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.
Twin pregnancies can be described or classified in two ways. The first is in relation to zygosity (the initial number of fertilized eggs that resulted in a twin pregnancy), and the second is in relation to placentation (the number of placentas and amniotic sacs).
Zygosity refers to the genetic relationship between your twins. It stems from “zygote,” which refers to the single fertilized egg. There are two types of zygosity:
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 ultrasound findings regarding chorionicity (as described below), blood type, or DNA analysis.
Placentation refers to the number of placentas (or chorionicity) and amniotic sacs (or amnionicity) that are present in the twin gestation.
Chorionicity
Chorionicity refers to whether each of your babies has their own placenta (known as dichorionic twins) or whether they share one placenta (monochorionic twins). It’s important to find out early on during your pregnancy whether your babies share a single placenta since this puts you and your babies at a higher risk of complications.
Amnionicity
Amnionicity refers to whether each of your babies is located within their own inner fluid-filled sac (known as diamniotic twins) or whether they share a single sac and thus have no membranes separating them (monoamniotic twins).
Relationship between zygosity, chorionicity, and amnionicity
Source: The Johns Hopkins Center for Fetal Therapy
Dizygotic (fraternal) twins would always be dichorionic and diamniotic, 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 share the placenta or amniotic sac, depending on when the single zygote splits, since the two babies will have to share all the structures that developed before the split occurs. Thus, if the egg splits at a very early stage (usually within the first three days following fertilization, before the placenta begins to form), each baby will have its own placenta and amniotic sac, which results in dichorionic diamniotic twins. If the egg splits at a later stage (between days 3-8, when the placenta has already begun to form but the amniotic sac has not), the resulting twins willshare the same placenta, resulting in monochorionic diamniotic twins. If the split happens even later (between days 8-13, at which time the amniotic sac was already formed), the two babies will share not only the placenta but also a single sac, resulting in monochorionic monoamniotic twins. Finally, in the rare event that the egg splits at an even later stage (after day 14, when the baby’s body has already started to develop), the two twins will share not only the same placenta and amniotic sac but also the body parts that were already formed at the time of the split, resulting in conjoined twins.
Both care providers and couples with twins occasionally get confused and assume that dichorionic twins (who have 2 separate placentas) are always non-identical (i.e., dizygotic). However, as described above and seen in the figure below, this is not always the case, and about a third of identical (monozygotic) twins will be dichorionic.
Twin pregnancies account for 1-2% of overall pregnancies. The incidence of triplets (and higher-order multifetal pregnancies such as quadruplets) is much lower, at approximately 0.08%.
The incidence of twin pregnancies is calculated by the number of twin pregnancies out of overall pregnancies per a given time period. It can also be estimated by Hellin’s Law (named after the German statistician Wilhelm Hellin), which is a mathematical formula. It says that for every 89 singleton prengnacies, there will be 1 twin pregnancy (1:89 ratio), for every 89 twin pregnancies there will be 1 triplet pregnancy etc.
In Canada, about 6000 sets of twins are born each year. In Sunnybrook, we take care of about 350 mothers with twins every year.
You are at increased risk of having a twin pregnancy if you:
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.
About 60% of twin babies will be born preterm (before 37 weeks of gestation). Being born early, especially before 32 weeks, can result in both short- and long-term complications. It’s not clear why twin babies are born early, but it’s believed to be, at least in part, due to the over-distension (or extra stretching) of the uterus.
Epidemiology
Below are some statistics adapted from Martin JA et al. (2021) showing the incidence of preterm birth and preeclampsia, as well as birthweight and gestational age at birth, by type of pregnancy.
| Outcome | Singletons | Twins | Triplets |
| Preterm birth<37 weeks | 8% | 60% | 99% |
| Preterm birth <34 weeks | 2% | 15-20% | 63% |
| Preterm birth <28 weeks | – | 4% | – |
| Average weeks of gestation at birth | 39 | 35-37 | 32-33 (quadruplets 30w) |
| Average birthweight | 3286g (7.2 lbs) | 2345g (5.2 lbs) | 1681g (3.7 lbs) |
| Birthweight <2500g (5.5 lbs) | 6% | 55% | 95% |
| Birthweight <1500g (3.3 lbs) | 1% | 9% | 34% |
| Preeclampsia | 4% | 8% | 10% |
Risks of preterm birth
Preterm babies are at higher risk of short- and long-term complications. At birth, they may have breathing and feeding difficulties, be at higher risk of infection or visual impairment, and have severe neurological and gastrointestinal complications. In the long term, they may experience growth delay or neurodevelopmental impairment.
Diagnosis
Preterm birth feels just like a regular birth, just early! Some things to look out for are contractions (which may feel like abdominal cramping, tightening, or period-like lower back pain), pelvic pressure, leakage of clear (amniotic) fluid, vaginal bleeding, or increased vaginal discharge. You should speak with your doctor or visit the hospital if you experience these symptoms.
