Stuck Twin

July 15, 2011

The stuck twin is part of the twin-twin transfusion syndrome. The easiest-to-understand explanation is that within the single placenta of a monochorionic twin pair, there are arteriorvenous communications between the circulation’s of the twins.

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The stuck twin is part of the twin-twin transfusion syndrome. The easiest-to-understand explanation is that within the single placenta of a monochorionic twin pair, there are arteriorvenous communications between the circulation’s of the twins. One twin (the donor) shunts blood to the other (recipient) twin. The donor does not grow well, it looses its amniotic fluid, thus cannot swallow, cannot urinate, and eventually it is shrink-wrapped in its amnion.

The recipient twin becomes plethoric in a polyhydramniotic sac, bounces around a great deal, swallows and urinates up a storm, and may develop cardiovascular disease.

This explanation is too simplistic. Certainly this problem occurs almost exclusively in monochorionic situations, but there are various theories as to the vascular mechanism. Also be aware that there are other causes of stuck twin and discordant growth: aneuploidy, infection, and structural defects.

The first and most important role of the sonographer is to make the diagnosis of monochorionic twins. This can best be done before 10 or 11 menstrual weeks, when you will clearly see one gestational sac and two amnions if the pregnancy is monochorionic and diamniotic. If it is dichorionic, you will clearly see two separate gestational sacs in the uterus.

Later in pregnancy the diagnosis of chorionicity becomes a little problematic. The most important thing is to be aware that it is your job (with twins) to make some comment about chorionicity. I have seen ultrasound reports that do not mention the subject. It is of vital importance, because monochorionic twins have a much higher rate of complications (including TTTS, but there are others) than dichorionic twins.

See your standard references if necessary to review how to make your best guess as to chorionicity later in pregnancy. If you diagnose boy and girl, it is not monochorionic, but if the twins are the same sex they are not necessarily monochorionic. The intertwin membrane, twin peak sign, status of the placentas all help in making your best guess. But before 11 weeks you can usually be certain.

TTTS is a very serious problem, occurs in about 32% of monochorionic pregnancies, and if untreated has as much as an 80% fetal mortality rate.

The most used treatment is serial amniocentesis of the recipient’s sac.

This case was a 26 year old LPN employee of our Clinic. She had one previous C Section, and that baby had renal UPJ stenosis, but did well.

The diagnosis of monochorionicity was made at the first transvaginal scan at 10 weeks 1 day menstrual age (it helps to work in a clinic). The next scan was done at about 20 weeks. We noticed that there was lots of fluid, which made for great imaging. We noticed that one baby, the one with the big stomach, full bladder that never seemed to empty (it takes us about two hours to scan twins), the thick cord, and the bilateral pyelectasis, was jumping all over the place. And the other twin was barely moving, and we could not find the stomach and the bladder. Suddenly a light went on! From then on our scans were trying to prove our idea that this was a stuck twin.

We never did find any amnion or intertwin membrane. The babies were just about the same size. And they both had upper limits of normal cerebral atria.

The mom was sent to an Army teaching hospital in the Northwest, and the only follow-up I received was when she telephoned her friends here at the Clinic. She stayed in the hospital for the rest of the pregnancy, did have serial decompression amniocenteses, the stuck twin became unstuck, and she delivered by C-Section at 33 weeks. Both babies weighed about 3.5 pounds. One was on a ventilator, and the other had too many platelets, had some blood removed, and saline infused. Both had normal karyotype.

I subsequently heard that one of the babies had to be cared for in an institution, and died about age 10 months. I have not heard about the other baby, and I do not know for sure if the "bad" baby was the donor or the recipient.

It is a difficult task to demonstrate a stuck twin on a still image. If you have the opportunity, try to do it. Try to show amnion stretching from promontory to promontory, such as knee to elbow. I was not able to find any sign of an amnion, anywhere.

Two references that I think are good (and it is a good idea to review the entire subject of twins) are: "Ultrasound and Multifetal Pregnancy", Edited by A Monteagudo and I. E. Timor-Tritsch, published by Parthenon 1998, part of Parthenon’s "Progress in Obstetric and Gynecologic Sonography Series".

Callen’s "Ultrasound in Obstetrics and Gynecology", Third Edition, Chapter Eight, "Ultrasound Evaluation in Multiple Gestation" by Harris J. Finberg.

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