Information for families - Sunnybrook Twins Research Centre
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Information for families

You’re expecting twins, now what?

The Twins Research Centre strives to empower and educate families about their twin pregnancy, labour and delivery. In this area, we’ll provide an overview of common terms you will hear, as well as some common questions and address concerns you may have with your twin pregnancy.

Pregnant woman doing research

Learning a new language: Common medical terms


This refers to the genetic relationship between your twins. It stems from “zygote” which refers to the single fertilized egg. There are two options:

  • Monozygotic twins, also known as identical twins, form when a single fertilized egg splits, resulting in the development of two individual embryos. Because they originate from the same set of cells, these individuals have the same DNA and often have remarkably similar physical appearances.
  • Dizygotic twins, also known as fraternal twins, form when multiple eggs are fertilized and develop. They may look alike or they may look completely different, the same as any siblings would.


Opposite-sex twins (male and female) are always dizygotic. However, in the case of same-sex twins, determining whether they are identical (monozygotic) or non-identical (dizygotic) may require additional information including ultrasounds findings regarding chorionicity (as described below), blood type or DNA analysis.


This refers to whether your babies share a placenta. It’s important to find out early on during your pregnancy as babies who share a placenta put you and your babies at a higher risk of complications. Dichorionic means each baby has a separate placenta and is inside a separate sac which has its own outer membrane. Monochorionic means the babies share a placenta and outer membrane.

Relationship between zygosity and chorionicity

Dizygotic (fraternal) twins would always be dichorionic, since each twin originates from a separate fertilized egg, each developing into a fetus with its own membranes and placenta. In contrast, monozygotic (identical) twins may be either dichorionic (2 placentas) or monochorionic (single placenta), depending on when the egg splits.


When the egg splits at a very early stage (usually within first three days following fertilization, and before the placenta begins to form), there is the possibility of each baby having its own membranes and placenta, which results in dichorionic twins. However, when the egg splits at a later stage (after day three, when the placenta has already began to from), the resulting twin will ‘need’ to share the placenta and membranes that were already formed prior to the switch, resulting in monochorionic twins.


Rarely, the egg splits at an even later stage, after the fetal body started to develop, in which case the two twins will be sharing the body parts that were already formed at the time of split, resulting in conjoined twins. This is illustrated in the figure below.

A common mistake is that dichorionic twins, where there are two placentas, are necessarily non-identical twins. This is not always the case, as about a third of the identical (monozygotic) twins will be dichorionic, as illustrated in the figure below.

 Twin Gestations: Dizygotic (fraternal) - Have one Chorionicity: Dichorionic (2 placentas). Risks: Preterm birth, Preeclampsia, Fetal growth restriction, Gestational diabetes Monozygotic (identical) - Have four Chorionicitys: Dichorionic (2 placentas) ~ 1/3
Risks: Preterm birth, Preeclampsia, Fetal growth restriction, Gestational diabetes Monochorionic diamniotic (1 placenta, each fetus with its own inner sac) ~ 2/3 Risks: Same as dichorionic twins, Twin-to-twin transfusion syndrome (TTTS), Twin anemia polycythemia sequence (TAPS), Selective fetal growth restriction (sIUGR) Monochorionic monoamniotic (1 placenta, both babies within the same inner sac) ~1% Risks: Same as monochorionic diamniotic twins, Cord entanglement Conjoined (Siamese twins) - rare

Why am I considered as having a “high risk” pregnancy?

When you’re carrying twins, you’re at greater risk for a number of complications. Because of this, you will be followed by an obstetrician or a special ‘high risk’ obstetrician called a “maternal fetal medicine specialist” or MFM. You are at a higher risk for:


  • Premature birth – this means delivering your twins before the end of 36 weeks of pregnancy. This happens in roughly half of all twin pregnancies. Premature babies may have more health problems.
  • Low birth weight – More than half of twins are born weighing less than 5 1/2 pounds, which is considered “low birth weight” or LBW. Babies born at a low birth weight are at increased risk of health problems after birth, such as vision and hearing problems, mental disabilities and cerebral palsy (with the risk greatly increasing if babies are born before 32 weeks or weighing less than 3 1/3 pounds).
  • Twin-twin transfusion syndrome or TTTS – This condition develops when a connection between the babies’ blood vessels allows one baby to get too little blood and the other too much. This complication is unique to monochorionic twins who have a single placenta and develops in approximately 10 to 15 per cent of these pregnancies. A doctor can treat TTTS with laser surgery to seal off the vessel connection or with amniocentesis to drain excess amniotic fluid.
  • Preeclampsia – This is a pregnancy complication characterized by high blood pressure and signs of damage to the liver and/or kidneys. You may have swelling, headaches and vision changes, in addition to the high blood pressure. If not treated, preeclampsia can deprive your baby of oxygen and nutrients and can also damage your organs.
  • Gestational diabetes – This condition means developing diabetes during your pregnancy and may result in your babies growing too big. Delivering a large baby increases your risk of labour and delivery complications, and your babies may have breathing problems and low-blodo sugar when they’re born.


This all may sound pretty scary, but the good news is you and your health care team can lower your risk and identify any problems earlier, rather than later. You can then work with your health care providers to manage and minimize the impact of any complications.

Do twin fetuses grow differently than singletons?

This is a common question from parents expecting twins. Twins’ growth rate is usually the same as a single pregnancy until around weeks 30 to 32, when they do slow down a little as the babies are competing more for nutrients. Your health care team will keep a close eye on your babies’ growth throughout the pregnancy. Keep in mind, your babies may weigh less as it’s more likely they will be born earlier.

Is it okay to exercise when I’m pregnant with twins?

There isn’t a lot of evidence to link restricting exercise and physical activity, or leaving work early, with preventing preterm labour in women pregnant with twins. However, some physicians recommend that women pregnant with twins should restrict physical exercise after 28 weeks.


We routinely monitor the length of your cervix during pregnancy. Having a cervical shortening increases the risk of preterm birth. In cases in which the cervix is found to be short or in the case of preterm contractions we will advise that you restrict physical activity.

How should I give birth – vaginal delivery vs. Caesarian-section?

The answer about how you will deliver your twins depends on several factors. The primary factor is the way the first twin to come out is positioned within your uterus. When the first twin is in a breech presentation, delivery should be via C-section.


When the first twin has their head down, the best way to deliver has been a matter of debate between obstetricians over the years. Some believe C-section is safer. Others recommend vaginal delivery only if the second twin is also positioned with the head down, and C-section if the second twin is in the breech position. Many other obstetricians believe that vaginal delivery is the best choice when the first twin is with the head down, regardless of the position of the second twin.


A large randomized controlled trial, led by Dr. Barrett (the Twin Birth Study) was conducted specifically to address this point. The trial provided evidence that as long as the first twin is with the head down, vaginal delivery is as safe as a C-section, and this is the approach that is taken at Sunnybrook.


A vaginal twin birth is somewhat different that the delivery of a single baby. The delivery of the first twin is essentially similar to that of a normal delivery in a singleton pregnancy. The main difference is that once the first twin is out, there is a small risk of complications for the second twin, including cord prolapsed, detachment of the placenta (placental abruption) due to the sudden decompression of the uterus after the delivery of the first twin, or change in the position of the second twin. Because of that, there is a risk of approximately five per cent that an urgent C-section will be needed for the second twin. For that reason, vaginal twin birth always takes place in the operating room, with the team being set up for urgent C-section if needed.

Are there good online calculators I can use for my pregnancy and determining potential due dates?

Check out this calculator from