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Almost one third of twins start as just one egg, which then divides and leads to a pair of wonderful identical twins! But depending on how late the egg splits, can lead to very different situations for you and your babies.
In most cases the egg splits early, which leads to a standard and generally uncomplicated pregnancy, but if the egg decides to split later, it can lead to situations like mono di twins.
Monochorionic Diamniotic twins have their own amniotic sacs, but share a placenta.
Because they come from the same egg, they are always identical. Sharing a placenta can lead to complications in your pregnancy, but recent advances in medicine have limited the risks of mono di pregnancies, so now it’s only considered a medium risk, rather than the high risk it would have been even ten or fifteen years ago.
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Generally, learning that your twins are mono di is nothing to worry about. The mono di twin survival rate is around 97%, with only around 15% of mono di cases developing into a situation that requires close monitoring or hospital intervention.
This means it’s incredibly likely that your mono di pregnancy will progress exactly like any other twin pregnancy, with no complications or problems arising.
And even if problems should arise, scientific advances give your twins the best chance of survival, with a relatively simple and safe medical procedure that’s available now that can deal with the major problem that faces this sort of pregnancy.
Because they share a placenta, complications and problems occur with more frequency; the main ones being Twin to Twin Transfusion Syndrome, (or TTTS) and Twin Reversed Arterial Perfusion sequence, (TRAP.)
Twin to twin transfusion syndrome is caused when there’s an imbalance in the blood flow from the placenta to each of your twins. Put simply, one of your twins is getting more blood than the other one, and this means they are also getting more oxygen and nutrients.
This can lead to complications with both twins. The twin receiving the lions share of the blood will produce more urine, which leads to a larger bladder and potentially even heart failure.
Because they’re receiving more blood and producing more urine, this can cause an excess of amniotic fluid, which causes the womb to expand and can lead to premature labor and delivery, which could lead to the loss of one or both babies.
The twin receiving less blood will develop anemia through lack of red blood cells, as well as producing less amniotic fluid, which in severe cases can cause the amniotic sac to stick to the fetus, and the fetus to potentially stick to the inside of the womb.
Even if the twin receiving less blood dies, the surviving twin is still at risk, because the shared blood vessels can cause major complications in blood flow, oxygen and nutrient levels. Over time, this can lead to growth deficiencies and other problems.
TTTS is generally treated with one of two methods, with a third option being taken only in the worst circumstances.
The first is amnioreduction, in which excess amniotic fluid is drawn from the larger twin’s amniotic sac, which reduces the chances of premature birth, decreases the strain on the mother and normally equalises blood flow between your babies.
If surgery is needed, a procedure called laser ablation is used, in which doctors seal off some of the blood vessels in your placenta to even up the amount of blood going to both twins.
In the rarest and most severe cases, it’s possible for doctors to close off the blood supply to the twin receiving less blood completely, so that the twin receiving the most blood can survive. This option is only ever taken when there is no other choice.
Thankfully, all of these options have incredibly high success rates, and caught early enough, TTTS is a situation that can be managed and dealt with, no matter what complications might occur.
In 1% of cases, mono di twins can develop into a situation called twin reversed arterial perfusion sequence, or TRAP.
What this means is that one twin develops without a heart, and normally many other internal organs, and is connected to the healthy twin with a large blood vessel.
Because they have no heart structure, the twin with no heart (called the acardiac twin) receives all their blood from the first twin, or pump twin. This causes the pump twin’s heart to have to work harder, because it is supplying blood to two bodies.
Treated properly, the survival rate for the pump twin is very high, with around 85% going on to live normal, healthy lives. Sadly, the acardiac twin cannot survive with no heart, and will not survive after birth.
Depending on how your twins TRAP sequence progresses, it may require surgical intervention, in the form of radiofrequency ablation. This procedure seals off the blood vessels in the acardiac twin, stopping the pump twin’s heart from having to work so hard.
Thankfully, even a procedure as delicate as this has a fantastic success rate, with a survival rate as high as 9 out of 10 for even the worst case scenario.
Other than this, once you know you’re expecting them, it’s normal to face a barrage of scans and tests. This is just to make sure that both of your babies are developing normally, with none of the complications we’ve just talked about.
Happily, as long as your babies are both healthy and don’t develop any of the syndromes we’ve discussed, you can expect a completely normal pregnancy.
Are you expecting mono di twins? Any interesting stories or encouragement for new mothers? Let us know all about it in our comments section.
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