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Twins | Interesting Twin Facts

Fraternal Twins vs Identical Twins

Most people don’t know much about twins and how they are conceived until a close friend or a family member finds out she is having twins, or until they find out they are expecting twins themselves. Twins are two offspring resulting from the same pregnancy, usually born in close succession. There are two types of twin pregnancies; fraternal (dizygotic) and identical (monozygotic). When a woman ovulate two eggs that are fertilised by two different sperm, she’ll have fraternal twins. When she ovulates one egg that is fertilised by one sperm, but then spontaneously splits in half afterwards, she’ll have identical twins. While having fraternal twins is often hereditary, having identical twins can happen to anyone. So this confirms the fact that just because there are no twins in your family, that doesn’t mean you won’t have twins! Let’s take a closer look at the two types of twin pregnancies and find out which is more common and what sets the two apart from each other.


Fraternal twins

Fraternal twins, also known as dizygotic or non-identical twins, are more common than identical twins. Each baby grows from a separate fertilised egg and by three weeks each zygote has developed into a ball of several hundred cells (a blastocyst) and is burrowing into the uterine lining.

Like normal siblings fraternal twins may look very similar, as genetically they are like any other siblings who happen to be the same age. However sometimes fraternal twins can look very different too – just like normal siblings. Fraternal twins can be the same sex or a different sex, unlike identical twins who are always the same sex.

Fraternal twins are more common for older women or for those with twins in their family, which means it’s hereditary. If your mother or other women in your family have a history of ovulating two eggs at the same time, chances are you can too. Also, for those using fertility drugs in order to help them fall pregnant, a twin pregnancy is a very common result.

Identical twins

Identical twins are very different from fraternal twins. Only one egg (zygote) from the mother is fertilised by one sperm from the father, and then the embryo splits and two foetuses will grow. Depending on when the embryo splits, you will have di/di, mono/di or mo/mo twins.
If the embryo splits within two days after fertilisation, the two foetuses will develop separate placentas and separate amniotic sacs. The result is dichorionic, diamniotic twins. All fraternal twins are di/di, but only 20-30% of identical twins are di/di.

In most identical twin pregnancies the embryo splits after two days, which means the foetuses will share the same placenta – as it has already formed – but they will have separate amniotic sacs. Monochorionic, diamniotic (mo/di) twins are very similar genetically.

When the embryo splits very late the twins will share both the placenta and the amniotic sac. Monochorionic, monoamniotic (mo/mo) twins are very rare – occurring in only 1% of twin pregnancies. A mo/mo twin pregnancy is a high risk pregnancy that requires special monitoring – you need to schedule an ultrasound with your practitioner every other week throughout your pregnancy and you’ll have to deliver by Caesarean section, usually by week 34. This is because the mo/mo twins share the same sac and their umbilical cords are therefore in close proximity to each other and can easily become entangled.

If the embryo splits extremely late – later than eight days – it will result in conjoined twins, but this is very rare. Mortality is highest for conjoined twins due to the many complications resulting from shared organs.

Semi-identical twins are also very rare. These “half-identical twins” are hypothesised to occur when an unfertilised egg cleaves into two identical attached ova and which are viable for fertilisation. Both cloned ova are then fertilised by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed – each of which have different paternal genes and identical maternal genes.

Taking a look at what’s most common when it comes to twins, you’ll find that fraternal twins of a different sex tops the list. In fact, you’ll find that around 40% of all twins born fall into this category. Second comes female fraternal twins, followed closely by male fraternal twins. Identical female twins are next, and identical male twins are last.


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