So if they're not identical, what would Tom and Jen's, and Tim and Jan's children look like?
Each of Tom and Jen's children would look different to the next — just like any brother or sister — and the same goes for Tim and Jan's children.
Sienna and Travis were born at Letterkenny University Hospital ahead of their due date, and now they share the exact same birthday as their twin sisters Samera and Tameka.
Four-year-olds Samera and Tameka were born on October 17th, 2013, and Travis and Sienna came into the world on October 17th 2017.
Since each pair of identical twins has identical DNA would both couple's children look the same? "[Two sets of] identical twins will not have identical offspring," says Professor Rodney Scott, head of medical genetics at the University of Newcastle.
It might sound like the beginning of a joke, or perhaps a romantic comedy starring the Olsen twins, but if a set of identical twins married another set of identical twins what would their children look like?
Moville couple Seamus and Elizabeth Doherty had their second set of twins this week on October 17th.What's more, some of the children might also be identical twins: this is because identical twin pairs like Tom and Jen, and Tim and Jan carry genes that mean they have a higher likelihood than non-twin couples of having children that are identical twins."It's not 100 per cent, but the probability that they will have identical twins is greater," says Scott."It's an inbuilt safety mechanism," explains Scott.So Tom and Jen's, and Tim and Jan's children wouldn't look the same, but their situation would still be a little strange: even though Tom and Jen's children would be cousins to Tim and Jan's children, all the children would be as closely related — or have the same genetic relationship — as brothers and sisters.“It’s like a wheelbarrow now, it’s all in front of us,” he added.But the happy unions weren't all down to incredible odds - one decade after they started dating Craig, 44, and Diane, 37, are returning to the Twin Day festival in Twinsburg, Ohio, where they met in 1998. "Those identical cells grow into identical twins," says Scott. The only way we get identical offspring is when a fertilised egg splits into two cells.These types of twins are called monozygotic twins, which literally means they come from the same cell: 'mono' meaning one, and 'zygotic' meaning a fertilised cell.Since monozygotic twins developed from the same cell, they have identical DNA.