A major limitation of cord blood transplantation is that the blood obtained from a single umbilical cord does not contain as many haematopoeitic stem cells as a bone marrow donation. Scientists believe this is the main reason that treating adult patients with cord blood is so difficult: adults are larger and need more HSCs than children. A transplant containing too few HSCs may fail or could lead to slow formation of new blood in the body in the early days after transplantation. This serious complication has been partially overcome by transplanting blood from two umbilical cords into larger children and adults. Results of clinical trials into double cord blood transplants (in place of bone marrow transplants) have shown the technique to be very successful. Some researchers have also tried to increase the total number of HSCs obtained from each umbilical cord by collecting additional blood from the placenta.

Much research is focused on trying to increase the number of HSCs that can be obtained from one cord blood sample by growing and multiplying the cells in the laboratory. This is known as “ex vivo expansion”. Several preliminary clinical trials using this technique are underway. The results so far are mixed: some results suggest that ex vivo expansion reduces the time taken for new blood cells to appear in the body after transplantation; however, adult patients still appear to need blood from two umbilical cords. More research is needed to understand whether there is a real benefit for patients, and this approach has yet to be approved for routine clinical use.