Pervasive Misunderstandings About the Nature of Identical Twins

When my husband and I learned we were having twins, our first question was, "Are they identical or fraternal?"  I used to think, like most people, that this was an easy question to answer via early ultrasound, that identical twins shared a containing membrane (ie. they were in the same sac) and fraternal twins did not.  So all you would need to do was decide whether the babies shared a sac or not to know if they were identical or fraternal.  But it's actually more complicated than that...

Just to review the basics, identical twins are formed when one egg is fertilized by one sperm, which then splits in two.  The twin embryos formed are consequently genetically identical to one another meaning that they will have the same sex and will look very similar.  The reason I say "very similar" instead of "identical" is that even identical twins have some small differences in appearance.  Any of us that have known a pair of identical twins know that their mothers, at least, can tell them apart by looking, as can, in most cases, their friends and family.  And their personalities and individual talents are often wildly variable.  How do we account for these
differences?  Well, while the two individuals may have been built from the same blueprints, subtle differences in their environment, small variations in the way those blueprints are followed, little "mistakes" in their formation make them different.  By the time identical twins are born, they have been growing separate from, if very near to, each other, for 9 months, and are already slightly different.

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,

An Introduction

My name is Amber.  I am 29 years old, 2 months into my 3rd year of medical school, 8 months married, and 20 weeks pregnant for the 1st time... with twins.  In my past life (that is my pre-marriage, pre-med school, pre-pregnancy life), I occasionally made posts on a blog entitled "Amber Opines".  The blog was largely a forum for me to respond as coherently as possible any time I felt that somebody was wrong on the internet.

My goal with this new blog is to narrow my focus to the two subjects which most preoccupy my thoughts at present: medicine and parenting.  I am a student of both, so I make no promise of special expertise.  But I do promise that any medical comments I make will be evidence-based and any opinions I express will be presented as such.  As for parenting, perhaps reading about my husband's and my misadventures will provide a few laughs.  Mostly I just want a forum to gush about my children to a willing audience.