Monochorionic twins are

monozygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of Twin Last Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two e ...

(identical)

twins Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of Twin Last Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two e ...

that share the same

placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...

. If the placenta is shared by more than two twins (see

multiple birth A multiple birth is the culmination of a multiple pregnancy, wherein the mother gives birth to two or more babies. A term most applicable to vertebrate species, multiple births occur in most kinds of mammals, with varying frequencies. Such births ...

), these are monochorionic multiples. Monochorionic twins occur in 0.3% of all pregnancies. Seventy-five percent of

twin pregnancies are monochorionic; the remaining 25% are ''dichorionic diamniotic''. If the placenta divides, this takes place before the third day after

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

Amniocity and zygosity

Monochorionic twins generally have two

amniotic sac The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of biological membrane, membranes that hold a developing embryo (and l ...

s (called Monochorionic-Diamniotic "MoDi"), but sometimes, in the case of monoamniotic twins (Monochorionic-Monoamniotic "MoMo"), they also share the same amniotic sac. Monoamniotic twins occur when the split takes place after the ninth day after fertilization. Monoamniotic twins are ''always''

(identical twins).Pregnancy-Info -- > Monoamniotic Twins
Retrieved on July 9, 2009 Monochorionic-Diamniotic twins are always monozygotic.

Diagnosis

By performing an obstetric ultrasound at a gestational age of 10–14 weeks, monochorionic-diamniotic twins are discerned from dichorionic twins. The presence of a "T-sign" at the inter-twin membrane-placental junction is indicative of monochorionic-diamniotic twins (that is, the junction between the inter-twin membrane and the external rim forms a

right angle In geometry and trigonometry, a right angle is an angle of exactly 90 Degree (angle), degrees or radians corresponding to a quarter turn (geometry), turn. If a Line (mathematics)#Ray, ray is placed so that its endpoint is on a line and the ad ...

), whereas dichorionic twins present with a "lambda (λ) sign" (that is, the chorion forms a wedge-shaped protrusion into the inter-twin space, creating a rather curved junction). The "lambda sign" is also called the "twin peak sign". At ultrasound at a gestational age of 16–20 weeks, the "lambda sign" is indicative of dichorionicity but its absence does not exclude it. In contrast, the placentas may be overlapping for dichorionic twins, making it hard to distinguish them, making it difficult to discern mono- or dichorionic twins on solely the appearance of the placentas on ultrasound.

Complications

In addition to a shared placenta, monochorionic twins also have their

circulatory system In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart ...

s intermingled in random and unpredictable circulatory anastomoses. This can cause disproportionate blood supply, resulting in twin-to-twin transfusion syndrome (TTTS) in 20% of MoDi pregnancies. This is the main complication of monochorionic twins. The 80% of MoDi pregnancies without TTTS still have high rates of birth weight discordance, fetal growth restriction, prematurity and resultant cesarean section deliveries. One twin may also fail to develop a proper heart and become dependent on the pumping activity of the other twin's heart, resulting in

twin reversed arterial perfusion Twin reversed arterial perfusion sequence, also called TRAP sequence, TRAPS, or acardiac twinning, is a rare complication of monochorionic twin pregnancies. It is a severe variant of twin-to-twin transfusion syndrome (TTTS). In addition to the twi ...

. If one twin dies in utero, blood accumulates in that twin's body, causing

exsanguination Exsanguination is the loss of blood from the circulatory system of a vertebrate, usually leading to death. The word comes from the Latin 'sanguis', meaning blood, and the prefix 'ex-', meaning 'out of'. Exsanguination has long been used as a met ...

of the remaining twin. In the case of monoamniotic twins the risk of complications is substantially higher because of additional potential umbilical cord entanglement and compression. However, the

perinatal mortality Perinatal mortality (PNM) is the death of a fetus or neonate and is the basis to calculate the perinatal mortality rate. ''Perinatal'' means "relating to the period starting a few weeks before birth and including the birth and a few weeks after bi ...

of monochorionic twins is fairly low.

References

{{Pathology of pregnancy, childbirth and the puerperium Twin Reproduction in mammals Zoology

Amniocity and zygosity

Diagnosis

Complications

See also

References