How many types of twins are there?

Identical Twins:

1 egg + 1 sperm / by 2 = identical twins
Identical twins (monzygotic twins) are twins that develop from a single egg that splits during the first 12 days following conception. Identical twins have the same DNA, and except for the rarest of rare cases (three that I’ve been able to find), are always of the same sex. Contrary to popular opinion, identical twins are not carbon copies of each other. Identical twins may look very similar, or they may have distinct physical differences. About 1/3 of identical twins share “mirror twin” characteristics.

According to the experts, there is no rhym or reason for identical twinning to occur. It is purely a chance occurance.

Fraternal Twins:

2 eggs + 2 sperm = fraternal twins
Approximately 75% of all twins are fraternal, or dizygotic, twins. Fraternal twins develop from two separate eggs. Your chances of having fraternal twins can be increased by a number of ways; a few of the most well-known reasons are:

1. There is a gene that causes hyperovulation that is passed down the genetic line, and when people ask if “do twins run in your family”, this is what they mean.
   
2. Women over 30 or more likely to have twins than those under thirty.
   
3. If you take fertility drugs or undergo other fertility treatments.
   
4. Recent research has indicated that a diet rich in animal protein may also contribute to an increase rate of twins.

Since fraternal twins come from two different eggs fertilized by two different sperm they can be opposite sex twins, and can look as similar – or dissimilar – to any other siblings. And yes, Virginia, fraternal twins can have two different fathers.

Read more about fraternal twins with two fathers . . .

Polar Twins:

1 egg / by 2 + 2 sperm = Polar Twins

The idea behind polar twins is that the egg splits before conception, creating an egg and a polar body. Both halves are then fertilized by different sperm, resulting in polar, or “half-identical” twins. Even though polar twins share 75% of the same DNA, there is no test that can be done to determine if twins are ‘polar’ or not, but this theory has been offered as an explanation as to why so many fraternal twins (Mary Kate and Ashley Olsen, for instance) look more like identical twins than some identical twins do.

Conjoined Twins:

1 egg + 1 sperm not/ = conjoined twins
There are two methods Conjoined Twinning can happen:

1. Fission Method: The most commonly known theory of why conjoined twins happen is that the egg splits more than 12 days after conception. The “split” stops before completely separating into two separate babies.
   
2. Fusion Method: Not as well known, fusion conjoining happens when twins become conjoined after the fertilized egg completely splits into identical twins. During any baby’s prenatal development, cells will seek out and attach to other cells of the same type (simply put, skin cells will find other skin cells to form skin, or bone cells will find other like cells to form a rib or thigh, etc.). Sometimes newly separated identical twins are lying so close to one another the cells find similar cells during development, but the found cells belong to the other twin, therefore “fusing” the twins together.

The chances of having conjoined twins is 1 in 85,000. The survival rate of conjoined twins is very slim – approximately 5 – 25% of all conjoined twins survive the first 48 hours. A lot of that depends on where the twins are born, and what treatment is available.Approximately 70% of conjoined twins are girls. Approximately 3 – 4 sets of conjoined twins born each year have a non-conjoined triplet born with them.

Two other types of conjoined twins are:

Parasitic twins:
Parasitic twins are asymmetrical conjoined twins, one twin being small, less formed, and dependent upon the other.

Fetus in fetu:

Fetus in fetu happens when an imperfect fetus is contained completely within the body of its twin.

Vanishing Twin

2 (or more babies) – 1 baby = vanishing twin

In some pregnancies that are confirmed as multiple pregnancies before 10 weeks gestation, (either by ultrasound or Doppler heartbeat monitor), one baby mysteriously vanishes without any symptoms of miscarriage before the second trimester. This is known as Vanishing Twin, or Vanishing Twin Syndrome.

Before the widespread use of ultrasound technology in the first weeks of pregnancy, a vanishing twin was detected (if detected) after the birth of its sibling, by examination of the placenta. With ultrasound now being used as early as four weeks into a pregnancy, vanishing twins are relatively common in multiple pregnancies diagnosed before 10 weeks. In fact, studies find that twins may vanish as much as 53% - 71% in multiple pregnancies diagnosed before ten weeks gestation. In multiple pregnancies confirmed with ultrasound after 15 weeks, there are 0% documented cases of a vanishing twin.