MULTIPLE BIRTHS FACT SHEET
What is the likelihood of having one of the developing embryos being lost in
twins or more? early pregnancy
In the UK about one in every 67 pregnancies What are the risks associated with
results in a multiple birth. There has been an multiple births?
upward trend in the numbers over the last 20
years. The rise is attributed to the use of Rates of still births are low but there continues
fertility treatments as well as the increased to be an increased risk for multiple births from
survival rates of premature babies and women 0.57% for singletons to 1.3% for twins, 5.1%
starting their families later. for triplets and 10% for quads (source: ONS,
2004). Several studies have also shown the
incidence of cerebral palsy to be higher in
Incidence of multiple births in the UK, 2004 multiple births, which is mainly attributable to
the higher levels of prematurity and lower birth
Wales Scotland N. I. U.K. weights in multiple births.
maternities 633,651 53,501 22,087 709,239 What is the likelihood of having a
Twins 9,294 757 330 10,381 full term pregnancy and how big are
the babies likely to be when born?
Triplets 147 8 7 162
Quads+ 5 0 0 5 Average Average
All length of birth weight
multiples 9,446 765 337 10,548 pregnancy
birth rate 14.91 14.30 15.26 14.87
Singletons 40 weeks 3.5kg
per 1000 Twins 37 weeks 2.5kg
maternities Triplets 34 weeks 1.8 kg
Quads 32 weeks 1.4kg
Source: ONS London, GRO Scotland and N.I.
The rate of triplet and higher order births has
declined since a peak in 1997/1998 though What are the different types of
there was a small rise in 2004. This is probably twins?
linked to the Human Fertilisation and
Embryology Authority change of policy in 2001, Monozygotic (MZ) twins are also known as
which reduced the maximum number of identical or uniovular. They occur when one
embryos being transferred to the mother in fertilised egg splits early in the pregnancy
each IVF cycle to two. (within 13 days of fertilization). There are no
known causal factors for conceiving MZ twins
and rates are constant worldwide, 3.5 per
Trends in the multiple birth rate
1000 births. Monozygotic twins are always the
England & Wales (1982-2004)
same sex, because division of the fertilised
ovum produces two genetically identical
Triplet (plus) deliveries/1,000 mats
0.5 individuals. Except in rare circumstances MZ
Twin deliveries/1,000 mats
0.4 twinning is not an inherited trait. Conjoined
twins are MZ twins which are the result
of an incomplete split about 13-16 days after
2 Twin-twin transfusion syndrome, an
0 0.0 imbalanced flow of blood from one twin to
another, occurs in 15% of MZ twins who share
a placenta. The implications of this are very
serious for the survival and health of both twins
More twins are conceived than actually and they would require close monitoring
delivered (the vanishing twin syndrome) with during the pregnancy.
Dizygotic (DZ) twins are also known as fraternal, (inner sacs) depending on when the fertilized egg
non-identical or binovular. DZ twins are conceived splits.
when the mother produces two or more eggs in the
same menstrual cycle which are then fertilized
Factors which predispose towards dizygotic
twinning include ethnic origin (highest in Nigerians
and lowest in Japanese), women who have
undergone ovulation inducing hormone treatment
for infertility, increased maternal age (until the late
30s), maternal height and weight and the number of
previous pregnancies. Dizygotic twinning is also an
inherited trait, passed on through the maternal side
of the family.
DZ twins are no more alike than any other brothers
and sisters and have an equal chance of being of
the same sex or different sexes. A mother who has
Diagram reproduced with permission from the Multiple Births
already had one set of DZ twins runs an increased Foundation
risk (approximately five fold) of having a second set.
DNA testing provides a 99% accuracy test for
Triplets and higher order children can be zygosity using a DNA sample collected from inside
monozygotic, dizygotic or a mixture of both. the babies’ mouth with a swab. The samples are
In the UK, one third of all twins are monozygotic and then examined for specific markers and
two thirds dizygotic twins. monozygotic twins will have identical DNA maps
while dizygotic twins will have different ones. This
How can you tell if the twins are test is not generally available on the NHS but details
monozygotic or dizygotic? for obtaining a private test can be found on the
Multiple Births Foundation website.
The simplest way is by looking at the children. All
different sex multiples are DZ and in older same sex For more information
children a comparison of the colour and the texture
of hair, the colour of the eyes, the shape of the Tamba aims to provide high quality information and
ears, teeth eruption pattern and formation and the mutual support networks for families of twins,
children’s fingerprints often give a good indication of triplets and more, highlighting their unique needs to
zygosity. However with babies this isn’t always easy all involved in their care. www.tamba.org.uk
to see and other methods can be more reliable.
Twins and Multiple Births. The Essential
The babies’ placenta(s) are helpful in determining Parenting Guide From Pregnancy To Adulthood
zygosity as well as the amnion or inner sac and the by Dr Carol Cooper, Vermillion
chorion or outer sac surrounding the babies (see Expecting More than One Tamba booklet -
diagram). There are three different types of placenta information and practical advice for parents
– separate, fused or single. expecting multiples
Expecting more than One Tamba video - parents
Separate placentas often indicate that the twins are of multiples discuss their experiences during
dizygotic. Sometimes though if one fertilized egg pregnancy, birth and the early days after the birth.
which produces monozygotic twins splits at a very
early stage in pregnancy (within the first few days) The Multiple Birth Foundation advises
separate placentas may be produced. To professionals working with multiple birth families
complicate things further these two placentas 0208 383 3519 www.multiplebirths.org.uk
sometimes may remain separate or fuse together.
Fused placentas sometimes occur when two The UK Twin to Twin Transfusion Syndrome
separate ones become implanted very close Association www.twin2twin.org
together in the womb. This can happen with either
dizygotic or monozygotic twins. Fused placentas If you wish to discuss any of the issues raised
sometimes cause confusion because they give the in this leaflet please contact the Tamba office or
appearance of a single one. Twinline, Tamba’s freephone helpline, a
Single placentas only occur in the case of national, confidential, listening and information
monozygotic twins. With single placentas there will service for all parents of twins, triplets and
always be one chorion (outer sac) around both more, and the professionals involved in their
babies but there may be either one or two amnions care (see details at the bottom of the first page).