Identical twins form from the same egg and get the same genetic material from their parents — but that doesn’t mean they’re genetically identical by the time they’re born.

That’s because so-called identical twins pick up genetic mutations in the womb, as their cells weave new strands of DNA and then split into more and more cells. On average, pairs of twins have genomes that differ by an average of 5.2 mutations that occur early in development, according to a new study.