Twins occur when two babies are born at the same birth.


Identical, or monozygotic, twins are of the same sex and are genetically identical and physically similar, because they both come from one ovum (egg), which, after fertilization, divides in two and develops into two separate fetuses. Fraternal, or dizygotic, twins occur when the mother produces two eggs in one monthly cycle and both eggs are fertilized. The conceptions may take place on two separate occasions and could involve different fathers.

Fertilized egg division which produces twins can either happen early or late in development. In the case of early separation, the two fetuses either share an amniotic sac or each has a separate amniotic sac. If the fetuses share an amniotic sac, they also share a placenta. If the two fetuses have separate amniotic sacs, they can either share a placenta or have two separate placentas. Twins can also result from a fertilized egg that divides slightly later in development. In this case, the twins share an amniotic sac and a placenta. It is from these cases of late separation that conjoined (Siamese) twins sometimes develop.

Fraternal twins, who are no more genetically alike than ordinary siblings, may be of the same or different sex and may bear some similarity of appearance. Fraternal twinning appears to be passed on by the female members of a family . If the mother is a fraternal twin herself, has fraternal twin siblings, or fraternal twin relatives on her side of the family, she is more likely to give birth to fraternal twins. If she has already given birth to fraternal twins, her chances of giving birth to fraternal twins again are four times greater than those of a woman who has not had fraternal twins. In vitro fertilization increases a woman's chances for having multiple birth.

The number of twins born in the United States rose between the early 1980s and the early 2000s. In 1980, there were 69,339 sets of twins born, and in 2002 there were 125,134 sets of twins born in the United States. According to data gathered by the Centers for Disease Control (CDC), there is considerable variation among the states in number and rate of twin births. In 1994, for example, the twin birth rate ranged from 19.8 per 1,000 live births in Idaho and New Mexico to 27.7 per thousand in Connecticut and Massachusetts. One factor that may influence the distribution of multiple births is whether the state provides insurance coverage for procedures such as in vitro fertilization (IVF) and other treatments to improve fertility. These procedures increase the chance of multiple births.

Ethnicity is another factor that may correlate to the twin birth rate. For 1994, the twin birth rate among non-Hispanic white mothers was 24.3 per 1000 live births; among non-Hispanic black mothers, 28.3 per 1000; and among Hispanic mothers, 18.6 per 1000. There are also significant differences internationally in the number of twins born with the rate in Belgium almost six times the rate in China.

The CDC also studies whether maternal age has any correlation with the rate of twin births. The data seem to suggest that mothers in states with rates of twin births higher than the overall rate for the United States are older on average, and mothers in states with rates of twin births lower than the overall rate for the United States are younger. Again, as in vitro fertilization is more widely done, the incidence of multiple births will increase.


Parents should avoid giving twins very similar names. Twins should be treated as two individuals and not as a package. They may need to be fed at different times and may develop skills at different rates. It is important to spend time with each twin separately so that they become used to being separated from each other for short times and know that they are each valued as individuals.


To help twins understand who they are as individuals, parents should avoid dressing both twins the same. It is preferable that each child receive toys that are geared towards their individual interests rather than each receiving the same toy.

School age

Sibling rivalry can be more intense in twins than in siblings of different ages. This is not unusual, because teachers, coaches, and even parents tend to compare twins. All children compare themselves to their siblings, and having others do this regularly can add to the pressure and stress of being a twin. Parents should consider arranging to have the twins put in different classes in school to help foster individuality. Each twin will probably have different skills, interests, and friends, and they should be encouraged to peruse activities separately if their interests diverge. Helping teachers, coaches, babysitters , and friends understand that it is important to treat the twins as two separate people can be very important. Friends should be encouraged to give separate gifts for birthdays and holidays, taking each child's special interests and talents into account.

Common problems

Twins often have a harder time developing their own independent identities than other children. Twins are more likely to have low birth weights or be delivered prematurely than single babies.

Parental concerns

Raising twins can be more challenging than raising two single children. The children may need to eat, sleep , and be changed at different times when they are infants. It can also be more expensive, because things like car seats and cribs must be purchased at the same time instead of reused for the second child. Some stores have special discounts for parents of twins.

When to call the doctor

Parents should call the doctor if one or both of their children seems ill, just as they would for any other child or children.



Noble, Elizabeth, with Leo Sorger. Having Twins and More: A Parent's Guide to Multiple Pregnancy, Birth, and Early Childhood , 3 ed. Boston, MA: Houghton Mifflin, 2003.

Pearlman, Eileen M., and Jill A. Ganon. Raising Twins: What Parents Want to Know, and What Twins Want to Tell Them. New York: Harper Resource, 2000.

Twin girls. Although many twins like to dress and act alike, especially at a young age, others try to differentiate themselves from each other, particularly in the teen years. ( Dennis Degnan/Corbis.)
Twin girls. Although many twins like to dress and act alike, especially at a young age, others try to differentiate themselves from each other, particularly in the teen years.
(© Dennis Degnan/Corbis.)


Brown, Judith E., and Marcia Carlson. "Nutrition and Multifetal Pregnancy." Journal of the American Dietetic Association 100 (March 2000): 343.


National Organization of Mothers of Twins Clubs. PO Box 438, Thompsons Station, TN 37179–0438. Web site:

Tish Davidson, A.M.


Dizygotic —From two zygotes, as in non-identical, or fraternal twins. The zygote is the first cell formed by the union of sperm and egg.

Monozygotic —From one zygote, as in identical twins. The zygote is the first cell formed by the union of sperm and egg.

Placenta —The organ that provides oxygen and nutrition from the mother to the unborn baby during pregnancy. The placenta is attached to the wall of the uterus and leads to the unborn baby via the umbilical cord.

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