Despite the glamour of twins, triplets and multiple pregnancies in the media, they are a high risk and they frequently result in poor outcomes. Throughout the world the twin birth rate is 4 twins per 1000 pregnancies (Ref. 18, p.861).
In the U.S. the frequency of twins is 12 in 1000 pregnancies. Twins are rare in Asians, are more common in whites and occur at a rate of 45 in 1000 pregnancies among black women.
Most of the twin pregnancies are “dizygotic twins” meaning that they originated from two different eggs. Age of the mother to be is another factor for twinning: women in their early 30’s have twins 3 to 4 times more often than women below the age of 20. Here are some reasons why twin pregancies add to mortality rates around the time of delivery and after. This is called “perinatal mortality”, which is the risk of death for the fetus and infant up to age 1 year.
These risk figures assume optimal care of the twin pregnancy with involvement of the neonatologist before, during and after the delivery as well as placement of the newborns in neonatal nurseries for high risk deliveries.
Death rates among twins
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Although twin pregnancies are often celebrated by the media, the survival statistics are not that glorious. From a medical point of view the overall perinatal mortality for a twin pregnancy poses a risk of 6 times higher than for a normal pregnancy. If only those twin pregnancies are considered that weigh more than 1000 grams at birth, the risk is still 3 times as high for twins versus single babies. The reason for these grim statistics are that twin pregnancies are associated with premature labor, with a higher rate of pre-eclampsia, genetic defects, growth restrictions in the uterus, breech presentations, placenta previa, premature placental separation as well as cord accidents. Cesarean section rates are higher in these pregnancies adding to the overall risk.
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The highest infant mortality rate is among twins born between 25 to 30 weeks (a gestational period of 38 to 42 weeks is normal). Mortality drops off beyond 30 weeks of gestation.
Because of the increased risk in twin and triplet pregnancies it is important for the physician to monitor for these multiple pregnancies early. One important tool is the prenatal ultrasound, which was introduced in the 1970’s and 1980’s on a large scale around the world. With improved ultrasonic equipment twin pregnancies can now be diagnosed as early as 6 to 7 weeks from the 1st day of the last period, but with certainty at 10 to 12 weeks.
100% of multiple pregnancies can be diagnosed, if two scans are done, one around weeks 15 to 16 and another one around weeks 32 to 33. Many women still believe that ultrasound would cause hearing damage or cancer in the fetus. This is false, and there is absolutely no evidence in the literature for that. However, this contributes to a higher mortality rate associated with twin deliveries, because of the reluctance of these women to under go ultrasonic examinations. Prenatal ultrasounds are safe and yield a lot of useful clinical information.
One downfall of early screening with ultrasounds is that medical science did not know until the early 1980’s that there is a phenomenon of a “vanishing twin”.
Studies along these lines have shown that between 7 and 12 weeks of gestational age in about 20% of early twin pregnancies one of the twins of early twin pregnancies “vanishes” through absorption. Nutritional factors, placental abruption and overcrowding have been implicated in this phenomenon. Because of the emotional impact it likely is better to postpone the first ultrasound until week 12 of the pregnancy unless there is a question about the exact timing of the first day of the last period (confusion about estimated due date). Placental abruption (thanks to aje.oxfordjournals.org for this link) is twice as common in twin pregnancies when compared to singleton pregnancies, as this publication indicates.
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