More about different types of twins
Fraternal (dichorionic) twins twins share the same prenatal environment. They each have their own genetic make-up, so they may not be more alike than any other brothers and sisters in the same family. Despite their differences they will share the special bond of being born on the same day and growing up together.
Fraternal twins are more common if the twins’ mother: has a history of twins in her family; is over 35 years old; has already had several babies; and/or, has been taking fertility drugs.
Identical, or monochorionic, twins occur in 25% of twin pregnancies. The latest research indicates that identical twins have a very similar genetic makeup (sharing about 88% of their DNA), resulting in both children looking alike. Identical twins are the same gender, may have similar finger prints, ear shapes, eye colour, hair colour and teeth imprints. They will often develop at similar rates and experience developmental stages simultaneously, for example, when they learn to walk and grow their first teeth. They may also have similar brain wave patterns. Mirror image identical twins have mirror image features, such as left and right hair crown swirl, left- or right-handedness and similar moles or body marks, but on different sides of the body.
What causes a fertilised egg to divide and create identical twins is still unknown.
Identical twins and chorionicity
Identical twins are categorised by chorionicity—depending on when the fertilised egg splits and how it splits, such as top/bottom or right side/left side. After an egg is fertilised it will develop a yolk sac (important for nourishing the forming embryo) and around the same time, the embryo’s placenta and chorionic sac begin to form. Finally, an amniotic sac surrounds the developing baby. There are different types of chorionicity for identical twins, depending on the timing of the afore-mentioned events.
Diamniotic dichorionic twins
If the egg splits before the placenta has formed, within three days of fertilisation, each baby will have their own placenta, chorionic sac and amniotic sac.
Diamniotic monochorionic twins
If the egg splits after the placenta has formed, after the third day following fertilisation, then the babies will share a placenta and chorionic sac, but will have their own amniotic sac. Monochorionic twin pregnancy is more common where assisted reproductive technology has been used.
Monoamniotic monochorionic twins
Monoamniotic twins occur when the egg splits after the amniotic sac has begun to form, around nine days after conception. As a result, both babies share an amniotic sac. Many monoamniotic twins lie very close to, or on top of, each other and early ultrasounds cannot tell if they are separate babies.
Monoamniotic twins are often misdiagnosed, especially at early scans. They are higher-risk than other diamniotic twins, but babies do survive. If you want to know more, the Monoamniotic Monochorionic Support Site, at http://www.monoamniotic.org, is an amazing support network and has a bulletin board with mo-mo mothers around the world.
If the egg splits later than 12 days after fertilisation, and does not split completely, then conjoined twins occur.