Fetal anemia occurs when a developing baby doesn’t have enough red blood cells or functional hemoglobin to carry oxygen to its tissues. The most common cause is a blood type mismatch between the mother and baby, but infections, genetic conditions, bleeding across the placenta, and complications in twin pregnancies can all play a role.
Blood Type Incompatibility
The single most frequent cause of fetal anemia is a mismatch between the mother’s blood type and the baby’s, a process called alloimmunization. This happens when the mother’s immune system encounters fetal red blood cells carrying proteins (antigens) it doesn’t recognize. The most well-known version involves the Rh factor: if the mother is Rh-negative and the baby is Rh-positive, the mother’s body may produce antibodies against the baby’s blood cells.
What makes this dangerous is the type of antibody the mother produces. Her immune system generates a small antibody called IgG, specifically the IgG1 and IgG3 subtypes, which are compact enough to cross the placenta. Once on the fetal side, these antibodies attach to the baby’s red blood cells and mark them for destruction. The baby’s body tries to compensate by ramping up red blood cell production, but if the immune attack is aggressive enough, it can’t keep pace. The result is hemolytic anemia, where red blood cells are destroyed faster than they’re replaced.
Rh incompatibility is largely preventable today with a routine injection given to Rh-negative mothers during pregnancy and after delivery. But other, less common blood antigen mismatches (such as Kell, Duffy, or certain ABO combinations) can trigger the same process and aren’t always screened for as aggressively.
Parvovirus B19 Infection
Parvovirus B19, the virus that causes “fifth disease” or “slapped cheek” rash in children, is the most significant infectious cause of fetal anemia. In adults, it often causes only mild cold-like symptoms or joint pain, so a pregnant person may not realize they’ve been infected.
The virus has a very specific target: it homes in on the cells in the fetal bone marrow and liver that are developing into red blood cells. Once inside these early red blood cell precursors, the virus hijacks their machinery to replicate, effectively killing the cells before they can mature. This creates a temporary but sometimes severe shutdown of red blood cell production. Cells at a mid-stage of development (around 6 to 15 days into their maturation) are most vulnerable to productive infection, while very early and very late precursors are more resistant.
Because fetal red blood cells have a shorter lifespan than adult ones, and because the fetus is growing rapidly and needs an expanding blood supply, even a temporary halt in production can cause a steep drop in red blood cell counts. In severe cases, this leads to fluid buildup throughout the baby’s body, a condition called hydrops. The good news is that parvovirus anemia is usually self-limiting. If the baby can be supported through the crisis, recovery is typical.
Alpha Thalassemia Major
The most severe genetic cause of fetal anemia is a condition called hemoglobin Bart’s hydrops fetalis, the extreme form of alpha thalassemia. Hemoglobin, the oxygen-carrying molecule in red blood cells, is built from paired protein chains. The fetal version normally uses two alpha chains and two gamma chains. In alpha thalassemia major, all four copies of the alpha gene (two on each copy of chromosome 16) are deleted.
Without any alpha chains, the fetus can’t build normal hemoglobin. Instead, the leftover gamma chains cluster together in groups of four, forming an abnormal hemoglobin called Hb Bart’s. This molecule grips oxygen so tightly that it essentially refuses to release it to the baby’s tissues. Even though it can pick up oxygen, it’s functionally useless for delivery. Hb Bart’s accounts for over 80% of the hemoglobin in affected fetuses.
The resulting profound anemia and oxygen deprivation typically causes hydrops in the second or third trimester, along with risks of neurodevelopmental problems and birth defects. This condition is most common in families of Southeast Asian or southern Chinese descent, where deletions of both alpha genes on one chromosome are more frequent.
Fetomaternal Hemorrhage
Sometimes fetal blood leaks across the placenta into the mother’s circulation, a process called fetomaternal hemorrhage. Small amounts of this mixing are extremely common and harmless. In over 99% of cases, fewer than 15 milliliters cross over. But when the volume reaches 25 to 30 milliliters, roughly 20% of the baby’s total blood supply, the loss becomes dangerous enough to cause fetal anemia, fluid buildup, or even death.
