How Often Does Vanishing Twin Syndrome Occur?
Physicians once considered it a somewhat rare condition but it now appears to happen rather frequently. Estimates are that it occurs in 21-30% of pregnancies involving multiples. The increase in numbers does not represent an actual increase in cases, rather, ultrasounds done earlier in pregnancy has brought an increased awareness to the condition. If an early ultrasound identifies multiple fetuses and then one of these fetuses subsequently dies and does not appear on a later ultrasound, it seems to have disappeared. If the early ultrasound had not been done, the fetal death may never have been known.
What Contributes to the Risk of Complications?
Most of the time the cause of this syndrome is not known but several possible causes are:
- fetal chromosomal abnormality
- advanced age of the mother as it relates to chromosomal abnormalities
- poor umbilical cord implantation (suspected as a risk factor)
What Complications Exist for the Mother?
If the fetal loss occurs during the first trimester the mother may have no symptoms at all or she may have mild bleeding and some cramping.
A greater risk of complications occurs if the death of the fetus occurs during the second or third trimester. These complications of vanishing twin syndrome include premature labor, serious bleeding problems, infection, difficulties during labor, and delivery by Caesarian section,
Emotional issues may also develop for parents that have lost a child in this manner. They may feel conflicted as they grieve the loss of one child while celebrating and worrying about the survival of another.
How Are Surviving Fetus(es) Affected?
If the fetal loss occurs during the first trimester the surviving twin usually has no problems at all.
However, if the death occurs in the second or third trimester the risk to the surviving fetus(es) increases. The chance of a preterm birth may as much as double. Surviving twins show a greater incidence of Cerebral palsy and some evidence points to a higher chance of congenital malformations as well. A low birth weight is very likely.
The same condition that leads to the death of one fetus may pose a risk of death to the other as well. Occasionally, the positioning of unabsorbed tissues of the dead fetus poses some risk to the surviving fetus(es).
In general, the later in the pregnancy that fetal death occurs, the higher the risk of complications of vanishing twin syndrome to the surviving multiple(s).
If the twin dies during the first eight weeks, it is usually completely absorbed leaving no visible trace (although histological evidence may still exist). If the fetal loss occurs later in the pregnancy, the tissue may not be fully absorbed. This may lead to one of several unusual, but not necessarily dangerous, conditions.
Tissue from a vanishing twin that is absorbed by the surviving twin may continue to exist as living tissue. A tumor (usually benign) may develop in the surviving twin to encase this tissue. These tumors are called teratomas.They can contain hair, bone, teeth, and rarely even some more developed organs. Whether or not a teratoma poses a risk depends on how large it becomes and where it is found in the surviving twin’s body.
Another unusual complication is a fetus papyraceus. This condition exists when, rather than absorbing the tissue, the growing surviving twin compresses the dead fetus against the uterine wall. It may become nearly paper thin. Rarely, a fetus papyraceus may be positioned in such a way as to complicate delivery and put the surviving twin at risk.
Vanishing Twin Syndrome: https://www.americanpregnancy.org/multiples/vanishingtwin.html
The Vanishing Twin: https://www.sonoworld.com/Client/Fetus/page.aspx?id=317
Vanishing Twin Syndrome: https://emedicine.medscape.com/article/271818-overview