Management of preterm birth
A mother’s cervix has to shorten and dilate during birth to allow for the baby’s head to pass from the uterus into the vaginal canal. Sometimes, the cervix shortens or dilates too early, a finding that increases the risk of preterm birth. At Sunnybrook’s Twins Clinic, we will measure your cervix via vaginal ultrasound every 2 weeks between 16 and 28 weeks gestation to detect if it is shortening or dilating. This allows us to detect such changes early on (before you experience any symptoms) and take actions that may decrease the risk of early delivery, such as the administration of vaginal progesterone (if the cervical length is below 25mm), or the use of a stitch to strengthen the cervix (known as cervical cerclage, if the cervical length is less than 15mm or the cervix is open).
Once preterm birth has already started, it cannot be stopped, but it can be slowed down to maximize the health of your babies. Drugs called tocolytics can be given to slow uterine contractions, allowing for more time to give your babies medications that help them thrive after birth. Examples are antenatal corticosteroids, which accelerate fetal lung development. If delivery is imminent, we may give you magnesium sulphate, which is a medication that helps protect the baby’s brain.
Fetal growth restriction (FGR) is defined as a failure for the baby to meet its growth potential due to a pathological process, most commonly a failure of the placenta to support fetal growth (known as placental insufficiency). In monochorionic twin pregnancies, a specific type of FGR occurs called selective fetal growth restriction (sFGR), which is discussed in more detail below (section 5).
Epidemiology
In singleton pregnancies, up to 10% of babies may be diagnosed with FGR. Twin babies grow slower than singletons starting at around 26-28 weeks of gestation. As a result, up to 30-50% of twin babies would be identified as small (and potentially growth-restricted) when their growth is evaluated using singleton growth charts. It is unclear, however, whether the slower growth of twins is due to a truly higher incidence of FGR due to placental insufficiency, or whether this is a benign physiologic adaptation of twin babies meant to reduce uterine over-stretching and, consequently, the risk of preterm birth.
More in the ‘How is FGR in twin pregnancies different than in singleton ones?’ section below!
Risks of FGR
Short term, FGR babies are at risk of mortality and morbidity, including respiratory, neurological, infectious, and gastrointestinal complications. Long-term complications of FGR include neurodevelopmental delay and increased risk of developing heart disease, high cholesterol, and diabetes later in life.
Diagnosis
When getting an ultrasound, the technician takes measurements of the fetal head circumference (size of the head), abdominal circumference (size of the tummy), and femur length. These measurements are used to calculate the estimated fetal weight (EFW). The EFW is then plotted on a growth chart against the gestational age, showing how big or small your baby is compared to other babies of that same gestational age. These growth charts are made from large population studies. If the EFW plots as less than the 10th percentile, then this condition is described as SGA (small for gestational age). In practical terms, being less than the 10th percentile means that compared to the rest of the population, 90% of babies at the same gestational age are larger than yours. In cases when the baby is below the 10th percentile (SGA), then FGR is suspected, which needs to be confirmed with additional testing such as ultrasound Doppler (which checks the blood flow to the placenta and in the baby’s blood vessels). Many SGA babies are not growth restricted, but are healthy constitutionally small babies meant to grow along this small size percentile. If your baby plots less than the 3rd percentile, then the diagnosis of FGR is very likely given that being below the 3rd percentile is much more likely to reflect underlying placental insufficiency.
FGR can also be classified by when it is detected during pregnancy. Early-onset FGR is detected before 32 weeks gestation, and late-onset FGR is detected at or after 32 weeks gestation. The reason for this distinction is that early-onset FGR tends to be more severe and is often associated with severe preeclampsia (more about this condition below in section 3!), while late-onset FGR tends to be more subtle and is often more challenging to diagnose.
Management of FGR
Management of FGR in dichorionic pregnancies is similar to the management of FGR in singleton pregnancies and includes the following:
How is FGR in twin pregnancies different than in singleton ones?
Twins are known to grow slower than singleton babies, but it remains unclear whether this represents a pathology (i.e. FGR from a failure of the placenta to support two babies) or normal benign adaptation in an attempt to decrease uterine stretching (and thereby decrease the risk of preterm birth). If it is a pathology, we should continue using singleton growth charts which are validated to identify FGR babies, but if it is a benign adaptation, we should use twin-specific charts to avoid overdiagnosis of FGR in twin pregnancies.
Therefore, our group looked further into this! We found that the use of twin-specific charts can reduce the number of twin fetuses wrongly diagnosed with FGR by up to 8-fold. They can also lead to a true diagnosis of FGR that is more clinically relevant to identifying twins who are at risk of complications due to placental insufficiency
Preeclampsia is a condition that is characterized by hypertension (high blood pressure) with proteinuria (protein in the urine) and/or injury to other organs, such as the liver or kidneys. In its most severe forms, it can also cause seizures (known as eclampsia). It is thought to be caused by a failure of the maternal blood vessels that supply the placenta (the spiral arteries) to adjust properly to pregnancy, resulting in insufficient blood supply to the placenta.
Epidemiology
Up to 10% of mothers with twins will be diagnosed with a hypertensive disorder of pregnancy, including either gestational hypertension or preeclampsia. Twin mothers are more than 2 to 4 times more likely to develop this disorder compared with singleton pregnancies. A study from our group found that 14.4% of twin mothers in Ontario developed a hypertensive disorder of pregnancy, compared with only 1.6% of mothers with a singleton pregnancy.
