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Are you twins ? Or do you have twins in your family ? Or are you expecting twins babies ?

Congratulations! As a parent or prospective parent of twins, you will experience special joys and challenges. Most twins experience a special emotional bond between each other. On the other hand, parents should assist twins in developing a completely individual identity.

Definitions
There are Identical Twins and Fraternal Twins.
Identical Twins are also called Monozygotic (Mz), meaning one egg. Their genes are identical. They are always of the same sex.
Fraternal Twins are called Dizygotic (Dz) or two egg. They share approximately 25 percent of their genes, as do any two brothers or sisters. One-half of fraternal twin pairs are boy-girl, one-quarter are boy-boy, and one-quarter are girl-girl.

Biology
You may be asking, "Why did we have twins?" No one else in your family may have had twins. If you have fraternal twins in the family, you may ask, "Why did we have identical twins?" To answer these questions, let’s talk about the biology of twins.

How twins are conceived ? This can occur in one of two ways:

The more common way (which accounts for two-thirds of cases) is for the two different sperm to fertilize two different eggs, resulting in what is called a Dizygotic (DZ) twin gestation. These twins are often called fraternal twins.

In this type of twinning each twin has its own sac of amniotic fluid and its own placenta (afterbirth). Dizygotic twins have two sets of membranes surrounding their amniotic fluid sacs (one inner amnion layer and one outer chorion layer), and therefore they are known as Dizygotic (diamniotic, dichorionic).

Each embryo has its own amnion and chorion. After the birth of your twins, your obstetrician may be able to tell if the twins are monozygotic or dizygotic from the membranes around the placenta.

In about one-third of twin pregnancies, one sperm fertilizes one egg, but this splits into two embryos resulting in what is known as monozygotic (MZ) twins. The two-cell mass passes down the Fallopian tube and attaches separately or together in the womb. These twins are often referred to as identical twins since they have the same genetic material.

The developing embryo is surrounded by a sack of two layers of thin tissue called membranes: an amniotic membrane (amnion) and around that a chorionic membrane (chorion) (see figure). Approximately one-third of MZ twins look just like fraternal twins on prenatal ultrasound since there are two separate amniotic sacs and two separate placentas. However, in two-thirds of identical twins, each twin has its own amniotic sac but shares a common placenta.

This type of MZ twinning is called monochorionic, diamniotic since there is an inner layer surrounding the amniotic sac of each twin, but there is only one common outer layer (chorion) surrounding both of the sacs. This type of twinning occurs in approximately one in 360 pregnancies. Monochorionic twins are at higher risk for complications since they share a common placenta.

Less than 1 percent of identical twins (about one in 2,400 pregnancies) will have one amniotic sac and one placenta for both twins. This type of twinning is referred to as  monochorionic, monoamniotic twinning. These twins are at very high risk for loss of the pregnancy due to entangled umbilical cords.

If the zygote splits early (1 to 3 days after fertilization), each embryo will have a separate chorion and amnion. The later the zygote splits after ovulation, that is at 4 to 8 days, the greater the likelihood of the embryos sharing one chorion. If the zygote splits after 8 days, the amnion may also be shared. But if the zygote splits after 14 days, the twins may be joined together at some part of their bodies and are called conjoined or Siamese twins.

Frequency
A couple has a 1 in 80 (1.2%) chance of delivering twins; monozygotic twins occur in 1 out of every 250 births (0.4%) and dizygotic twins in 1 in 170 (0.6%) in the U.S. A woman who is herself a fraternal twin has a 1 in 10 (10%) chance of delivering twins. If she is an identical twin she has only a 1 in 170 (0.6%) chance of delivering twins. A mother of twins has a 1 in 20 (5%) chance of delivering twins again. A father who is a twin has the same chance of fathering twins as a nontwin father.

Cause of Twinning
The cause of monozygotic twinning is unknown. Less than 20 families have been described with an inherited tendency towards monozygotic twinning. (People in these families have a nearly 50 percent chance of delivering monozygotic twins!) Some evidence suggests that the environment of the womb causes the zygote to split in most cases.

Dizygotic twins are slightly more likely in Blacks, in women between 30 and 40 years old, in tall and heavy women, in women who conceive soon after stopping birth control pills, and in women who have had several previous pregnancies. Women who have been taking fertility drugs have the greatest chance of multiple births, a 10 to 20 percent or 1 in 10 to 1 in 5 chance.

