The Multiple Birth Dictionary. An overview of common multiple birth terms.

twin terminology

Please note:  This is recommended to be used as an overview only and has been compiled from a variety of web resources. For further clarification on any term and any medical advice, please speak to your health care professional.  

Some terms are more ‘general’ baby terms however I have been requested to include them.

Please contact us if you would like a term added or updated. Thank you.


Adjusted age – See ‘corrected age’

Advanced maternal age – Pregnancy when the mother is 35 years old or older.  Sometimes referred to as a ‘geriatric pregnancy’.

AMBA – Australian Multiple Birth Association was formed as a volunteer organisation in 1974. AMBA is the only national charity in Australia focused on improving the lives of twins, triplets or more and their families.

Amniotic sac – a bag of fluid inside a woman’s womb (uterus) where the unborn baby develops and grows. It’s sometimes called the “membranes” because the sac is made of 2 membranes called the amnion and the chorion.


Breech – A breech baby is in a bottom-down position.

BLW – Baby-led weaning (BLW) is a method of adding complementary foods to a baby’s diet of breastmilk or formula. A method of food progression, BLW facilitates the development of age appropriate oral motor control while maintaining eating as a positive, interactive experience. Baby-led weaning allows babies to control their solid food consumption by “self-feeding” from the very beginning of their experiences with food.


Cephalic – When the baby lies within the uterus with its head facing the birth canal

Chronicity – The type of twins. Knowing the chronicity of twins can help assess risk and determine a treatment plan to provide proper prenatal care for an optimal outcome of the pregnancy.

Chronological age – A baby’s age as determined by counting the number of days, weeks, or years from the day of birth.

Conjoined Twins – Identical twins (Monozygotic twins) joined in utero.

Cryptophasia – The secret language of twins

Corrected age – Corrected age (or adjusted age), is your premature baby’s chronological age minus the number of weeks or months he was born early. For example, a one-year-old who was born three months early would have a corrected age of nine months.


DCDA – In a dichorionic diamniotic (DCDA) twin pregnancy, each twin has its own chorionic and amniotic sacs. DCDA twins are usually dizygotic but can also be monozygotic.

DDH – Developmental dysplasia of the hip (DDH) is an abnormal development of the hip joint.

DiDi twins – Dichorionic Diamniotic twins (see Dichorionic Diamniotic (DCDA) twins for more information).

DVP – The deepest vertical pocket (DVP) depth is used to assess amniotic fluid volume during an ultrasound.

Diamniotic Twins – Twins who have developed in two separate amniotic sacs. These twins can be either or monozygotic or dizygotic.

Dichorionic Twins – see DCDA

Dizygotic Twins – Twins formed from two different zygotes. Commonly referred to as ‘fraternal twins’.

Dizygous Twins – See Dizygotic twins

multiple birth dictionary


EBF – Exclusively breastfeeding

EBM – Expressed breast milk

EFWD – Estimated foetal weight discordance 

Embryo Reduction – See ‘selective reduction’.

EP – Exclusively pumping

Evil Twin – The evil twin is an antagonist found in different fictional genres.


Fetal Reduction – See ‘selective reduction’.

Fetus Papyraceous – a mummified parchment-like remains of a dead twin (fetus) that is retained in-utero after intrauterine death in the second trimester.

FF – Formula fed / Formula feeding

Footling breech – when one or both of the baby’s feet point downward and will deliver before the rest of the body.

Fraternal Twins – See Dizygotic Twins

FTM – First time Mum      OR

FTM – Full time mum (See SAHM for more details)


GDM – Gestational diabetes mellitus (GDM) is diabetes first recognised during pregnancy

GTT – The glucose tolerance test (GTT) is a medical test. The pregnant mother drinks a glucose drink and then has a blood test which measures how quickly the glucose is removed from the blood.


HOM – Higher Order Multiples (HOMs) are triplets, quadruplets, quintuplets or more.

multiple birth dictionary


Identical twins – develop from one zygote, which splits and forms two embryos.

IUGR –   Intrauterine Growth Restriction is a condition where the unborn baby is smaller than it should be because it is not growing at a normal rate inside the womb.


Marginal cord insertion – In Marginal Insertion of Umbilical Cord, the umbilical cord inserts into the edge of the placental disc of the developing foetus.

MCA Doppler – Foetal middle cerebral arterial (MCA) Doppler assessment is used to assess and foetal cardiovascular distress.

MCDA  – A monochorionic diamniotic (MCDA) twin pregnancy is a subtype of monozygotic twin pregnancy. These foetuses share a single chorionic sac but have two amniotic sacs and two yolk sacs. 

MCMA – A monochorionic monoamniotic (MCMA) twin pregnancy is a subtype of monozygotic twin pregnancy.  These foetuses share a single chorionic sac, a single amniotic sac and, in general, a single yolk sac.

