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Multiple Pregnancy

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What is a multiple gestation pregnancy?

A multiple gestation pregnancy (multifetal gestations) is a pregnancy with twins, triplets or more babies. Doctors describe these pregnancies based on the number of placentas (a baby’s oxygen and blood supply, also called chorions) and amniotic sacs (space that contains the baby and amniotic fluid) in the pregnancy. These categories include:

  • Dichorionic-diamniotic twins (di/di twins): Each baby has its own placenta and amniotic sac. These twins can be identical or nonidentical. They have limited effect on each other except for sharing the same womb.   
  • Monochorionic-diamniotic twins (mo/di twins): The babies each have their own amniotic sacs but share one placenta. These twins result from a single fertilized egg, meaning they are genetically identical.
  • Monochorionic-monoamniotic twins (mo/mo twins): The babies share one amniotic sac and one placenta. These twins result from a single fertilized egg, meaning they are genetically identical.
  • Conjoined twins: Identical twins who are joined at one or more parts of their body. This is a rare form of a monochorionic-monoamniotic pregnancy.  
  • Higher-order multiples: A pregnant person is carrying three or more babies. These pregnancies will have different combinations of placentas and amniotic sacs.

Generally, the more babies in a pregnancy, the higher the risks. Potential complications of multiple pregnancies may include:  

  • Fetal growth restriction: This condition describes when babies don’t grow at the expected rate during the pregnancy. In monochorionic pregnancies, this is often referred to as selective fetal growth restriction.
  • Congenital anomalies: These are structural or functional problems of development present at birth.
  • Prematurity: This is defined as delivery of the pregnancy before 37 weeks. The more babies, the earlier delivery usually occurs.
  • Twin-twin transfusion syndrome (TTTS): TTTS occurs in monochorionic pregnancies in which the placenta develops abnormal blood vessel connections between the babies, leading to serious complications.  
  • Twin anemia-polycythemia sequence: This is a rare complication of monochorionic pregnancies where one baby has a very low amount of blood cells (anemia) and the other has a very high amount of blood cells (polycythemia). 
  • Stillbirth: A stillbirth is one in which the baby dies during pregnancy or delivery. This can occur due to many different factors during the pregnancy such as cord entanglement in monoamniotic pregnancies. The cause is not always understood.

Multiple gestation pregnancies also carry risks for the pregnant person, including:

  • Gestational diabetes: This is high blood sugar that may require treatment. Without treatment, this condition can be dangerous to the pregnant person and babies.
  • High blood pressure/hypertension: High blood pressure in pregnancy can lead to a condition called preeclampsia.

People who are pregnant with multiples also have increased risks during delivery, including bleeding and a higher chance of cesarean section (C-section).

What causes a multiple pregnancy?

A multiple pregnancy occurs when either:

  • More than one egg gets fertilized (dizygotic). These babies are not identical.
  • One egg gets fertilized, then splits into two or more (monozygotic). These babies are identical.

Anyone can get pregnant with multiples, but it’s more likely after age 30. A multiple pregnancy is also more common among people who undergo fertility treatments and in certain families.

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