While the majority of human babies come into the world one at a time, multiple births often occur. With the advent of prenatal ultrasound, twins, triplets and higher-order multiples rarely come as a delivery room surprise. But twins and higher-order multiple births continue to fascinate and interest people.
Incidence
Twins occur at a rate of 32 per 1,000 births. Triplets and higher-order multiples (triplets or more) comprise 1.5 out of 1,000 births. Black women have more multiples than other ethnic groups; Asians have the lowest. Women have an increased incidence of multiples as they age; the chances increase from 3 percent at age 25 to 5 percent, nearly double, at 35, Keep Kids Healthy states.
Women who have undergone fertility treatment have the highest rate of all. The twin rate in the United States rose 70 percent between 1980 and 2004 as a result of fertility treatment, the March of Dimes reports, and higher-order multiple rates quadrupled. Less than 20 percent of triplet and higher gestations are spontaneous, Mothers of Supertwins reports.
Types
Two babies born at a time are twins; three are triplets, four are quadruplets, five are quintuplets, six are sextuplets, seven are septuplets and eight are octuplets. Multiples can be identical, which means they developed from the same fertilized egg that split at a very early gestational age, or fraternal. Identical twins have the same exact genetic makeup, are always the same sex and look very similar. Fraternals can be the same sex or boy-girl, and may look nothing alike.
Triplets and higher-order multiples may be all identical, all fraternal or a mixture; triplets, for example, may consist of two identical and one fraternal. As of 2010, the most famous set of all identical babies were the Dionne quintuplets: five identical girls born in Canada in 1934.
Rarities
Twins have been born that actually have different fathers, a condition known as heteropaternal superfecundation, the fertilization of eggs from two different sexual encounters. Twins have also been born several days or months apart when a woman goes into preterm labor and only one baby is delivered. High-order multiples in 2010 are usually the result of fertility treatment and are almost always fraternal.
As of 2010, the highest-order identical multiple group ever born were the Dionne quintuplets. Up to nine babies have been born at one time, but none of the babies have survived. One set of octuplets and several sets of septuplets have survived, all the result of fertility treatment. Conjoined twins, identical twins born from a fertilized egg that doesn't split completely, occur in 1 in 50,000 to 80,000 births. The Dionne quintuplets are the only known set of identical quints to survive.
Risks
Babies born from multiple pregnancies have an increased risk of prematurity and birth defects. According to the March of Dimes, twins deliver on average at 35 weeks, five weeks early, while triplets deliver on average at 32 weeks, and quadruplets at 29 weeks. Identical twins may both get sick from twin to transfusion syndrome, where one twin has too much blood and the other has too little.
Women carrying multiples also have a higher risk of gestational diabetes, high blood sugar in pregnancy and pregnancy-induced hypertension, which is a rise in blood pressure and fluid retention that can lead to seizures and preterm delivery. Cord accidents are also more common in twins, who can become entangled in each other's umbilical cords as well as their own.
Misconceptions
Multiples don't always look alike and don't appear to have an increased ability to communicate telepathically, although they may make up their own language, Babyzone states. The chance of twins does not skip a generation, and there's no gene for identical twinning, so only fraternal twins run in families.
