Ultrasound scanning techniques applied in early pregnancy have

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					                                          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
                                    Eng &
                                                                                                                            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.
                                                                                                                                Source: ONS

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)
                               16                                              0.6
                                                                                                                            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
                               8                                               0.3
                                                                                                                            of an incomplete split about 13-16 days after
                               6                                                                                            conception.
                               4                           Twins
                                                           Triplets+           0.1
                               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).