The cellular machinery for separating chromosomes is unusually unstable in human eggs. This makes the eggs prone to having abnormal numbers of chromosomes, which can result in pregnancy loss and genetic disorders.

When cells divide to make eggs or sperm, chromosome pairs separate owing to spindle-shaped cellular machinery. Melina Schuh at the MRC Laboratory of Molecular Biology in Cambridge, UK, and her colleagues observed this process in more than 100 live egg cells from women undergoing fertility treatments. They found that the chromosome segregation period was unusually long, lasting about 16 hours. In many egg cells, the spindles were unstable, causing the chromosomes to lag behind during separation, and increasing the risk that they would not reach the correct side of the spindle before the cells divided.

Science 348, 1143–1147 (2015)