About 126,500 babies are born in the U.S. each year as part of twins or more, to approximately 63,000 families.
Fifty-three percent of all twins are born prematurely (before 36 weeks).
Seventeen percent of women pregnant with multiples develop pre-eclampsia.
Sixty-four percent of multiples are born at low birth weights.
In-vitro fertilization leads to multiples in approximately 30 percent of cases.
According to Hellin’s Rule, formulated in 1895, spontaneous (naturally conceived) twins occur once in every 89 births.
In 2001, 3.1 percent of all U.S. births were twins. The rate was roughly half in Europe. In the United States, one in 50 people is a dizygotic (fraternal) twin. One in 150 is a monozygotic (identical) twin.
If a woman has already given birth to dizygotic twins, she has a 4 times higher chance of conceiving another set of twins—one in 3,000 births.
A woman who is a dizygotic twin has a 1-in-17 chance of having a set of twins.
The fertility drug Clomid increases the chance of having twins to 1-in-10.
Approximately 70 percent of all twins result from fertility treatments.
African-Americans are more likely to conceive twins than Caucasians. Asians are less likely to conceive twins.
Women between the ages of 35 and 39 are more likely to conceive twins.
The more children a woman has, the more likely she is to have a set of twins.
Taller women conceive twins more often than shorter women.
Armadillos almost always give birth to monozygotic quadruplets. One type of armadillo often produces monozygotic octuplets.
Conjoined (Siamese) twins result from the egg splitting near the end of the second week following fertilization.
Identical twinning is 9 times more likely to happen if a woman becomes pregnant while breastfeeding another newborn.
Some scientists believe that women who consume more milk have a greater chance of having multiples because she is ingesting more insulin-like growth factor (IGF), which is commonly fed to cows to increase their milk and beef production.
About 25 percent of identical twins are mirror-image twins.
About 95 percent of all multiple births in the United States are twins.
Thirty-four out of every 1,000 births in the United States are multiples.
Between 1980 and 1998, the rate of triplets and higher-order births in the United States increased by 400 percent.
A woman has a 1 in 65 million chance of conceiving identical quadruplets.
The time for an egg to split into identical twins is very short. If the egg doesn’t split into two separate but identical eggs within the first 14 days after conception, it never will.
The record for the number of fetuses in the human womb at once is 15.
For every 400 sets of fraternal twins, one set will be made up of twins who have different fathers.
About 50 percent of twins are delivered by Caesarean section.
Women who eat meat or dairy products are 5 times more likely to have twins than women who are vegans.
Identical twins are more likely to be female than male, especially if they are conjoined.
Monozygotic twins are more likely to miscarry than dizygotic twins.
Since 1980, the number of triplets has increased tenfold.
Twins at birth are hospitalized twice as long as singletons, and medical costs are three times higher during the first five years of life compared with singletons.
Triplets are at higher risk for cognitive delays during the first two years of life.
Medical costs for a triplet pregnancy are estimated at $200,000.