Do Twins Run In Your Family?

One of the many questions people like to ask when they learn that I am pregnant with twins is, "Do twins run in your family?"  The question is actually a bit trickier than most people realize.  Three considerations must be made:

First, only fraternal twins count when it comes to twins "running in families".  Why?  Because when we say something "runs in the family", we are suggesting that there is some genetic predisposition to display the trait in question, be it obesity, blue eyes or schizophrenia.  In the case of twins, what is inherited is actually a genetic predisposition towards "hyperovulation".  Typically, from puberty to menopause, women release one egg every month. If that one egg is fertilized, and that zygote (fertilized egg) successfully implants, a singleton pregnancy typically occurs.  If the single zygote splits before or shortly after implantation, identical twins are formed.  The chances of a zygote splitting into identical twins is equal between all women, and has nothing to do with genetics.  Fraternal twins, however, require that a woman release more than one egg during ovulation,
in which case, both eggs are likely to be fertilized (because where there's one sperm, there are generally 100 million more) and a fraternal twin pregnancy may occur.  The chances of a woman naturally releasing more than one egg during ovulation are related to genetics (ie. if a woman's mother was a hyperovulator, it's more likely that she will be too).  So identical twins do not run in families (or if they do, it is merely a coincidence having nothing to do with genetics and having no bearing on future generations likelihood of producing more twins) whereas fraternal twins may run in families in the form of an inherited tendency to hyperovulate.

The second consideration is that only the mother's side of the family counts.  Since fraternal twins are produced as a result of hyperovulation, the babies' father is not at all responsible for its occurrence, and it is irrelevant whether twins run in his family.  Contrary to what this t-shirt proclaims, every man capable of fertilizing one egg is equally capable of fertilizing two.  In fact, were he to find a woman who was ovulating a dozen eggs at once, any non-sterile man could just as easily father dodecatuplets.  The only implication that twins in the father's family has is on the potential for his daughters to produce twins, since he may be passing on the genes for hyperovulation from his side of the family to his children.

Finally, twins conceived using fertility drugs and/or IVF should not be counted when deciding whether twins "run in the family" since these treatments greatly increase the chance of conceiving multiples independent of genetics.  Many fertility drugs act to "boost ovulation" meaning that they help some women who fail to ovulate well or at all to ovulate.  Often, these drugs work so well that they induce hyperovulation and twins may be conceived.  IVF, of course, involves harvesting eggs from the mother, fertilizing them outside of her body, then placing the fertilized eggs into her uterus in the hopes that they will implant and result in a successful pregnancy.  Twins (or greater order multiples) are often produce from IVF if more than one embryo is transferred to the uterus and successfully implants.

So, in my case, there is one prior set of twins in my family, and they are fraternal and were conceived naturally.  My twins are also fraternal and conceived naturally.  So I can say that yes, twins "run in my family".  And my female cousins should probably watch out because they, too, may be hyperovulators.