Identical twins don't run in families, but fraternal twins can
Fraternal twins come from two separate eggs, and a gene that makes some women release extra eggs can pass down the generations. Identical twins arise when a single fertilized egg splits, which seems to happen at random, so every couple has the same chance of having them.
Twins are by far the most common kind of multiple birth. The US Centers for Disease Control and Prevention count more than 132,000 sets a year out of 3.9 million births, roughly 1 in 30, and twins make up 97% of all multiple births there. Fraternal twins share about 50% of their genes, like any brothers or sisters, while identical twins share 100%. A father cannot raise his partner's odds of fraternal twins, but he can pass the egg-releasing gene on to his daughters.
Each extra baby shortens the pregnancy. The average is 38 weeks for one baby, 36 for twins, 32 for triplets and 30 for quadruplets, and the risks linked to early birth grow with every sibling. It was only in the twentieth century that more than four babies from a single birth first survived infancy. Hellin's law, an old rule of thumb, puts natural twins at about 1 in 89 births and triplets at about 1 in 89 squared.
The extremes are headline-making. Natural quintuplets are estimated at 1 in 55,000,000 births, and the first known to survive infancy were Canada's identical Dionne sisters, born in 1934. Britain's Walton sextuplets, born in Liverpool in 1983, were the first surviving all-female set. The McCaughey septuplets of Iowa in 1997 and the Suleman octuplets of California in 2009 were firsts for their numbers, and in May 2021 Halima Cissé, a woman from Mali, gave birth to nine babies in Morocco, all still living two years later.
Multiple births have become more frequent since the late 1970s. In Canada, the number of multiple-birth babies rose 35% between 1979 and 1999, and by 2011 an estimated 36% of US twin births and 78% of triplet and larger births followed assisted reproduction. Women over 35 are also naturally more likely to conceive more than one baby, possibly because of higher levels of a hormone that stimulates the ovaries.
Source: Multiple birth
