A WOMB WITH A VIEW
"Jenny van Dongen at VU Amsterdam in the Netherlands and her colleagues have found that identical twins carry a characteristic pattern of alterations to their DNA, known as epigenetic changes, that isn't seen in people who didn't have twin embryos. These variations happen in early pregnancy and last into adulthood."
https://www.newscientist.com/article/2291665-dna-markers-reveal-if-you-shared-a-womb-with-twin-that-didnt-survive/