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Resources Centre / Twins Tips Introduction / Twins Tips / Twins & Multiple Birth Statistics 2008 (Feb 2010)
Twins & Multiple Birth Statistics 2008 (Feb 2010)

These are the recently released statistics on Multiple Births in the UK for 2008 obtained from the Office of National Statistics (Released February 2010). General Registry Office Scotland and GRO Northern Ireland. The multiple birth rate continues to rise, and for the first time, seeing more than 12,000 sets born - a total of 12,179 sets were born in 2008 of which twins accounted for 11,989 sets or 98.4% of all multiple births. Total multiple births including twins, triplets, quadruplets/quintuplets increased by 453 sets (around 4%) on the previous year, although remains around 1.5% of all maternities.

England & Wales

Scotland
N. Ireland
UK
All maternities
701,297
60,041
25,378
786,716
Twins
10,680
953
356
11,989
Triplets
174
9
6
189
Quads
1
0
0
1
All multiples
10,855
962
362
12,179
Multiple birth rate per 1,000 maternities
15.5
16.2
14.26
15.48

In the UK about one in 34 babies is born a twin or triplet (in 1980 this was 1 in every 52 children) and 1 in 65 pregnancies results in a multiple birth. There has been a significant increase in the actual numbers of all multiple births from 10,694 to 12,179 and the overall multiple birth rate (per 1,000 maternities) has increased from 14.95 to 15.48.

The twinning rate fell from 13.0 per 1000 maternities in 1951 to 9.6 in 1980 and has since risen steadily. Scotland has the highest rate of multiple pregnancies, with 1.62% of all pregnancies resulting in a multiple birth.

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). Figures highlight that multiples are more frequently born to older mothers, around 62% of twins and 70% of triplets are born to women aged 30 years+, compared to 48% of singletons.

Twinning rates vary in different countries but the Monozygotic Rate (Identical Twins) is generally constant aross the world at 3.5 per 1000 maternities, or 1 in every 285 pregnancies. The Dizygotic Rate (fraternal/non-identical twins) is affected by assisted reproductive techniques and treatments such as fertility enhancing drugs or IVF but also by a number of pre-disposing factors such as maternal age, race, number of previous pregnancies, maternal height and weight and family history of fraternal twins (through the maternal line - Mother, Grandmothers etc).

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.

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 identical 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 but in 2008 the number of triplets born showed a rise on the previous year, from 149 sets to 189 sets (from 1 in 5130 pregnancies in 2007 to 1 in 4162 in 2008). This rate has been falling 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 Multiple Births

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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