Over the past generation, the number of twin births has increased in the United States, so that compared to the historical baseline there are now over a million extra twins. Since the 1980s, the trends towards delayed childbirth and increased utilization of fertility assistance have raised the twin birth rate from about 2% to over 3% of babies born.
The majority of twins are fraternal, occurring when the mother releases two separate eggs that are both fertilized. The birth rate of identical twins, which occurs when a single fertilized egg spontaneously splits into two eggs, has not changed and is only 1 in 285 pregnancies. In other words, it used to be that half of twins were identical, but nowadays only 1 in 3 sets of twins are identical.
Genetically, fraternal twins are the same as any other siblings, they just happen to be born at the same time. Both fraternal twins and siblings have a 25% of having matching antigen types. That is a 1 in 4 chance of being a perfect match for a cord blood transplant or any other stem cell therapy that requires HLA matching.
The fact that twins share the same womb means that they tend to be smaller babies. Since the 1960s, it has been established that the average birth weight of full-term babies in the United States is about 8 pounds (3.6 kg) and babies smaller than 5.5 pounds (2.5 kg) are classified as low birth weight. However, within the past decade research has changed medical ideas about what is normal for a twin. Singleton babies are expected to spend a full 39-40 weeks in gestation, but studies show that in order to avoid growth restrictions it is best for twins to be born at 37 weeks. The average birth weight of twins born at 37 weeks or later is 5.5 pounds.
Nonetheless, the fact that twins are smaller babies does mean that they have smaller placentas and less umbilical cord blood. Parents of twins are often concerned about cord blood banking: On the one hand they worry that their babies may experience the learning delays that are often associated with early birth and low birth weight, and which can now be treated with cord blood therapy. On the other hand, they worry that the collected cord blood will not be enough to be useful for therapy.
The bank Cryo-Cell has conducted the only study of cord blood collections from multiple births in the setting of family cord blood banking. Since 3 out of every 100 births are twins, for their study they pulled data from their last 300 multiple births and their last 10,000 singleton births. Not surprisingly, the median volume of cord blood collected was 10% smaller in multiple births. The good news was that the cord blood units from multiple births had nearly 4 times more Total Nucleated Cells (TNC) cells and nearly 8 times more CD34+ cells than the minimums required for storage.
The reassuring conclusion of the Cryo-Cell study is that cord blood units collected from multiple births have sufficient numbers of stem cells to treat children for cerebral palsy and other conditions identified in infancy, under current dosing guidelines.