Multiple Births Foundation


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Improving baby safety by reducing multiple births - the HFEA calls for national strategy

The Multiple Births Foundation (MBF) welcomes the HFEA call for a professionally-led, coordinated national strategy to reduce the number of multiple births following fertility treatment. 

The MBF provides advice, information and support to multiple birth families and training and education for professionals about their special needs and problems and  how to meet them. 

Although there are many joys for these families, far less well know are the higher risks for the mothers and babies. Prematurity and low birth weight are the main reason for increased mortality and disability for these children.

At the MBF we support families in all circumstances. Sadly, much of this involves those who are bereaved or caring for one or more children with cerebral palsy and other health and developmental problems.  We see the heart break and life long suffering which is so often underestimated or forgotten by professionals and the public.

With the increasingly reassuring evidence that careful and individual selection of patients for single embryo transfer can maintain good pregnancy rates, we strongly support the HFEA proposal to develop the many facets of a strategy to reduce multiple births which will have the confidence of all involved.

We look forward to working with the HFEA, professional bodies and other organisations to achieve this.

If you require more information please contact the MBF offices.

General Information

Multiple Births statistics in the UK for 2018


England & Wales (ONS London)

Scotland (GRO Scotland)

N.Ireland (NISRA)

UK Total

Total Maternities
All multiples
Quads and more
Not Listed
Not Listed
Not Listed
Not Listed
Twinning rate/1000 mats
Triplet rate/1000mat
Multiple Birth Rate/1000 mats

Obtained from the Office for National Statistics, General Registry Office Scotland and GRO Northern Ireland.

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.

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.

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, Monoamniotic).

Causes of Twinning
MZ - causes still unknown. (There are slightly more than expected sets resulting from infertility treatment (drugs alone or IVF). The MZ twinning rate had been constant worldwide at about 3.5 per 1,000 maternities but there has been a slight rise recently.
DZ - the main factors associated with DZ twinning are infertility treatment, race (highest in Black Africans and lowest in Orientals), maternal age, parity, maternal height and weight and maternal family history of twins.

Triplets and Higher Multiples
The largest surviving complete set is of septuplets (in US). Three complete sets of sextuplets survive in the UK (6 girls in 1983, 3 boys and 3 girls in 1986 and 1 boy and 5 girls in 1993). 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 triplet rate has fallen annually since 1998 probably due to more judicious treatment for infertility including a reduction in the maximum number of embryos that can be transferred in an IVF cycle. 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.

Recommended reading: Three, Four and More, published by HMSO 1990. This national study of triplet and higher order births included the obstetric, paediatric and social aspects of all families with triplets and higher order births born in the UK 1979 - 1986 (except 1981).

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

Mortality Rates (England and Wales 2015)
per 1,000 live births

  Stillbirth Perinatal Neonatal Postnatal Infant
All 4.3 6 2.2 0.7 2.9
Twins 9.8 17.9 10.7 2.6 13.3
Triplets 19.8 49.6 34.4 left blank on ONS site 36.4
All Multiple Births 10 18.8 11.4 2.5 14.0
Figures taken from the ONS: Infant mortality (birth cohort) tables in England and Wales - 2015

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.

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

Further Reading:
MBF Guidelines:
Facts about Multiple Births
Multiple Pregnancy
Special Needs in Twins and More
The First Five Years and Beyond

MBF booklets:
Fetal Reduction
Individuality in Twins
Language Development in Twins
Monochorionic Twins
Relating to Twins
Selective Feticide
The Twin (and Triplet) with Special Needs
When a Twin or Triplet Dies

Twins, Triplets and More: Their Nature, Development and Care by Elizabeth Bryan, Multiple Births Foundation, 1996.
Entwined Lives: Twins and What They Tell Us About Human Behavior by Nancy Segal, EP Dutton, 1999.
The Lone Twin: Understanding Twin Bereavement and Loss by Joan Woodward, Free Association Books Ltd, 1998

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The Multiple Births Foundation
Hammersmith House Level 4, Queen Charlotte's & Chelsea Hospital, Du Cane Road, London, W12 0HS
Telephone: 020 3313 3519 E-mail:
Registered Charity number: 1094546  Limited Company Number: 4426289

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