Saturday, March 13, 2010

The Zygocity Question

"They're so cute! Are they Identical?" is a question that we - and all twin parents - get all the time. I find it incredibly interesting that even parents of obviously fraternal boy/girl twins get this question too. But to be completely honest, I was guilty of asking this question of twin parents before Eli and Jude entered my life and still am now. It's typically the first question that flies out of my mouth before I can even reign it in when I meet a new set of twins... unless they look significantly different, that is. What can I say, it's interesting! The sheer fact that a single fertilized embryo potentially split into two separate, but genetically identical people is simply fascinating to me. So, naturally the first question I asked my ultrasound technician (after the shocked "Are you sure?" and "Can you check again?") was "Are they Identical?" Little did I know how complicated this question really was.

The ultrasound tech told me that day that she was almost 100% sure they were fraternal, or dizygotic. She showed me the membrane between the two babies and the two placentas and explained that it was highly unlikely that they were identical, or monozygotic, given those circumstances. I didn't know very much about twinning at the time so I just nodded and assumed she was right.

Fast forward to after Eli and Jude were born, while we were still in the hospital. I started getting the identical question twice a day in the NICU - once for each nurse assigned to their case. One nurse said to me, "I see a lot of babies and they all generally look alike, but yours look A LOT alike. Are you sure they weren't di-di twins?" I didn't have any idea what she meant but I explained what the ultrasound tech had told me and she nodded but said that the chances of identical twins being in separate amniotic sacs with separate placentas is not as rare as one might think. I was surprised and did some research. It turns out that about 25% of identical twins are dichorionic-diamniotic but most are diagnosed as fraternal. The only way to be sure of fraternal twins is if you have one of each gender. More suspicious facts were that the boys were only 2 oz apart in weight and half an inch different in length at birth - which the doctors say is normal, but seems to be quite a coincidence to me - and have only grown more similar in looks as they've gotten older. They also have hit all of their developmental milestones within weeks, sometimes days, of each other. This is less common, but also still normal for fraternal twins. Needless to say, Travis and I have been suspicious of their original dizygotic diagnosis for a while. And it makes answering strangers' questions awkward because you never know what to say or how much information to share. Everyone seems to have their own opinion, even passers-by who only take a quick glance at the boys' faces!

Finally, all of our questions are going to be answered. Our home genetic zygocity test kit arrived today. It's hard for me to imagine that in a little more than a week we'll know the answer to the ever-present zygocity question! So, I put up a final poll to see what you all think. Are Eli and Jude fraternal, identical, or somewhere in the middle? I'll post the answer when we find out.


  1. Boy, that's a tough one b/c they DO look very similar. And it's hard to tell much from pictures since I haven't seen them in person. But I'm going to go with fraternal, but that's just a guess! Good luck!

  2. Grandma Lolli say definitely identical! :) Can't wait to hear the results.