Twin pregnancy, why does it occur? When we think of twins most of us conjure up images of cute, identical babies in matching outfits. We think of deep sibling bonds and pranks played on unsuspecting adults. But there is a serious side to twins that can be lost in the rather stereotypical images we have.

Having twins can be a lot of fun and full of incredible joy. But, higher risks are associated with twin pregnancies and births, and a great deal of hard work follows their arrival. Good care during pregnancy; birth and the postnatal period can certainly reduce many of the risks associated with twins.

Twin pregnancies: Why does it occur?

Multiple pregnancies (more than one baby conceived) accounted for 1.5% of all births in Australia in 1998 (Australian Institute of Health and Welfare, 2000).

There were 14.5 twins born for every 1000 mothers giving birth. This is a significant increase from the rate of 9 twins per 1000 births in 1997. Triplets have also increased. Overall in 1998 in Australia there were 3645 twin pregnancies, 104 triplet pregnancies and 2 quadruplet pregnancies.

Multiple pregnancies have increased in Australia during the last 20 years. This is largely due to the increased number of older mothers getting pregnant, the increased use of fertility drugs, and assisted conception technologies such as In Vitro Fertilisation (IVF) where more than one fertilised embryo is transferred back to the mother.

The chances of multiple pregnancy peaks amongst mothers aged 35 to 39 years.  Due to the associated risks of twin pregnancies, and the improved success of IVF, professionals encouraged women undergoing an IVF procedure to have only one embryo transferred. Professionals rarely recommended transferring more than two embryos as the risks are so great.

Identical and non-identical twins

There are two types of twins: identical (monozygotic) and non-identical (also known as fraternal or dizygotic).

Identical twins develop from one of the mother’s ovum and one of the father’s sperm and they make up around one-third of all twins. During the first 13 days after conception, the zygote splits in two growing to become two embryos.

Identical twins share one placenta and sometimes even the same amniotic sac. Identical twins are always of the same sex and are genetically identical, which leads to their similar physical characteristics. The occurrence of identical twins is not hereditary. Identical twins occur by pure chance.

In extremely rare cases a zygote may split in excess of 13 days after conception.

This can lead to an incomplete division resulting in conjoined or ‘Siamese’ twins where the two embryos, and resultant babies, are physically connected to each other.

Non-identical twins result when the two sperm from the father fertilised the two ovum that the mother’s ovaries release (rather than the usual one). For this reason, non-identical twins can be a different sex and they are no more genetically similar than brothers and sisters born at different times.

Non-identical twins make up around two-thirds of all twins and have two separate amniotic sacs and placentas, though the placentas can fuse together and seem like one.

There is a tendency for non-identical twin pregnancies to be hereditary. So a history of twins in the mother’s family increases the likelihood of a twin pregnancy. Women with one set of non-identical twins are also more likely than the general population to have twins again.