Some Facts about Multiples

Did you know?

• One birth in 80 is a twin birth.

• The rate of monozygotic twins (identical twins, or twins from a single egg) is relatively constant in different cultures and geographic areas. Monozygotic twin births occur at a rate of 3.5 per 1000 births all around the world. However, the rate of spontaneous dizygotic twins (fraternal twins, or twins that result from two fertilized eggs) varies from a low of 3 per 1000 births in Eastern Asia to a high of 40 per 1000 births in Nigeria. The North American rate is about 8 per thousand births. This does not include assisted pregnancies, which are a major cause of multiple births.

• In Canada, there are 41,000 multiple-birth children five years of age and under.

• More than 4000 sets of twins are born in Canada each year. More than 75 sets of triplets, quadruplets and quintuplets are born each year.

• Older women are more likely to conceive multiples.

• There are more male-male twins than female-female twins, but among triplets, there are more girls than boys.

• It is estimated that 15 to 17 percent of multiple births are the result of fertility treatments.

Are they Identical?

It is natural for parents to want to learn all they can about their babies and this includes their genetic makeup (or zygosity). You may be wondering if your multiples will be identical or fraternal. Even if your babies have already been born, the zygosity of the babies may not be clear.

Since identical twins are never truly identical (for example, fingerprints are always different), it is more accurate to talk about monozygotic and dizygotic twins.

Dizygotic twins are the result of two fertilized eggs.

How can you tell?
There are many different sources of information that can help you and your doctor determine zygosity.

Blood type is usually determined after delivery. If your babies have different blood types, they are dyzygotic twins.

Ultrasound during pregnancy can reveal information about the structure of the placenta, which can provide clues to zygosity. Two placentae do not always mean that the babies are dyzygotic; two placentae may develop if the egg splits before it implants in the wall of the uterus. However, analysis of the placentae after the birth may help answer your questions.

If you are still left with more questions than answers, DNA testing can be used. Monozygotic multiples have the same DNA, while dyzygotic twins would be no more similar than any other siblings.

Back to Multiples Index