Genetics, to no one's surprise, play a huge role in pregnancy. From the sex to eye and hair color, genes are the essential building blocks that piece together your perfect bundle. The biggest factor that determines whether or not you'll have twins is, of course, how many eggs are fertilized. If there are two, you'll have fraternal twins, and if there's only one, they'll be identical. The thing is, twins appear to run in families. Coincidence, or does biology have a say? Are twins hereditary, or do they occur purely by chance, repeatedly, and in the same families? Turns out, it depends. Yay, science!
While fraternal twins do seem to run in families, identical twins do not. In a study published in American Journal of Human Genetics, researchers studied genetic variations — the kinds that would infer which mothers are likely to have twins in the first place — to better predict when and why twins may be conceived. While identical twins are genetically the same (because they've split from one egg, fertilized by one sperm), fraternal twins are created in two separate eggs, with two separate sperm, and, therefore, are more likely to occur. So regardless of a woman's chances at conceiving twins, one thing is for certain: your chances of bringing fraternal twins into the world are higher than your chances at conceiving identical twins.
Science Magazine, in conjunction with The American Association for the Advancement of Science, says that in a major data-scan of over 2,000 mothers, scientists from eight countries have identified the very genes that increase a woman's chance of having twins: one has to do with hormones, and the other is about how the ovaries react to those hormones. Stanford at The Tech Museum of Innovation cites if you add to this a family history of twins — especially fraternal — a woman becomes "2.5 times more likely" to have twins herself.
The major catch to all of this is how much of a role the mother's genes play. This means, because the woman's body releases the eggs, if there are two (or one egg that splits), it would be her daughter that has an increased chance of having multiples — not her son. The genes can be passed to grandchildren, though. Sound complicated? It is!
With multiples being so risky in general, scientists are busy working on additional studies that further help identify who's more likely to have them. Though, as an article on NPR points out, women who undergo fertility treatment are more likely to conceive twins, due to the hormones that stimulate the ovaries, causing multiple eggs to be released at one time.
When it comes down to it, fraternal twins are more likely to be hereditary than identical twins. However, neither can be predicted 100 percent as, again, every woman's reproductive system is in charge of dropping those eggs at any rate it wants.
The human body is pretty damn fascinating, right?!