According to Statistics New Zealand in 2006 there were 845 sets of liveborn twins (1,690 babies) registered in 2006 and
10 sets of liveborn triplets (30 babies). So it is safe to say that there are a fair number of twins and triplets in our neighbourhoods. Yet the public generally don't tend to know much about multiples beyond the fact that they are either identical or fraternal and are bound to be "double trouble" !! So let's take a closer look at what twins are and the different twinning variations.

Are they Identical?

This question is one that almost all parents of twins will grow to loathe. Especially if they have boy/girl twins and have to explain to people how given certain anatomical differences there is NO way they can be identical, only to be asked again, "Are you sure they aren't identical?" So what exactly are the different types of twins and multiples?

Identical Twins (Monozygotic)

Identical twins are the result of a single egg (mono zygote) being fertilised by a single sperm and subsequently splitting. Identical twins are always same sex combinations boy/boy or girl/girl. Identical twins are largely believed to be a random occurrence and therefore non-hereditary. The way identical twins form in the uterus depends largely on the timing of the egg's division and implantation.

Dichorionic diamniotic (di/di) twins
If the fertilized egg splits soon after fertilization (within approx. 72 hours) the embryos will develop in the same manner as fraternal twins -each with its own placenta, chorion (outer sac), and amnion (inner amniotic sac).

Monochorionic diamniotic (mono/di) twins
If the fertilized egg splits between days 4 and 8, the twins are likely have a shared chorion and placenta, but have separate amnions.

Monochorionic monoamniotic (mono/mono or mo/mo) twins
If the fertilized egg splits after day 9, the twins will likely share the placenta, chorion, and amnion.

Conjoined twins
An incomplete split of the fertilized egg will result in conjoined twins.

Fraternal Twins (Dizygotic / Dichorionic diamniotic - di/di)

Fraternal twins occur when the mother produces two eggs at the time of ovulation which are then fertilised by two sperm. Fraternal twins can be same sex or boy/girl combinations. Fraternal twinning can be hereditary through the mother's side of the family. Many fertility treatments also increase the likelihood of fraternal twins as does the age of the mother (older mothers are statistically more likely to produce twins.) Fraternal twins develop in the uterus separately. They each have their own chorion, amnion and placenta. However, sometimes it may be difficult to identify the separate placentas if they have implanted close to each other in the uterus and have subsequently fused.

Triplets or more

Triplets and other higher order multiples can be fraternal, identical or a mix. For example, some triplet sets may contain a pair of identical twins with a third fraternal twin, while others may be made up of 3 fraternal twins. The odds of having identical triplets is believed to be one in 200 million!!