Risk factors include placental abnormalities like placenta previa or placental abruption, maternal trauma, multiple pregnancies, and procedures like amniocentesis that can disrupt the placental barrier. Gestational diabetes and connective tissue conditions may also increase the risk by damaging the placental barrier. Despite these known associations, more than 80% of significant hemorrhages (above 30 milliliters) have no identifiable cause.
Fetomaternal hemorrhage can be acute or chronic. A sudden, large bleed is more immediately dangerous than a slow, ongoing leak, because the baby has no time to compensate. Chronic cases sometimes go undetected until routine monitoring picks up signs of anemia.
Twin-to-Twin Transfusion Syndrome
In about 10 to 15% of monochorionic twins (identical twins who share a single placenta), an imbalance develops in how blood flows between them. This is twin-to-twin transfusion syndrome, or TTTS. Because the two babies share a placenta with no barrier between their circulations, blood vessels in the placenta almost always connect them. Normally these connections balance out, but sometimes they don’t.
In TTTS, an artery from one twin (the “donor”) feeds into the placenta, but instead of the corresponding vein returning blood to that same baby, it routes the nutrient-rich blood to the other twin (the “recipient”). The donor progressively loses blood volume while the recipient gets overloaded. The donor twin becomes anemic, and its body responds by redirecting what little blood it has to the brain and heart at the expense of less critical organs like the kidneys. Over time, this can lead to kidney failure and dangerously low amniotic fluid around the donor twin.
A related condition called twin anemia polycythemia sequence (TAPS) involves a slower, more chronic transfer that creates a significant difference in hemoglobin levels between the twins without the dramatic fluid shifts seen in TTTS.
Fetal Tumors
Rarely, certain fetal tumors can cause anemia through indirect mechanisms. Sacrococcygeal teratomas, tumors that develop at the base of the baby’s tailbone, are the best-known example. These tumors often become extremely vascular, meaning they develop dense networks of blood vessels. The heavy blood flow through the tumor acts as a short circuit in the baby’s circulation, forcing the heart to pump harder to keep up.
Anemia in these cases can result from two pathways: bleeding within the tumor itself, or arteriovenous shunting, where blood bypasses the normal capillary network and cycles too quickly through the tumor without effectively delivering oxygen. Both scenarios can push the baby into high-output heart failure, where the heart is working overtime but can’t meet the body’s demands. Solid, highly vascular tumors carry the greatest risk. Predominantly cystic teratomas, by contrast, tend not to develop the blood vessel density that triggers this cascade.
How Fetal Anemia Is Detected
Fetal anemia can’t be diagnosed with a simple blood draw the way adult anemia can, because drawing blood from a fetus is invasive and carries risks. Instead, doctors use a specialized ultrasound technique that measures how fast blood flows through an artery in the baby’s brain, specifically the middle cerebral artery. When a baby is anemic, its blood becomes thinner and flows faster. A peak blood flow speed at or above 1.5 times the expected average for gestational age suggests moderate to severe anemia, with about 86% sensitivity and 71% specificity in babies who haven’t yet received treatment.
This measurement is less reliable after a baby has already received one or more blood transfusions in the womb, because transfused adult red blood cells behave differently than fetal ones, changing the blood’s flow characteristics.
Treatment With Intrauterine Transfusion
When fetal anemia is severe enough to threaten the baby’s health, the primary treatment is an intrauterine blood transfusion. Using ultrasound guidance, a needle is inserted through the mother’s abdomen into the umbilical cord or the baby’s abdominal cavity, and compatible red blood cells are transfused directly to the baby. Some babies need only one transfusion, while others require several over the course of the pregnancy.
The procedure carries a complication rate of roughly 1 to 5%, with temporary slowing of the baby’s heart rate being the most common concern. Transfusions performed before 34 weeks carry a higher complication rate than those done later in pregnancy. In one study comparing early and late transfusions, 20% of babies transfused before 34 weeks experienced a complication within one week, compared to none in the group transfused at 34 weeks or later. Perinatal death after the procedure is rare but possible, particularly in very early or complex cases.
For conditions like parvovirus infection, a single transfusion or short series may be enough to bridge the baby through the crisis until its own bone marrow recovers. For ongoing causes like alloimmunization, repeated transfusions may be needed every two to four weeks until the baby is mature enough for delivery.