I found this article on twinsuk.co.uk.  It gives a great overview of what I believe could well be a very pervasive issue:

In recent years. enhanced use of ultrasound early in pregnancy has increased the frequency of diagnosis of twin pregnancy. and unfortunately. has produced a heightened awareness of the phenomenon of Vanishing Twin Syndrome (VTS).

Here are the answers to some questions that about this condition.

What is Vanishing Twin Syndrome?

Vanishing Twin Syndrome occurs when one of a set of twin fetuses apparently disappears from the womb during pregnancy. usually resulting in a normal singleton pregnancy.

What really happens?

One of the fetuses in a twin pregnancy spontaneously aborts. usually during the first trimester; the fetal tissue is absorbed by the other twin. the placenta. or the mother. thus giving the appearance that the twin “vanished.”

How is it diagnosed?

Here’s a typical scenario: A mother undergoes a routine ultrasound early in her pregnancy. for example at six or seven weeks gestation. Two fetuses are detected. The mother is told she is having twins. When the mother returns to the doctor six weeks later. only one heartbeat can be heard with a Doppler scan (This is a scan designed to measure blood flow; it can be done to see if your placenta is working normally.) Another ultrasound is performed. Only one fetus is identified.

In other cases. a pregnant mother experiences symptoms that would seem to mimic miscarriage; however the single baby in her womb remains unaffected.

How often does it happen?

Scientists have confirmed that the number of twin conceptions greatly outnumbers the number of actual twin births. Some estimates offer that 1 in 8 people started life as a twin. while in reality only 1 in 70 actually are a twin. In “Having Twins.” author Elizabeth Noble claims that 80% of twin pregnancies result in the loss of one or both babies. Other studies predict that Vanishing Twin Syndrome occurs in 21 – 30% of all multiple pregnancies in the United States and a recent study in the UK estimated that 1 in 20 of us were once a pair of twins . It is estimated that Vanishing Twin Syndrome will play a role in 50% of assisted ovulation pregnancies. It is estimated that of the 133 million people born in the world in the year 2000. at least 7 million should have had a twin.

Why is it happening more frequently?

Although it would seem that incidences of Vanishing Twin Syndrome are increasing with alarming frequency. it is simply that the detection of the phenomenon has increased. Advancements in ultrasound technology allow modern doctors (and parents) the exciting opportunity to peek into the womb. As more doctors routinely use ultrasound in the first trimester. more multiple pregnancies are identified. And a certain percentage of those will be affected by Vanishing Twin Syndrome. In the past. many women experienced VTS without ever knowing it.

What causes it?

Just as there is no clear attributable cause for most miscarriages. there aren’t always reasons or explanations for the loss of a fetus in a multiple pregnancy. In some cases. the fetus is unviable due to chromosomal or placental abnormalities. Some studies suggest that because these abnormalities are more common in older women. Vanishing Twin Syndrome occurs more often in mothers of advanced age. Vanishing Twin Syndrome occurs with equal frequency in monozygotic and dizygotic twins. although the complications of sharing a placenta between monochorionic monozygotic twins may contribute to the condition.

What are the symptoms?

There might not be any symptoms. However. some mothers experience some mild cramping. bleeding or pelvic discomfort. similar to miscarriage. Decreasing hormone levels may also indicate that one fetus has been reabsorbed.

What is the treatment?

Generally. neither the mother nor the remaining fetus will require any kind of medical treatment. When VTS occurs in the first trimester. the mother usually goes on to experience a normal pregnancy and delivers a healthy singleton. However. in situations where a fetus dies in the second or third trimester. the mother may experience pre-term labour. infection or haemoragging. In those cases. doctors will prescribe treatment appropriate for those conditions.

What are the ramifications for the mother?

Physically. none. But emotionally. the mother may be feeling an awkward combination of grief over the loss of one baby and relief for the viability of the surviving baby. It is important for the parents to grieve in a way that feels appropriate. acknowledging the loss of a child as well as the loss of their identity as parents of multiples.

What are the ramifications for the surviving twin?

In most cases of first trimester Vanishing Twin Syndrome. there is no physical impact on the surviving twin. A healthy womb experience followed by a normal delivery should be expected. A late pregnancy occurrence of VTS does have some implications for the surviving fetus. just as for the mother. Occasionally. remnants of the re-absorbed fetus are found in the survivor. in the form of a tertoma tumor containing bone. hair. teeth or tissue fragments. Researchers have found that after 20 weeks. the surviving fetus has an increased risk of cerebral palsy. And asynchronous death may also be a risk if the twins are monozygous and sharing a vascular connection.

There is a great deal of speculation about the psychological and emotional impact of Vanishing Twin Syndrome. Some survivors report feelings of longing. guilt. or grief or problems with relationships or sexuality.

What happens when the twin doesn’t really vanish?

Sometimes. remnants of the unviable fetus are found in the mother. placenta or surviving twin. This is most likely to occur during the second or third trimester. Although usually the fetus will be partially re-sorbed and retained. the death of one twin at around 15 – 20 weeks may result in a fetus papyraceous. a tiny paper-like. flattened fetal remnant. A tertoma tumor containing bone. hair. teeth or tissue fragments is also an indication of a Vanishing Twin.