MFM – A maternal-foetal medicine (MFM) specialist is a doctor who helps take care of women having complicated or high-risk pregnancies, such as multiple birth pregnancies.

Mirror Image – A type of twinning that may show characteristics with reversed asymmetry. This may include opposite dominant handedness, dental structure, or even organs.

MoDi twins – Monochorionic-Diamniotic twins (see Monochorionic-Diamniotic  (MCDA) twins for more information).

Monozygotic – See MZ

MoM – Mother of multiples

MoMo twins – Monoamniotic twins (see Monoamniotic (MCMA) twins for more information).

MZ – A monozygotic (MZ) twin pregnancy results from the division of single zygote following fertilisation and shares similar genetic materials. These twins are therefore always of the same gender.


PPROM – Preterm Premature Rupture of Membranes during pregnancy (see PROM)

Presenting twin – The twin that will be born first.

Preeclampsia – A pregnancy complication characterized by high blood pressure and signs of damage to another organ system, most often the liver and kidneys.  The possibility of this condition occurring is increased in a multiple birth pregnancy.

PROM – Premature Rupture of Membranes during pregnancy occurs when the membraned sac holding your baby and the amniotic fluid breaks open before you’re actually in labour

nicknames for twins in utero


SAHD – Stay At Home Dad. When the father stays home to look after the children and the mother works to financially support the family.

SAHM – Stay At Home Mum.  Traditionally, SAHM refers to the mother staying at home with the kids while the father works to financially support the family.

Selective Reduction – the practice of reducing the number of fetuses in a multiple birth pregnancy.

Singleton – A single child born from a single pregnancy

sIUGR – Selective intrauterine growth restriction occurs when there is unequal placental sharing which leads to suboptimal growth of one twin.

SNS – The Supplemental Nursing System allows supplemental nutrition to babies while they are being breastfed.

Superfetation – the simultaneous occurrence of more than one stage of developing offspring in the same animal. It is not believed that it occurs naturally in humans. There have been 10 reported cases of possible superfetation in humans.

Superfecundation –  The fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse, which can lead to twin babies from two separate biological fathers.


Tandem breastfeeding – The Australian Breastfeeding Association uses the term ‘tandem feeding’ to describe concurrent breastfeeding of siblings who are not twins. The children may feed together or at separate times. However in the multiple birth community this term is colloquially used when referring to the simultaneous breastfeeding feeding of twins.

TAPS – Twin anemia polycythemia sequence (TAPS) is a form of 
Twin to twin transfusion syndrome that can complicate monochorinic twin pregnancies. Unequal blood counts cause the twins to suffer from an imbalance in red blood cells and haemoglobin.

TOPS – Twin oligopolyhydramnios sequence.  The main diagnostic criterion for TTTS is the presence of oligohydramnios in the donor twin and polyhydramnios in the recipient twin, the so-called twin oligopolyhydramnios sequence (TOPS).

TRA – Twins Research Australia brings twins and researchers together for vital health research that benefits everyone.

Transverse Lie – When the baby is lying sideways across the stomach, rather than in a head-down position.

Trizygotic – Trizygotic or fraternal triplets are formed when three separate eggs are fertilised by three separate sperm.

TTTS – Twin-twin transfusion syndrome (TTTS) is a rare, serious condition that can occur in pregnancies when identical twins share a placenta. Abnormal blood vessel connections form in the placenta and allow blood to flow unevenly between the babies. One twin − called the donor – becomes dehydrated; and the other − called the recipient − develops high blood pressure and produces too much urine and over fills the amniotic sac. 

Twin 1 – Antenatally twins are labelled as 1 or 2 according to their location relative to the birth canal during prenatal ultrasound.

Twin A – Antenatally twins are labelled as A or B according to their location relative to the birth canal during prenatal ultrasound.

Twin growth discordance – a term used to describe a significant size or weight difference between the two foetuses of a twin pregnancy.

Two vessel cord – Most babies’ umbilical cords have three blood vessels: one vein, which brings nutrients from the placenta to baby, and two arteries that bring waste back to the placenta. A two-vessel cord has just one vein and one artery.


UA Doppler – Umbilical arterial (UA) Doppler assessment is used in surveillance of fetal well-being in the third trimester of pregnancy to monitor the babies for IGUR or the mother for suspected pre-eclampsia.


Vanishing Twin – A vanishing twin, also known as fetal resorption, is a fetus in a multi-gestation pregnancy which dies in utero and is then partially or completely reabsorbed.

Velamentous cord insertion – is diagnosed when the umbilical vessels insert into the membranes before they reach the placental margin.


Zygosity – the genetic relationship between two twins.

Zygote – the union of the sperm cell and the egg cell. Also known as a fertilized ovum


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