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Resources Centre / Twins Tips Introduction / Twins Tips / Twins & Multiple Birth Statistics 2007 (Feb 09)
Twins & Multiple Birth Statistics 2007 (Feb 09)
Newly released statistics on Multiple Births in the UK for 2007 obtained from the Office of National Statistics (Released February 2009). General Registry Office Scotland and GRO Northern Ireland. The multiple birth rate continues to rise, with a total of 11,726 sets being born in 2007, twins accounted for 11,573 sets. Total multiple births including twins, triplets, quadruplets and quintuplets increased by around 3.5% on the previous year.

Multiple Births Statistics for the United Kingdom

1984

2002

2004

2005

2006

2007

Total Maternities

726,295

661,603

709,316

715,556

741,067

764,387

Sets of Twins

7,255

9,740

10,455

10,533

11,165

11,573

Sets of Triplets

93

194

163

159

149

149

Sets Quads/Quins

5

4

5

2

7

3/1


Types of Twins
Monozygotic (MZ) (identical twins, monozygous, uniovular) 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" twins) 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 identical (MZ )and 2/3 fraternal (DZ).

Zygosity
Whether the twins are identical (MZ) or non-identical (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.& ;

Placentation
All DZ fraternal twins and one third of MZ identical 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, monoamniotic).

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& ;the number& ;of triplets born remains the same for 2007 as the previous year. It would be expected that the rate for higher order multiple births 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.

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


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