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Kindly sponsored by:- 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 land 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 ties (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 15 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 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 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. www.tamba.org.uk 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 www.multiplebirths.org.uk 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 www.twin2twin.org 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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