Twin to Twin Transfusion Syndrome

What is twin to twin transfusion syndrome and how can it place your babies at risk?

Twin pregnancies can occur as either dizygotic (fraternal twins) or monozygotic (identical twins).  In the case of a dizygotic (fraternal) pregnancy the twins are the result of fertilization of two separate eggs.  In the case of monozygotic (identical) pregnancy the twins are the result of fertilization of one egg that splits at some point after fertilization.

In either case, each fetus may be surrounded by two sacs during pregnancy, an inner amniotic sac and an outer chorionic sac.  All dizygotic (fraternal) twins are di-chorionic and di-amniotic, meaning that there are two separate chorionic sacs and two amniotic sacs present.  Di-chorionic twins will also have separate placentas for each twin.

Potential health risks to identical twins

Monozygotic (identical) twins can have two chorionic sacs (referred to as di-chorionic) and two amniotic sacs (referred to as di-amniotic), or one of each, depending on the timing of when the embryo splits.  If there is one chorionic sac, it is described as mono-chorionic.  Of significance in mono-chorionic cases, the twins will share a single placenta.

When identifying twins, it becomes critically important to determine whether they are di-chorionic or mono-chorionic so as to determine the number of placenta.  Twins that share a placenta (mono-chorionic) are at an increased risk of developing a complication of the placenta known as twin to twin transfusion syndrome or TTTS.

In such a situation where there is a sharing of a single placenta, depending on the development of the fetus, the blood supplies of mono-chorionic (identical) twins can become connected so that the twins share a circulation.  Twin to twin transfusion syndrome occurs when, depending on the number, type and direction of the inter-commuting blood vessels (anastomoses), there can be an unbalanced flow of blood between the fetuses. That is to say, one twin gains some blood while the other loses blood (which gives reference to the name).

Donor and Recipient Twins

In some cases this may result in only some mild difference in development, but in more severe cases, where the condition progresses, it may result in severe harm to one or both of the fetuses.  In such severe cases, the twin that is losing blood to its sibling (described as the donor twin) will not grow as well and will typically have less amniotic fluid, and the twin receiving the blood (described as the recipient twin) has the potential for increased growth and increased amniotic fluid as its urine output increases as a result of the additional blood flow.

Five Stages

If left untreated, TTTS may develop through five stages.  As it progresses and becomes more severe, significant harm can result to the fetuses in terms of serious developmental issues, cardiac failure, and ultimately the demise of one or both twins.

Potential treatment

Various treatments for this condition have existed over time but the current most accepted treatment is selective fetoscopic laser photocoagulation (SFLP) therapy of the communicating placental blood vessels.  This procedure involves the use of laser ablation therapy to interrupt the blood vessels that allow exchange of blood between the fetuses.  The chance of demise or other serious adverse outcome to the fetuses can be greatly reduced through such therapy, provided that it is undertaken at an appropriate and timely stage.

Proper diagnosis critical

For these reasons, the proper identification of mono-chorionic (identical) pregnancies, which is usually to be done through the use of ultrasound at 11-13 weeks, is critical to the identification and ultimate treatment of TTTS.  Follow up and careful monitoring of twin pregnancies that are identified as mono-chorionic (identical) is also critical for the proper and timely treatment of TTTS.

Therefore in some instances, a failure to have identified a mono-chorionic (identical) pregnancy and/or to properly monitor and follow up for the development of TTTS, may result in a preventable birth injury should harm result to one or both of the twins.