How Do You Know You’re Having Twins?
The pregnant mother of twins first hears the statement "I bet you’re going to have twins," from family members who have had twins. The twin pregnancy causes earlier occurrence of pregnancy symptoms such as nausea, frequent urge to urinate, swelling, and weight gain. Fetal movement may be so increased that some mothers say they must have an octopus inside. The obstetrician may feel two different heads or note an increasing fundal height beyond that which is expected for dates. An ultrasound test can safely confirm the suspicions. Advance notice allows the parents time to prepare emotionally and practically for the twins’ birth.

Parental Adjustment before the Birth
Parents may worry even more than usual knowing that twins are on the way. Delivery complications occur more frequently in twin births than nontwin births, including premature (early) delivery and small birth weight. While her doctor can advise her, the expectant mother is the only one who can make the daily commitment of rest and proper nutrition to insure optimal health of the twins.

If you know in advance that you are expecting twins, now is a good time to meet other parents of twins for emotional support and practical advice. A chapter of the Mothers of Twins Club (MOTC) may exist in your community. You will need to buy various items in duplicate before the babies arrive: two bassinets or cribs, 1 twin stroller (the face-to-face type allows easy passage through doors), two baby chairs, clothes, and so forth. Dad hopefully will have time off around the delivery. In addition, have a friend or relative to help with housework when you come home from the hospital.

Prepare your other children for the new arrivals. Let them get used to their new bed and send them to preschool well before the babies arrive. The older children will have enough change to deal with in their new twin siblings.

Birth
The birthing process is more complicated with twins, so it’s unlikely that your obstetrician will advise alternative birthing centers. Vaginal delivery is often possible, especially when the babies are in the head-down position.

Just a few words about birth defects. Happily, Siamese twins occur rarely, about 1 per 100,000 births and can easily be detected by prenatal ultrasound. Monozygotic twins have a somewhat higher risk of having birth defects, so your obstetrician will make a point of checking the placenta to determine whether your twins are monozygotic or dizygotic.

Your pediatrician or family doctor will examine the twins carefully to make sure there are no birth defects. If one twin is significantly larger than the other, the physician will check the blood counts. Sometimes one monozygotic twin transfers too much blood to the other, leaving the first twin anemic and small and the second ruddy and large. This can only happen to monozygotic twins, and only when they share chorions. That’s why the term "identical" can be misleading, since monozygotic twins may differ because one of them has a birth defect or there may be a difference in their blood quantity. Bowed legs and bent ankles may occur in twins due to the crowding in the womb. However, these problems usually disappear completely as time passes.

The chances are that your birth will not have serious complications.

Are My Twins Identical or Fraternal?
We believe that it is important to answer this question correctly at birth and not just guess it. If it’s known that newborn twins are monozygotic, the physical exam must be especially careful, since monozygotic twins who differ in size may have birth defects. Identifying monozygotic twins at birth is important because it can improve medical care throughout life. For example, if one monozygotic twin develops a disease such as diabetes or epilepsy, the second twin has a higher than average chance of getting the same disease and should be carefully checked.

Also, twin behavior is different in monozygotic compared to dizygotic twins. Knowing the specifics is important in parenting. Identical twins have closer relationships than fraternal. They cooperate more. Fraternal twins develop their individuality easier because of clearer physical, emotional, and genetic differences.

It is possible to determine whether your twins are monozygotic or dizygotic. This is best done after you’ve delivered and are still in the hospital. If the sexes differ, the twins are dizygotic. If they’re the same sex, the obstetrician, after carefully examining the placenta, will send it to the pathologist for detailed study. If there is only one shared chorion, they are monozygotic twins. If there are two chorions, then they may be monozygotic or dizygotic, and a simple blood test can be done to compare blood cell groups, for example, A, B, O, and others you may not have heard. Many laboratories can do this relatively inexpensively. If red blood cell groups differ, then the twins are dizygotic.

Other characteristics can determine zygosity (monozygotic vs dizygotic): fingerprints, teeth imprints, electrocardiogram. However, these tests are difficult and are less accurate than the blood tests. Detailed comparison of physical characteristics may be fairly accurate. For example, in one study, where mothers reported that their twins looked like two peas in a pod, these twins were identical, based on blood types.

Can you recognize the name of the twins correctly ?

(compiled from various sources based on an article from Jonathon J. Evans, M.D)
images from Texas Childrens Hospital.