There are several "types" of twins, but the two most common are fraternal and identical.

Fraternal Twins
Fraternal twins – medically termed as dizygotic twins – occur when two offspring are born of the same pregnancy and develop from two separate ova that were released from the ovary and fertilized at the same time. Fraternal twins may be of the same or opposite sex and will differ genetically as well as in appearance.

"Fraternal twins are twins that come from different eggs," says Barbara Hanson, a professor of genetics at Canisius College in New York. "There are women who are known as 'multiple ovulators.' These women do not ovulate one egg each month – they ovulate multiple eggs each month. As two eggs are released at the same time, women who release more than one egg a month are 10 times as likely to give birth to twins than those who release only one. Multiple ovulation is one of the main reasons behind the birth of fraternal twins."

The occurrence of fraternal twins varies, and there are several factors that contribute to the presence of these twins, including ethnic origin and maternal age. The highest occurrence of fraternal twins is found in black women, while the lowest is found in Asian women. Women between the ages of 35 to 39 tend to give birth to fraternal twins twice as many times as those not within this age group.

According to Hanson, another reason for the development and birth of fraternal twins is the use of fertility medications. These medications are used to stimulate the release of the egg from the ovaries, and it is quite common for more than one egg to be released at a time. "Fertility drugs are responsible for at least 35 percent of all multiple births – especially the birth of fraternal twins," says Hanson. "As fertility drugs are used for periods of time to achieve the desired result – pregnancy – the body may become over-stimulated and release more than one egg. If both or all of the eggs released are fertilized, a multiple birth results."

"I have a boy and a girl, so my twins are fraternal," says Trina Lambert, a freelance writer from Englewood, Colo. "Although I ovulated regularly, I was unable to stay pregnant. My doctors gave me [a fertility drug] for three months in a row before I was able to sustain a pregnancy. I don't know if I would have conceived twins anyway."

Identical Twins
Identical twins – medically termed as monozygotic twins – occur when an ovum splits into equal halves during early embryonic development, resulting in two offspring of the same pregnancy. Identical twins are always of the same sex, have the same genetic makeup, possess identical blood groups and will resemble each other physically. Unlike fraternal twins, identical twins occur equally in all races and age groups.

"What happens when identical twins form is that the egg is fertilized and it starts going through the cell divisions during the early part of fetal development," says Hanson. "For some reason, which we don't totally understand, the cells separate during this division, giving way to two fully-formed ova, which will develop into identical twins."

Other Twin Types
In addition to fraternal and identical twins, other types of twins may present themselves as well. These types are conjoined and unequal twins.

Conjoined twins are identical twins who did not completely separate during fetal development, and the offspring remain attached to each other and may even share body parts or organs.

"What happens with conjoined twins is that the egg does not separate completely," says Hanson. "It is the degree of separation that determines how much they share ultimately. They can be joined at the head, or more commonly, the abdomen or pelvis. Sometimes they share legs; sometimes they can share liver, heart, kidneys or other internal organs. Sometimes they can be easily separated, and sometimes it can be a very complicated operation. However, there are conjoined twins who have never been separated and have grown and developed into healthy, well-adjusted adults."

Unequal twins are two separate fetuses born of the same pregnancy in which only one of the pair is fully formed. The other fetus shows varying degrees of developmental defects. The differences that characterize unequal twins do not have to be that of a serious nature, and it may only be that one of the pair is larger by a minimum of 2 pounds or 1 inch.

"Size is the major factor with unequal twins," says Hanson. "However, in most cases, one of the pair will suffer from several physical problems, such as underdeveloped lungs or heart. It is thought that unequal twins result from an unequal separation of the ova during development, which leads to one of the pair receiving more nutrients, oxygen and room in the uterus. Unequal twins are rare and make up only about 2 percent of all multiple births."

The Role of Genetics
When having twins, parents often find themselves subject to questions regarding heredity and the birth of their twins. The question that I've gotten is, 'Do twins run in the family?'" says Robin Frazier, a stay-at-home mom from Brampton, Cambridgeshire, UK. "And I always answer, 'They do now.'"

"People ask all the time if I had twins in my family background," says Lambert. "I also hear from people who tell me they have a family background of twins and that it makes them nervous about getting pregnant. My twins were the first set of living twins in my family."

So what part does heredity play in the development of twins? According to Hanson, it depends upon the type of twin. "The twins that run in families are fraternal twins," says Hanson. "As some women naturally ovulate more than one egg per month, this trait tends to run in families. However, identical twins come from the same egg and do not run in families. There is no concrete link between a genetic inheritance and identical twins, as no one knows when or why certain eggs will split to cause twins while others do not. Maybe once people understand this, parents of multiples won't have to explain their twins."

The next time you see parents walking with their twins in a stroller or cooing at them while shopping, look carefully before asking them questions. You may be able to answer your own questions by knowing what twins are, why they happen and the difference between fraternal and identical. Besides, if you have twins, you need all the spare time you can get.

Want to see more?

• Two at Once: Becoming a Parent of Twins
• Elisa's Babies Today Diary, Toddlers Today Diary and Children Today Diary
• Two at Once: Pregnant With Twins
• Talk about it!

About the Author: Carma Haley Shoemaker is a contributing editor for iParenting Media and the mother of three.