Types of Twins

Once you announce to the world that you are expecting twins (or shortly after they are delivered), one of the most common questions you will hear from family and strangers is, "Are they identical?" You will soon find that most people are fairly uneducated about the different types of twins and how they are formed.

Identical Twins

Identical twins or monozygotic twins are the result of one egg and one sperm. Prior to implantation, the 46 chromosomes double and the fertilized egg splits into two equal halves. Each twin develops from an identical half of this original fertilized egg. About one-third of twins are identical with about half girl-girl combinations and half boy-boy ones. Obviously, boy-girl twins cannot be identical because they are not the same sex but this isn't common knowledge. Just ask any parents of boy-girl twins!

More about identical twins. . .

Fraternal Twins

Fraternal twins or Dizygotic twins are more common, making up two-thirds of all twins. They are formed when the mother releases two eggs during ovulation, which are fertilized by two different sperm. About half of all fraternal twins are girl-boy combinations. Boy-boy and girl-girl combinations make up the remainder in a fairly even split. Fraternal twins may or may not look alike and share anywhere between 25 to 75 percent of their genes.

More about fraternal twins. . .

Supertwins Triplets, quadruplets, and other multiples can be a combination of monozygotic and dizygotic twinning or just one of these types. For instance, triplets are often the result of the fertilization of three separate eggs or a combination of monozygotic twins and one single baby. The number of females typically increases with the number of multiples in a set. The use of fertility drugs has increased the incidence of supertwins in recent years. In 2000, almost three percent of all multiple births were triplets or higher-order multiples.

Your feelings: Finding out it's twins