The language, words and terminology used when referring to multiples can be confusing. Here, we help you with an A to Z guide on common terms used in the world of multiples.
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A to Z of multiples explained
The amnion is a membrane that closely covers the embryo when first formed. It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo or fetus. The number of amniotic sacs is important in determining the type of multiple pregnancy. See DCDA and MCDA twin pregnancy
The outermost of the two fetal membranes (the amnion is the innermost) that surround the embryo. The chorion develops villi (vascular finger-like projections) and develops into the placenta.
Refers to the number of placentae of a pregnancy. This is most accurately determined between weeks 6 and 9 of gestation, sometimes up to week 12. Because twins and multiples are usually described according to two main things;  how many eggs they develop from and  if they share a placenta in their mother’s womb, chorionicity is an important consideration in determining twin type. See Dichorionic Diamniotic (DCDA), Monochorionic diamniotic (MCDA)
and Monochorionic monoamniotic (MCMA)
The age of your babies since birth. Your childrens' birthdays are celebrations of their chronological age.
Corrected age (or adjusted age) is the age that premature babies would be if they had been born on their due date. For example, babies who were born two months early (who now have a chronological age of six months) have a corrected age of four months. Corrected age is useful while following your babies' growth and development. For most premature babies, you will only need to correct their age until they are aged two or three years (corrected!), as by this time most children have caught up developmentally to their peers.
The medical term for twins formed from the simultaneous fertilisation of two eggs by two separate sperm. See fraternal twins
Dichorionic Diamniotic (DCDA)
A DCDA twin pregnancy is where each twin has its own placenta and amniotic sacs.
A heritable change in gene expression without changes to the DNA sequence. What this means in practice is that all of the cells in our bodies have exactly the same DNA within them, but they can be expressed in very different ways. For example, one cell may turn into a liver cell, another into a muscle cell and another into a fat cell, despite all of them having identical DNA. The difference in how these cells develop is due to epigenetic influences.
Fetal Growth Restriction (FGR)
Fetal growth restriction is an obstetrical complication meaning that a baby is not growing well in the womb. It is often defined as a fetal weight lower than the tenth percentile for a particular gestational age. FGR is also known as Intrauterine Growth Restriction
and is sometimes referred to as small for gestational age, or SGA.
The common term for twins formed from the simultaneous fertilisation of two eggs by two separate sperm. See dizygotic twins
Higher Order Multiples / Gestations (HOM)
Higher-order multiples or gestations are multiple pregnancies with three or more fetuses. Common terms are triplets, quads, quins, etc, depending on the number of fetuses.
3 fetuses = triplets
4 fetuses = quadruplets / quads
5 fetuses = quintuplets / quins
The common term for twins formed from the fertilisation of one egg by one sperm and where the embryo then splits in two. See monozygotic twins
Intrauterine Growth Restriction (IUGR)
Intrauterine growth restriction is an obstetrical complication meaning that a baby is not growing well in the womb. It is often defined as a fetal weight lower than the tenth percentile for a particular gestational age. IUGR is also known as Fetal Growth Restriction
and is sometimes referred to as small for gestational age, or SGA.
Kangaroo care or skin-to-skin care is a special way both mums and dads can spend time holding their babies. Babies wear only a nappy and are placed in an upright position directly on their dad’s bare chest or between mum’s bare breasts. The baby’s head will be turned to the side and then a blanket is placed on top.
Monochorionic diamniotic (MCDA)
MCDA twins share a single placenta but have separate amniotic sacs. MCDA twins are a subtype of a monozygotic twin pregnancy.
Monochorionic monoamniotic (MCMA)
Monochorionic twins share a single placenta and are in a single amniotic sac. A very rare occurence, they are also referred to as MoMo twins. MCMA twins are a subtype of a monozygotic twin pregnancy.
The medical term for twins formed from the fertilisation of one egg by one sperm and where the embryo then splits in two. See identical twins
Neonatal or newborn intensive care unit
A neonatal or newborn intensive care unit (NICU) combines advanced, life-supporting equipment with trained health care professionals, designed to meet the unique needs of premature and sick newborns. Some of the babies are critically ill, while others may need specialised care and observation as they grow. NICUs have specialist doctors, nurses, other professionals and equipment to care for premature and sick babies.
The term used to describe when a baby is born early. For most women, pregnancy lasts around 40 weeks. Babies that are born between 37 and 42 weeks are considered full-term and babies born before 37 weeks are considered premature. Other terms often used for prematurity include ‘premmie’, ‘prem’ and ‘pre-term’.
The practice of reducing the number of fetuses in a multiple birth pregnancy. For example, reducing quadruplets to a twin or singleton pregnancy. The procedure is also called multifetal pregnancy reduction.
A term used to refer to a single baby born as a result of a single, non-multiple pregnancy.
A special care nursery (SCN) is where newborn babies can be cared for by specially trained staff. Your babies may need this care even if they can maintain their own body temperature and generally breathe on their own.
Twin-to-twin transfusion syndrome (TTTS)
Twin-to-twin transfusion syndrome is a condition of the placenta that develops only with identical twins that share a placenta. Blood vessels connect within the placenta and divert blood from one fetus to the other. It happens in about 3 in 20 twins with a shared placenta.
For twins, zygosity refers to the degree of genetic similarity within each pair. Zygotes are produced by fertilisation between two haploid cells; the ovum and the sperm cells. If twins are the same sex and each had a placenta or if there is doubt about the type of placenta, then they could be monozygotic (identical
) or dizygotic (fraternal
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