Multiple Births Fact Sheet - PDF

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                                                                                                      Multiple Births Fact Sheet
          What is the likelihood of having twins or                                                                                                              What are the risks associated with multiple
          more?                                                                                                                                                  births?
          In the UK about one in every 65 pregnancies                                                                                                            Rates of still births are low but there continues
          results in a multiple birth. There has been an                                                                                                         to be an increased risk for multiple births from
          upward trend in the numbers over the last 20                                                                                                           0.57% for singletons to 1.3% for twins, 5.1% for
          years. The rise is attributed to the use of                                                                                                            triplets and 10% for quads (source: ONS,
          fertility treatments as well as the increased                                                                                                          2004). Several studies have also shown the
          survival rates of premature babies and women                                                                                                           incidence of cerebral palsy to be higher in
          starting their families later. Multiples are also                                                                                                      multiple births, which is mainly attributable to
          born more frequently to older mothers.                                                                                                                 the higher levels of prematurity and lower birth
                                                                                                                                                                 weights in multiple births.
          There has been a significant increase in the
          actual numbers of all multiple births in the UK                                                                                                        What is the likelihood of having a full term
          and the overall multiple birth rate.                                                                                                                   pregnancy and how big are the babies likely
                                                                                                                                                                 to be when born?

                                                Incidence of multiple births in the UK, 2006                                                                                           Average       Average birth
                                                                                                                                                                                       length of     weight
                                                                     Eng &                                                                                                             pregnancy
                                                                     Wales                    Scot-                       N. I.                    U.K.               Singletons       40 weeks      3.5kg
                                     All ma-                                                                                                                          Twins            37 weeks      2.5kg
                                     ternities                       662,915                  55,108                      23,044                   741,067
                                                                                                                                                                      Triplets         34 weeks      1.8 kg
                                     Twins                           9,992                    858                         315                      11,165
                                                                                                                                                                      Quads            32 weeks      1.4kg
                                     Triplets                        138                      10                          1                        149

                                     Quads+                          7                        0                           0                        7
                                                                                                                                                                    Source: ONS
                                     All multi-
                                     ples                            10,137                   868                         316                      11,321        What are the different types of twins?
                                     Multiple                                                                                                                    Monozygotic (MZ) twins are also known
                                     birth rate                      15.29                    15.75                       13.71                    15.28
                                     per 1000                                                                                                                    asidentical or uniovular. They occur when one
                                     materni-                                                                                                                    fertilised egg splits early in the pregnancy
                                                                                                                                                                 (within 13 days of fertilization). There are no
           Source: ONS London, GRO Scotland and N.I. NB: Figures
          include live and still births
                                                                                                                                                                 known causal factors for conceiving MZ twins
          The rate of triplet and higher order births has                                                                                                        and rates are constant worldwide, 3.5 per1000
          declined since a peak in 1997/1998 though                                                                                                              births. Monozygotic twins are always the same
          there was a small rise in 2004. This is probably                                                                                                       sex, because division of the fertilised ovum
          linked to the Human Fertilisation and                                                                                                                  produces two genetically identical individuals.
                                                                                                                                                                 Except in rare circumstances MZ twinning is not
                                                       Trends in the U.K. multiple birth rate, 1991-2006                                                         an inherited trait. Conjoined     twins are MZ
                                    16                                                                                                                           twins which are the result of an incomplete split
                                                                                                                                                                 about 13-16 days after conception.
Multiple Births/1,000 maternities


                                    14                                                                                                                           Twin-twin transfusion syndrome, an
                                    13                                                                                                                           imbalanced flow of blood from one twin to
                                    12                                                                                                                           another, occurs in 15% of MZ twins who share a
                                    11                                                                                                                           placenta. The implications of this are very
                                    10                                                                                                                           serious for the survival and health of both twins
















                                                                                                                                                                 and they would require close monitoring
                                                                                                                                                                 during the pregnancy.
          Embryology Authority change of policy in 2001,                                                                                                         Dizygotic (DZ) twins are also known as
          which reduced the maximum number of                                                                                                                    fraternal, non-identical or binovular. DZ twins
          embryos being transferred to the mother in
          each IVF cycle to two. Scotland has the
          highest multiple birth rate
are conceived when the mother produces two or
more eggs in the same menstrual cycle which are then
fertilized separately.

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       Diagram reproduced with permission from the Multiple
same sex or different sexes. A mother who has al-          Births Foundation
ready had one set of DZ twins runs an increased risk
(approximately five fold) of having a second set.          DNA testing provides a 99% accuracy test for
                                                           zygosity using a DNA sample collected from inside the
Triplets and higher order children can be monozy-          babies’ mouth with a swab. The samples are then
gotic, dizygotic or a mixture of both.                     examined for specific markers and monozygotic twins
In the UK, one third of all twins are monozygotic and      will have identical DNA maps while dizygotic twins will
two thirds dizygotic twins.                                have different ones. This test is not generally
                                                           available on the NHS but details for obtaining a
How can you tell if the twins are monozygotic or           private test can be found on the Multiple Births
dizygotic?                                                 Foundation website.
The simplest way is by looking at the children. All dif-
ferent sex multiples are DZ and in older same sex          For more information
children a comparison of the colour and the texture of     Tamba aims to provide high quality information and
hair, the colour of the eyes, the shape of the ears,       mutual support networks for families of twins, triplets
teeth eruption pattern and formation and the chil-         and more, highlighting their unique needs to all
dren’s fingerprints often give a good indication of zy-    involved in their care.
gosity. However with babies this isn’t always easy to
see and other methods can be more reliable.                Twins and Multiple Births. The Essential
                                                           Parenting Guide From Pregnancy To Adulthood
The babies’ placenta(s) are helpful in determining         by Dr Carol Cooper, Vermillion
zygosity as well as the amnion or inner sac and the        Expecting More than One Tamba booklet -
chorion or outer sac surrounding the babies (see           information and practical advice for parents expecting
diagram). There are three different types of placenta –    multiples
separate, fused or single.                                 Expecting more than One Tamba video - parents of
Separate placentas often indicate that the twins are       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 complicate          professionals working with multiple birth families
things further these two placentas sometimes may           0208 383 3519
remain separate or fuse together.
Fused placentas sometimes occur when two sepa-             The UK Twin to Twin Transfusion Syndrome
rate ones become implanted very close together in          Association
the womb. This can happen with either dizygotic or
monozygotic twins. Fused placentas sometimes                If you wish to discuss any of the issues raised in
cause confusion because they give the appearance           this leaflet please contact the Tamba office or
of a single one.                                           Twinline, Tamba’s freephone helpline, a national,
Single placentas only occur in the case of monozy-         confidential, listening and information service for
gotic twins. With single placentas there will always be    all parents of twins, triplets and more, and the
one chorion (outer sac) around both babies but             professionals involved in their care (see details at
there may be either one or two amnions (inner sacs)        the bottom of the first page).
depending on when the fertilized egg splits.

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