Birth Definitions

Monochorionic Twins Definition


Monochorionic twins are a type of identical twins where both babies share the same placenta. This happens when the fertilized egg splits into two separate embryos early on in development, usually within the first week. 

Monochorionic twins are at higher risk for certain complications during pregnancy and delivery than other types of twins due to unequal sharing of blood, blood volume, placental nutrients, or a combination of these. These include:

1) Monoamniotic Twins: Also known as “mono mono” twins, these babies share not only the same placenta but also an amniotic sac. This means they have little room to move around and may become tangled in each other’s umbilical cords, which can be dangerous or even fatal. Monoamniotic twins occur in about 1% of all monozygotic pregnancies (identical twin pregnancies).

2) Twin-to-Twin Transfusion Syndrome: In this condition, blood flow from one fetus to another is unequal. The fetus with less blood flow (the “donor”) becomes anemic while the fetus with too much blood flow (the “recipient”) can develop heart problems and high levels of bilirubin in their bloodstream. Twin-to-twin transfusion syndrome occurs in about 10% – 15% of all monozygotic pregnancies complicated by a shared placenta (monochorionic). It is thought to be caused by abnormal vascular connections between fetuses sharing a single placenta.

3) Higher-Order Multiples: When more than two embryos split from a single fertilized egg, it’s called higher-order multiple gestation or HOMG. A HOMG pregnancy can involve three or more fetuses sharing one chorion, which again puts them at increased risk for developing twin-to-twin transfusion syndrome. 

4) Premature Delivery: Because monochloric multiples are often born prematurely, they may have developmental delays or require special medical care after birth due to their premature status.

5) Congenital Anomalies: Identical twins who share a chorion are also at increased risk for having congenital anomalies. Some examples include cleft lip/palate, neural tube defects, cardiac abnormalities, and limb deficiencies.

This image shows how often the term ‘Monochorionic Twins’ is used in relation to other, similar birth terms:

6) Intrauterine Growth Restriction: One common complication seen with monozygotic multiples is intrauterine growth restriction (IUGR), where one baby doesn’t grow as well as expected compared to their co-twin  

7) Asymmetric IUGR refers specifically to when one baby has severe IUGR while the other remains relatively unaffected.

8) Fetal Death : Unfortunately, fetal death is also common among monozygotic multiples, occurring in about 20% – 30% of cases. In most instances, only one fetus is affected and the other baby goes on to be born healthy. 

9) Chorionic Hematoma: A chorionic hematoma is a collection of blood within the fetal membranes (the chorion). It can occur in any pregnancy but is more common in twins and higher-order multiples. Chorioamniotic hemorrhage/hemorrhage into amniotic sacs (CAA/CAM), which are related conditions, can also occur. These usually happen when there’s trauma to the placenta or umbilical cord during delivery 

10) Placental Abruption: This occurs when part or all of the placenta separates from the uterine wall before delivery and can be life-threatening for both mother and baby. Placental abruption occurs more often in twin pregnancies, especially later on in pregnancy.

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