Vanishing twin syndrome

Until around 1945, we did not know about Vanishing Twin Syndrome. But it goes without saying that although it was only formally recognised and named in the mid-forties, many women over the preceding generations must have been aware they were pregnant with twins but had miscarried one of them. Sometimes this became obvious when the placenta was examined after birth and the midwife or obstetrician told the mother about the other baby, which did not develop.

Where does it “vanish” to?

When Vanishing Twin Syndrome occurs, the remains of the foetal tissue are absorbed by the remaining twin, the mother or even the placenta. Now, with the common use of ultrasounds during pregnancy, embryos can be seen at their earliest, cellular level. This has meant that from almost the moment of implantation women can potentially become aware that they are pregnant.

Early confirmation of multiple pregnancy is relatively easy when 2 or more embryonic sacs, rather than just 1, are seen on the ultrasound screen. The earliest stage when foetal heartbeats can be visualised is at around 6 weeks of gestation. But when subsequent ultrasounds after the initial one are carried out and only one viable sac can be seen, or only one heartbeat heard, then the diagnosis of Vanishing Twin Syndrome is given.

Diagnosing a twin pregnancy is more common since the widespread use of fertility assistance and closer pregnancy monitoring. When checking and counting is done on the numbers of fertilised eggs implanted and, their subsequent growth so carefully monitored, it is easier to see when something has gone wrong.

Why does vanishing twin syndrome happen?

The word “vanishing” is rather a magical term for what is, essentially, a rather sad occurrence. Because the twin and pregnancy sac is reabsorbed by the mother, the word “vanishing” is used to describe this. If the miscarriage happens very early on in the pregnancy, there are no outward signs.

The truth is that we are not 100% sure why it happens, but many explanations have been put forward.

These include:

• Major genetic or chromosomal abnormalities, which mean that the embryo is simply not viable
• Improper cord implantation

How frequently does vanishing twin syndrome occur?

It is thought that conception with twins or multiples occurs much more often than we are aware of. Women over the age of 30 are more likely to experience Vanishing Twin Syndrome. A conservative estimate is that it happens in around 21-30% of multiple pregnancies.

What can I do to prevent vanishing twin syndrome from happening?

There is nothing that can be done to prevent this from occurring. The major cause is thought to be an interruption to the embryo’s developing genetic structure. This is completely out of the control of the mother or her partner.

In cases of repeated miscarriage for no obvious reason, genetic counselling may become necessary. Speak with your healthcare practitioner if this is an option you wish to investigate.

What are the symptoms of vanishing twin syndrome?

If it occurs in the first trimester of pregnancy, there are often no symptoms at all. The mother continues to feel pregnant because she still has sufficiently high levels of hCG, the pregnancy hormone, to sustain the viable embryo.

However, if she has conceived through fertility treatment and is having regular blood tests, there is likely to be a reduction noted in her circulating pregnancy hormones. This may be the earliest indication that a mother was pregnant with twins or triplets, but that one of them has miscarried.

Unlike a miscarriage though, the pregnant mother may not have any vaginal bleeding or uterine cramping, though some still may. This really depends on the stage of pregnancy when the miscarriage occurs.

It’s easy for mothers to interpret vaginal bleeding as a complete miscarriage and an indication that they are no longer pregnant. But if twins or triplets have been conceived and only one of them miscarried, then the other twin/ triplet is likely to mature in the usual way.

Unless a mother has been told she is pregnant with twins or multiples, she may not even be aware she has miscarried with one of them. If, however, one of the embryos dies in the second or third trimester there can be some increased risks with the remaining one. Premature birth can become a higher risk as can the risk of cerebral palsy.

If the miscarriage occurs before 8 weeks of gestation, the water and fluids from the miscarried embryo are reabsorbed, and there is nothing to be seen. But if the miscarriage happens after this time, the tiny, compressed foetus can sometimes be seen at the birth of the other twin as they are both expelled from the womb. This may be the first indication that twins were initially conceived though only one matured to delivery.

Sometimes a second or third trimester miscarriage of one twin can cause significant obstetric problems. Premature labour, infection, bleeding and an increased risk of birth complications can all occur with a late stage miscarriage of one twin.

What about the remaining twin?

After the death of one twin, the other, viable one is usually not impacted. Though this depends on the reason why the miscarriage occurred in the first place. If it was because of the individual embryo’s chromosomal or genetic make-up, then this will not necessarily impact on the other twin – particularly if they were dizygotic or fraternal twins.

In the majority of cases, the surviving twin is not affected by the miscarriage of the other. They continue to grow and mature to full gestation without any complications. However, recent studies have found that around 7% of women that have experienced Vanishing Twin Syndrome will deliver the other baby or babies prior to 28 weeks of gestation. There is also an increased risk of the surviving baby or babies having low birth weight and the health risks associated with this. There is also potentially a greater risk of the surviving twin/ triplet having health issues later in life.

If your healthcare practitioner is aware of your Vanishing Twin Syndrome they will monitor your pregnancy extra carefully. If you are an Rh Negative blood group your blood will be tested for antibodies. You may need to have an injection of Anti-D after your baby is born to protect any future babies you may carry.

What is the treatment for vanishing twin syndrome?

There is no specific treatment. If the other twin continues to mature and develop towards maturity, there is no management for the twin that has died. If it becomes clear that both twins are no longer viable and no heartbeats can be seen, then a dilatation and curette of your womb may become necessary.