Cord blood is the blood that remains in the umbilical cord and placenta following birth. It contains the infant’s red blood cells, which carry the unique ABO and Rh factors. Therefore, cord blood is definitively the baby’s blood type. Analyzing this sample immediately after delivery provides an accurate reading of the newborn’s specific blood group, which is established through genetic inheritance long before birth.
Genetic Determination of Infant Blood Type
The infant’s blood type is determined by genes inherited from both parents, independent of the mother’s circulation. Blood type is classified by the ABO system and the Rh factor, controlled by separate genes.
The ABO system involves three possible alleles: A, B, and O. A and B are co-dominant, while O is recessive. An infant inherits one ABO allele from each parent, resulting in one of the four main blood types: A, B, AB, or O.
The Rh factor is determined separately. The Rh-positive allele is dominant over the Rh-negative allele. A baby is Rh-positive if they inherit at least one Rh-positive gene. An infant is only Rh-negative if they inherit the Rh-negative gene from both parents. This genetic makeup is fixed at conception and will not change based on the mother’s blood type during pregnancy.
Separating Infant and Maternal Circulations
A common misunderstanding is that the baby’s blood type must match the mother’s because they share a circulatory system. The placenta acts as a sophisticated barrier that prevents the mixing of the mother’s and baby’s red blood cells. Fetal and maternal blood flows come into close proximity within the placenta’s intervillous space, separated only by a thin placental membrane.
This placental barrier is selectively permeable. It allows for the exchange of nutrients, oxygen, and waste products but prevents the passage of large blood cells. This physical separation ensures that the fetus’s distinct blood type, determined by its own genes, remains separate from the mother’s blood. In rare instances, small amounts of fetal blood cells can enter the maternal circulation, typically during delivery, which is a mechanism related to Rh incompatibility.
Why Testing Cord Blood Type Matters
Testing the blood from the umbilical cord immediately after birth provides medically actionable information about the newborn’s health status. The primary reason for testing is to assess the risk of hemolytic disease of the newborn (HDN). If the mother is Rh-negative and the cord blood test reveals the baby is Rh-positive, it confirms a potential for Rh incompatibility, which can lead to complications in future pregnancies.
The cord blood sample is also analyzed for signs of ABO incompatibility, which occurs when the mother is type O and the baby is type A or B. This incompatibility increases the risk of neonatal jaundice, caused by a buildup of bilirubin from the breakdown of red blood cells. Knowing the baby’s blood type helps medical staff monitor the infant for this condition and intervene quickly if bilirubin levels become too high. Cord blood is also a source of hematopoietic stem cells, and the blood type is recorded if parents choose to bank the cord blood for potential future medical treatments.