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Explainer: twins, triplets, quadruplets and more

The Good sisters, born in Bristol in 1948, were the first quadruplets in the world to survive a caesarean-section birth. Paul Townsend

Are they twins? Are they identical? Are they natural? Which one is older? These are some of the all-too-familiar, somewhat invasive and highly personal questions posed by well-intentioned strangers to parents with multiples the world over.

A multiple birth occurs when more than one foetus is carried to term in a single pregnancy. Twins are the most common type of multiples (accounting for 90% of all multiple births) compared with higher order multiples (triplets, quads or more).

Identical multiples are rarer than fraternal multiples, as they are the result of a single fertilised egg spontaneously splitting and forming two foetuses. Identical multiples are the same gender and blood type and also have the same hair and eye colour. These are the multiples you often have trouble telling apart.

Fraternal multiples are the result of two (or more) eggs being fertilised by two different sperm. Fraternal multiples can be different genders and may be no more alike than if they were singleton siblings carried in a single pregnancy and born years apart. Higher order multiple pregnancies involving more than two babies can be fraternal, identical or a mixture of both.

The rate of multiple births increased from 1.1% of births in 1985 to 1.7% in 2005, then dropped to 1.5% of pregnancies in 2010. The rise in multiple birth rates between 1985 and 2005 is thought to be largely the result of increased use of assisted reproductive technology.

But there are many other factors that can influence the chance of a multiple birth. In the case of fraternal twins (clinically referred to as dizygotic twin pregnancies), factors include:

  • a genetic tendency to hyper-ovulate;

  • being of West African descent;

  • being taller and/or heavier than average; and

  • having experienced several previous pregnancies.

Maternal age is another factor and, given the tendency of women in recent decades to delay having a family until their early 30s (or sometimes later), it’s not surprising that the rate of multiple births for women over the age of 35 years is 20% compared to only 6% in women under the age of 20 years.

To date, the predisposing factors of monozygotic (identical) twinning remain unknown.

While the rate of multiple births may have steadied, the number of pregnancies resulting in a multiple birth has continued to increase since the 1970s – in 1990 there were 3,200 multiple births in Australia and by 2010 this increased to 4,500, a 43% increase, meaning you are still seeing more multiples out and about.

A multiple pregnancy is fraught with risks and complications. The most common risk is premature birth, which is defined as the birth of a baby before the developing organs are mature enough to allow normal postnatal survival – generally before 37 weeks. Significant progress has been made in the care of premature infants but not in reducing the prevalence of preterm birth, the cause of which remains unknown.

A multiple pregnancy almost never lasts the 40 weeks that a single pregnancy usually takes, and over half of multiple pregnancies are delivered before 35 weeks. The average birth weight of a twin is 2,400 grams, for a triplet it’s 1,750 grams and 1,300 for a quadruplet. Compare this to 3,500 grams for singleton births.

When you factor in prematurity and shorter gestation, birth weights of multiples can plummet to the region of 600 grams. As a result, long periods of hospitalisation are common in the early days of a multiple’s life, as well as an increased risk of illness, mortality, and of long-term complications including disabilities and impediments in growth.

There are also logistical challenges within neonatal intensive care units (NICUs) in dealing with the arrival of sets of multiples, as inevitably the opportunity to keep the babies and the family together is linked to the availability of beds within the special care nurseries of our hospitals. New parents of multiples are often faced with the prospect of being separated from one (or more) of their babies, which makes bonding and breastfeeding impossible.

The desire to keep a family together can result in one person being transferred to the closest hospital with enough NICU beds - which in a recent case of a Sydney family with triplets meant a trip to Brisbane for a number of months, depriving them of the support of friends and family that’s so important for all new parents.

Nor are parents of multiples exempt from impact. It’s no wonder that sleep deprivation is a recognised form of torture. It should come as no surprise that surveys report 76% of all mothers with newborn multiples feel constantly exhausted compared with only 8% of mothers with single babies. And the stress placed on relationships most likely arises from the fact that mothers of multiples are five times more likely to suffer post-natal depression.

Families with multiples have an increased rate of separation, and in families with triplets, there’s a 50% divorce rate. The physical and mental stresses that families of multiples live with day-to-day are exacerbated by the financial challenges of clothing, feeding and educating two or more children at the same time.

Despite the significant difficulties faced by both multiples and families of multiples, being a member of this special club is a privilege, and to watch firsthand the amazing bond that multiples have is an honour. But just consider these factors and stresses before you bombard new parents of multiples with questions!

Ashlee Tenberge from North West Sydney Multiple Birth Association contributed to this article.

See more Explainer articles on The Conversation.

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