What are Semi-Identical Twins & Why are They so Rare?

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Us twin moms often have a laugh at people who ask if boy/girl twins are identical. Just to make it clear… if your kids are packing different junk, they are not identical. Another way to be sure is if your unborn babies are sharing a placenta. So what happens if you spot boy/girl twins sharing the same placenta at your ultrasound? What are semi-identical twins?

What are Semi-Identical Twins and Why Are They so Rare?

Every now and again a bizarre but awesome twin story hits the news. Twins that turn out to have different fathers; Twins who appear to be of a completely different race. The science of twins is fascinating, and the case of the semi-identical twins is no exception.

The Puzzle of Semi-Identical Twins

In 2014, an Australian first-time mom went for a routine appointment at 6wks of pregnancy. During her ultrasound, twins were discovered sharing a single placenta. The fact they shared a placenta would ordinarily confirm them as identical twins. But at a later anatomy scan, Mom was found to be carrying one boy and one girl… not so identical after all.

A mid-trimester amniocentesis was performed on each amniotic sac to investigate. A comparison of the Twins’ DNA showed chimerism in both twins. This means they were carrying an unusual combination of DNA. Normally this would occur when one twin absorbs the other in utero, but here we had two babies intact… so what happened?

The DNA test showed both twins carried the same DNA on the Mother’s side. But when it came to the Father, the DNA they shared wasn’t quite the same. It is a possibility simultaneous fertilization occurred, two sperm arrived at exactly the same time. Usually, this kind of heterogonesis would result in miscarriage but this little pair made it work!

There were of course concerns. Would the babies be born with genital ambiguity, gonadal dysgenesis or Turner Syndrome? The pregnancy continued as normal, until at 33wks fetal growth discordance (one twin growing quicker than the other) led to an early but healthy birth by C-Section. Hooray for healthy semi-identical twins!

identical twins why does the fertilized egg split title
Why does the fertlized egg split to make identical twins?

How Are the Semi-Identical Twins Now?

Although the report on this case was published in February of 2019, the semi-identical twins are now 4yrs old and generally healthy. The girl twin did have some complications… she suffered a blood clot which led to the amputation of her arm at 4wks old, and at 3yrs was confirmed with gonadal dysgenesis which led to the surgical removal of her ovaries to reduce the risk of cancer.

Still, both twins are reported to be developmentally normal and doing well!

How Rare is Sesquizygotic Twinning?

Sesquizygotic twins are extremely rare, but just how rare is yet to be determined. One in 1,000 births are identical twins but there have only ever been two recorded sets of semi-identical twins. The first case was reported in 2006; one boy twin and one hermaphrodite twin. Similarly to the second case, both shared the same DNA from their mother but a mix of DNA from their father.

“We know this is an exceptional case of intermediate twins, also known as a third type of twinning,”

Professor Fisk, former President of the International Fetal Medicine and Surgery Society.

The question I have is… how many undiagnosed cases are there? This type of DNA cocktail would normally not lead to a viable pregnancy, but that does not mean there are not many more cases of sesquizygotic twins out there. In both cases, it was the visible difference in genitalia that triggered further investigation. What if you’re carrying two semi-identical boys? Can that even happen? I will continue to dig for details because it truly is fascinating.

Semi-identical twins
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I recently carried out a ‘23 & Me’ Ancestry and Health test, and learned some hidden issues within my own DNA. As Twin Zygosity tests become more popular, I wonder if more cases of semi-identical twins will appear in the news? Gotta love the wonders of wacky twin science!

Sources / Further Reading

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About the author

Katherine is an electrochemist, hiking enthusiast, and family lifestyle blogger. As a mom of three, including twins, she enjoys DIY, travel, and eating good food. British born, Katherine moved to the US in 2014, and now called Las Vegas home.

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