Twins for Kate? The princess's hyperemesis gravidarum (a condition worthy of further research) has led to speculation that she is having twins. It is true that a multiple pregnancy is associated with a 7.5% increased chance of HG. My own anecdotal evidence is a bit different -- I had significant nausea and vomiting with both of my pregnancies, and it was actually worse with my singleton the second time around than my first twin pregnancy. But, people are speculating about where the royal baby was conceived, what the princess should eat, the name of the royal baby, the maternity styles we will see, so it is not surprising that our endless appetite for royal gossip on both sides of the pond has fueled twin talk.
With this speculation about twins, though, comes the misconceptions about twin zygosity and twin inheritance that have persisted (she could have twins because William's uncle Charles has identical twins!), even among skilled medical professionals, even as twins have become much more common, increasing 76% over the past 30 years to 3.3% of births in the U.S.
In 2004, when I was pregnant with twins, after the dropping of the jaws and the widening of the eyes and the "What are you talking about, I am not having twins!" part was out of the way, doctors told me I had two sacs and two placentas, so the twins were fraternal. Those doctors are not alone -- in a 2004 study, 81% of doctors said they thought two placentas meant fraternal twins. There were no twins that I knew of in my family, but my husband has some twins in his. My grandmother asked me why I took fertility drugs while I was in law school (I didn't). Other family members said for sure it must be my husband's family's influence.
At the time, I did not think about it much -- I was taking the bar, and then I was a first-year associate at a law firm pregnant with twins, so I didn't have time for many thoughts extraneous to immediate tasks at hand (topping the list: eating when I finally was able to and billing hours). And then, when the twin girls were born over 10 weeks early and were in the Neonatal Intensive Care Unit for two frightening months, questions of their zygosity were put on the back burner as we attended to the more important issues of digesting and breathing.
But at some point, as many moms of same sex (even different sex) twins can relate, after the fifth question that day about whether your twins are identical, you really want to know whether your kids share the same DNA. About a month after they came home and their health stabilized, as I was pumping breast milk for them, I started to do some research about identical and fraternal twins. My babies looked the same and had the same blood type -- could they be identical? Why did I have twins, anyway? Because of the babies' paternal twin great aunts? If I had another baby would I have an increased chance of having twins again?
One thing is clear. Two placentas did not mean that my same-sex twins were fraternal. Researchers say that 25-30% of identical twins have two placentas -- meaning, in layman's terms, the fertilized egg split early. According to the New York Times, a British study conducted earlier this year found that about 15% of twin parents studied were misinformed about the zygosity of their twins (i.e. whether their twins were identical or fraternal).
What about "twin genes"? Fraternal twins who are conceived without artificial intervention are often the result of hyperovulation, a genetic trait the mother inherits that causes her to ovulate multiple eggs. (Sorry, dads, you don't ovulate.) Researchers also think fraternal twins are more common in older women.
Why a fertilized egg splits -- resulting in identical twins -- remains a mystery. There does not seem to be a genetic component. The general twin rate has skyrocketed, due mainly to fertility treatments which increase the chance of fraternal twins, but the rate of identical twins has remained relatively constant at about 3 in 1000, half as common as fraternal twins.
It looks like Prince William's family history has no bearing on whether his wife has twins. And, contrary to popular belief, it is impossible to tell if same-sex twins are identical without a DNA test. When my twins were 4 months old, I had them tested through a genetic testing service. I swabbed their cheeks with a cotton swab and sent the samples back to the lab. The lab said they were monozygotic. My egg had split, no thanks to my or my husband's family history. (My grandmother insists I must have at least taken too many vitamins.)
At 8 years old, my twins look a bit different -- one is taller; one wears glasses with a mild prescription. If I had to guess now, I would have thought they were fraternal. Turns out that genes are complicated things -- they may express themselves differently or mutate.
I did not want to guess, though. I wanted to know my twins' zygosity as best I could, not just because I wanted to answer the constant questioners, but because I wanted to know myself. And perhaps this knowledge will help them if they encounter any health issues in the future as genetic research advances. The fact that I had only one egg also eased my mind a bit when I decided to go for a second pregnancy, which, thankfully, was not twins.
That is not to say having twins is not an incredible and singular experience -- it is. And if the princess does have twins, the twin mom club will welcome her. Can't wait to see the designer double stroller.