Twin Types

Types of Twin

There are two types of twinning.

Monozygotic (identical) and dizygotic (fraternal) twins.

Fraternal twins can happen when two different eggs join two different sperms.  Each baby then has its own cord and placenta.  These twins may be of the same or opposite sex.  They will not be identical; indeed they will be no more alike than brothers or sisters born into the same family.  This type of twinning is more common if there is a family history of twins.  They are also more likely to occur if you are over age 35 or if fertility drugs have been used to help you become pregnant.

Identical twins are produced when an egg and a sperm join together but then divide into two once they have joined.  These twins may share a single placenta and very occasionally may also share the same amniotic sac.

Dichorionic Diamniotic is the term for fraternal twins with two placentas and two amniotic sacs.

Monochorionic Diamniotic is the term for identical twins with one placenta and two amniotic sacs.  The fertilised egg splits between 4 and 8 days.

Monochorionic Monoamniotic is the term for twins sharing one placenta and one amniotic sac.  The egg splits between 8 and 12 days.

Conjoined Twins are extremely rare.  They originate from a single fertilised egg.  The developing embryo starts to split into identical twins within the first two weeks after conception but the process stops before it is complete leaving a partially separated egg which develops into a conjoined foetus.

Chances of Twin Conception

Most twins are Fraternal.  Identical twins are rare.  The likelihood of having identical twins is about one set per 250 births.  This is a consistent statistic across the world.  Identical twins are not dependent on heredity, race, age or number of pregnancies.  Fraternal twins however are said to be influenced by race, heredity, maternal age and number of previous pregnancies and are even more common due to fertility drugs.

If you are a twin or twins run in your family you are slightly more likely to conceive twins yourself.  Women who are fraternal twins have one in 60 chance of conceiving twins themselves.

Apparently, the older you are the higher chance you have of conceiving fraternal twins.  A study found that women over 35 produce more follicle stimulating hormones than younger women.  The more pregnancies you’ve had the greater the chances of having twins.  If you have already had a set of fraternal twins you are twice as likely to have another set in future pregnancies.  If the firstborn are twins you are about five times more likely to repeat twinning.

Twins are more common in large and tall women.

Fertility treatment increases the odds of twins as the drugs administered stimulate the ovaries increasing the odds that a woman will release several eggs at the same time.  Approximately 20% of women who fall pregnant using fertility drugs will become pregnant with more than one baby.  Fertility drugs and other treatments primarily increase the chances of having fraternal twins, where two eggs are fertilized by two different sperm.  However, there is some evidence that women who undergo certain treatments also have a higher rate of identical twins.

Identical Twins are described as “spontaneous” and somewhat of a mystery.