow to ramp up red blood cell output when oxygen levels drop. In a fetus, the liver produces most of this hormone. After birth, the kidneys gradually take over that role.
This liver-to-kidney switch begins during the final third of pregnancy and continues after delivery, finishing around 40 days after birth. The handoff is gradual: as the kidneys gain the ability to produce erythropoietin, the liver’s output slowly decreases rather than shutting off abruptly. Thyroid function and changes in the baby’s oxygen supply both influence the timing of this transition.
When Blood Production Occurs Outside the Marrow
In a healthy newborn, the bone marrow handles blood production on its own. But certain conditions can push the body to reactivate blood cell production in the liver, spleen, or other organs. This is the body’s backup plan when the marrow can’t meet demand. Severe anemia, chronic infections, and inherited blood disorders like thalassemia or sickle cell disease are common triggers. The underlying issue is always the same: bone marrow dysfunction or overwhelming demand forces the body to recruit other tissues for help.
If extramedullary hematopoiesis is detected in an infant, it’s a sign that warrants investigation rather than something considered part of normal development. In a fetus, it’s expected. In a newborn, it points to stress on the blood-forming system.