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Multiple Births Statistics 2006
Newly released statistics on Multiple Births in the UK for 2006 obtained from the Office of National Statistics. General Registry Office Scotland and GRO Northern Ireland. The number of twins being born in the continues to rise. with a total of 11.165 sets being born in 2006. the latest year for which statistics are available.

England & Wales

Scotland

N. Ireland

UK

All maternities

662.915

55.108

23.044

741.067

Twins

9.992

858

315

11.165

Triplets

138

10

1

149

Quads+

7

0

0

7

All multiples

10.137

868

316

11.321

Multiple birth rate per 1.000 maternities

15.29%

15.75%

13.71%

15.28


In the UK about one in 32 babies is born a twin or triplet (in 1980 one in 52) and 1 in 65 pregnancies is a multiple birth. There has been a significant increase in the actual numbers of all multiple births in the from 10.694 to11.321 and the overall multiple birth rate (per 1.000 maternities) has increased from 14.95 to 15.28.

The twinning rate fell from 13.0 per 1000 maternities in 1951 to 9.6 in 1980 and has since risen steadily to 14.72 in 2002. 2003 was the first year showing a decline (14.67). Scotlandhas the highest rate of multiple pregnancies. with 1.58% of maternities resulting in a multiple birth; whilst the chances of having multiples in is currently lower than it has been over the past 6 years.

The twinning rate varies with the age of the mother. From 6.3 in under 20 years old. rising to 21.7 in 35-39 years. and 56.7 in the over 45s (compared to 6.3 in mothers over 45 years in 1980). The latest figures highlight that multiples are more frequently born to older mothers - 62% (62% "05) of twins and 77% (67% "05) of triplets are born to women aged 30 years+. compared to 48% (48% "05) of singletons.

Twinning rates vary in different countries with the Monozygotic rate (identical twins) generally constant aross the world at 3.5 per 1000 maternities. with an increase due to fertility treatment through drugs or IVF. The Dizygotic rate (fraternal/non-identical twins) is affected by fertility treatments. maternal age. race. number of previous pregnancies. maternal height and weight and family history of fraternal twins (through the maternal line).

Types of Twins
Monozygotic (MZ) (monozygous. uniovular. "identical") arising from the splitting of a fertilised egg (zygote) during the first 14 days after fertilisation. They have the same genetic make-up (and therefore also the same sex).
Dizygotic (DZ) (dizygous. binovular. fraternal or "non-identical") resulting from the fertilisation of two independently released ova by two different sperm. There are equal numbers of same and unlike sex pairs who are genetically no more alike than are any two siblings. About 1/3 of twins in the UK are MZ and 2/3 DZ.
Zygosity
Whether the twins are MZ or DZ can be determined by analysis of DNA samples from blood or other tissues. All unlike sex twins are DZ. All monochorionic twins are MZ. Thus DNA analysis is required for the approximate 50% of twins who are like sex with dichorionic placentas.

NB. More information about zygosity can be found on the "Are they Identical?" page on this website. the MBF also has a leaflet we can send you. see publications page.

Placentation
All DZ twins and one third of MZ twins have separate inner (amnion) and outer (chorion) sacs and are thus dichorionic. diamniotic. Two thirds of MZ twins have a single outer sac and two inner (monochorionic. diamniotic). About 1% of twins will share their inner sac (monochorionic. diamniotic).

Triplets and Higher Multiples
The triplet rate in the UK used to be about 1 in 10.000 maternities. This figure more than quadrupled between 1970 and 1998 following the introduction of ovulation induction and multiple embryo transfer in the treatment of subfertility.

The number of triplets has been falling since 1998 and although in 2004 the number of triplets born in and showed a small rise on the previous year. this increase has not continued (149 sets in 2006). It would be expected that this rate would fall following the national guidelines from the HFEA that recommend that only two embryos are replaced during IVF conceptions in women under 40 years. Some further unrecorded triplet and higher order pregnancies will have been reduced to twins or a single fetus by medical intervention. referred to as multifetal pregnancy reduction. early in the pregnancy.

The Human Fertilisation and Embryology Authority (HFEA) national data for 1/4/2000 to 31/3/2001 showed that in the UK 1579 twins and 109 triplet births resulted from IVF. The HFEA policy states that no more than 2 embryos should be transferred in an IVF cycle except in exceptional circumstances. There are no restrictions in the use of ovulation inducing drugs.

Data on Multiples
Average pregnancy length
  • Singletons 40 weeks
  • Twins 37 weeks
  • Triplets 34 weeks
  • Quadruplets 32 weeks
Average birthweight
  • Singletons 3.5kg
  • Twins 2.5kg
  • Triplets 1.8kg
  • Quadruplets 1.4kg
Mortaility Rates (England and Wales 2004)

Perinatal

Stillbirth

Infant

Singletons

5.4

7.7

4.5

Twins

14.8

28.2

21.6

Triplets

51.8

83.3

61.8


The high rate amongst multiples is largely due to their prematurity and low birthweight. Mortality is higher in MZ than DZ twins mainly due to the 2/3 of MZ twins with monochorionic placentas and a shared blood circulation. A higher incidence of congenital abnormalities is another factor.

Development
General development in twins (provided they have not suffered complications from prematurity) is similar to singletons. The area most at risk of delay is language. Twins are more likely to be delayed in their language development; boys more so than girls. Up to 40% of twins develop a twin language. otherwise known as idioglossia or cryptophasia or "the secret language of twins").

Disabilities of most kinds are more common in multiples mainly. but not entirely. due to prematurity and low birthweight

Cerebral Palsy Rate per 1000 livebirths*

Singletons

1.6

Twins

7.4

Triplets

26.7


*Western Australia figures
Data used in this article was obtained from NOS. Multiple Births Foundation and Tamba



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