The Different Types of Twins Explained

It took me several years working as an obstetrical nurse to really understand the different types of twins.

Fraternal versus identical twins: what’s the difference?

Fraternal (or non-identical) twins are formed by the fertilization of two eggs by two different sperm.  These babies will have their own placenta and their own amniotic sac.

Identical twins are formed from the splitting of one embryo.  They can have separate placenta and amniotic sac, same placenta and separate amniotic sacs, or same placenta same amniotic sac.  I know, Confusing right!? Don’t worry, stay with me- it is all going to become clear by the end of this article!

The three main types

1. Dichorionic/Diamniotic (di/di)

The most common type of twins is Dichorionic/Diamniotic (di/di).  Di/di twins are the lowest risk type of twins as both babies have their own placenta and amniotic sac.  Basically, they are just sharing your womb and nothing else. Di/di twins can be either fraternal or identical.

If a mom pregnant with di/di twins has not gone into labour on her own, and there are no other health concerns with mom or babes, most health care providers are happy to wait until 37-38 weeks before considering induction of labour.

2. Monochorionic/Diamniotic (mono/di)

Monochorionic/Diamniotic (mono/di) is the next most common type of twins.  These babies share the same placenta but have separate amniotic sacs, therefore always being identical.  Mono/di twins are considered mid-risk in the twin world due to the risk for twin-to-twin transfusion syndrome (TTTS).  TTTS is a condition where blood flows from one twin to the other. This results in one baby getting to much blood and the other baby not getting enough.  TTTS can an affect the health of both babies.

Health care providers will discuss options with moms pregnant with mono/di twins if they have not gone into labour by 36 weeks.  Since the placenta is shared, the concern is the placenta may not be able to continue to function well enough. The placenta needs to supply both babies with all the nutrients and blood flow needed past this gestation.

Di/di and mono/di twins can be delivered vaginally depending on the position of the babies and the policies at the hospital where they will be delivered.  

 3. Monochorionic/Monoamniotic (mono/mono)

The third type of twins we see are Monochorionic/Monoamniotic (mono/mono).  Mono/mono twins are the rarest and the highest risk type. These babies are always identical as they share the same amniotic sac and placenta. Along with the risk for TTTS, these babies are also at a high risk for entanglement since they share an amniotic sac.  As the babies move around, their umbilical cords are at risk of becoming tangled and compressed. This would result in less nutrients and blood flow through the cord and could become fatal to one or both babies.  

Health care providers would recommend a Cesarean section around 34-35 weeks gestation for mono/mono twins.  Recently, the survival rates for mono/mono twins are much higher due to a laser treatment that can be done to create a barrier between the babies inside the amniotic sac.  


Whatever the route is for delivery, the babies will be closely monitored and the birth may be more medically managed then if you were giving birth to one baby.  Health care providers will continuously monitor the babies during labour. You will have IV access in place. An epidural may be recommended for pain management, especially if the position of second twin is not head down as the health care provider will be able to assist with the twin’s delivery much easier and quicker if an epidural is in place.  Occasionally, one twin may be born vaginally, but the second twin is born by Cesarean section. This is not common, but can happen.


Twin pregnancies always have increased risk over singleton pregnancies.  No matter what type of twins you are pregnant with there is an increased risk for preterm labour.  According to Wikipedia, around half of all women expecting twins gives birth before 37 weeks. With the increased demand on the body, twin mamas are at a higher risk for all the things that come with any pregnancy, especially increased blood pressures, risk for premature births, low birth rates.  To make sure both babies are growing adequately, twin mamas are followed closely by their health care provider and by ultrasound.

If you are pregnant with twins reach out to a Mama Coach in your area for a private tailored prenatal class!!  Any one of us would be happy to help you feel prepared for the rest of the pregnancy, delivery, and bringing home 2 babies!!

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