Risks of preeclampsia
Preeclampsia can be dangerous for both the mother and her babies. Babies can become growth-restricted, be born preterm, or have low amniotic fluid (oligohydramnios). Mothers are at risk of life-threatening complications, including liver and kidney injury, placental abruption, and development of seizures (eclampsia), which is an obstetrical emergency.
You are at higher risk of developing preeclampsia if you were diagnosed with preeclampsia during a previous pregnancy, if you are older, or if you have preexisting conditions such as chronic hypertension, kidney disease, or high body mass index. See the section below on prevention for a better understanding of how we can reduce the risk of recurrence of preeclampsia.
Diagnosis
Most mothers will be asymptomatic when they are diagnosed with preeclampsia and are found by chance to have high blood pressure and protein in the urine (proteinuria). In these cases, we would usually send blood work (such as blood count and liver and kidney function tests) to determine the severity of preeclampsia. In addition, Sunnybrook is one of the few centers in Ontario that has a special test (sFlt-1/PlGF ratio) that can confirm (or rule out) the diagnosis of preeclampsia in unclear cases and predict the risk of it becoming more severe.
Symptoms that should make you immediately seek medical attention include severe headache, blurry vision, abdominal pain, confusion, or shortness of breath.
Management of preeclampsia
The only cure for preeclampsia is delivery of the babies and the placenta(s). Management of preeclampsia depends on the severity of the disease, with the overall goal being to defer delivery as much as possible to minimize the risks of prematurity to the babies, while balancing the risks of the condition to the mother.
In cases of mild preeclampsia, delivery is commonly deferred until 37 weeks. Until then, we advise mothers to monitor their blood pressure at home and come to the hospital if the blood pressure increases above 150-160 mmHg systolic or 95-100 mmHg diastolic, or if they experience symptoms such as headache, blurry vision, or epigastric pain.
Blood pressure medications are often prescribed to control blood pressure levels (most commonly a drug called labetalol). These medications are safe in pregnancy! We also monitor the mother’s blood work periodically to check liver and kidney function to make sure that the preeclampsia hasn’t evolved to a more severe form.
In cases of severe preeclampsia, delivery may be indicated earlier, especially if the blood work becomes abnormal. HELLP syndrome (which stands for Hemolysis, Elevated Liver enzymes, and Low Platelets) is the most severe form of preeclampsia (although there is some academic debate as to whether they are two different conditions). Symptoms most commonly start between 28 to 37 weeks of gestation, but can also start later in pregnancy, and 30% of cases develop post-partum. Rest assured that this is an uncommon complication.
Can preeclampsia be prevented?
Research has shown that starting to take low-dose aspirin before 16 weeks of gestation (162 mg daily at bedtime) can substantially reduce the risk of severe preeclampsia. We will usually recommend aspirin if you have additional risk factors for preeclampsia, such as age over 35 years, elevated BMI, history of hypertension, diabetes, or a history of preeclampsia in a prior pregnancy. Ask your obstetrician before starting any medication.
I have preeclampsia, what should I do?
Regularly check in with your obstetrician for monitoring and know the signs which mean you should go seek medical attention immediately: severe or persistent headache, visual changes, new shortness of breath, right upper quadrant or epigastric pain. Additionally, any decrease in fetal movement, vaginal bleeding, abdominal pain, rupture of membranes, or regular uterine contractions should also be a sign to go to your nearest medical centre.
How is preeclampsia in twin pregnancies different than singleton ones?
Gestational diabetes mellitus (GDM) is defined as glucose intolerance (high blood sugar) that is first recognized during pregnancy. GDM is believed to be caused by a combination of maternal predisposition (such as high insulin resistance or mild failure of the pancreas to secrete insulin) along with the physiologic increase in insulin resistance that occurs in pregnancy under the influence of pregnancy hormones.
Epidemiology
GDM affects approximately 6 to 10% of all pregnancies in North America, and specifically 3 to 9% of twin pregnancies. A large study by our group found that twin pregnancies are more likely to be affected by GDM, although this finding was limited to cases of mild GDM that can be controlled by diet (GDMA1), and was not observed for the more severe form of GDM that requires medications or insulin (GDMA2).
Risks of GDM
Since glucose readily crosses the placenta, elevated maternal blood glucose results in elevated glucose levels in the baby, as well. This, in turn, may accelerate fetal growth and result in a large baby (known as macrosomia). After birth, babies may experience a sudden drop in their blood sugar levels known as hypoglycemia, which can be dangerous.
Mothers who have been diagnosed with GDM are more likely to develop type II diabetes later in life. Several studies have also found a link between babies born to mothers with GDM and the development of obesity and type II diabetes in the baby later in life.
Diagnosis
Your obstetrician will refer you for a blood test at around 24 weeks of gestation. Briefly, the test involves drinking a sugary orange drink, waiting one hour, and then having a blood draw. Your blood will be tested for sugar levels, and depending on the number, you may receive the following results: 1) a diagnosis of GDM, 2) a confirmation that you do not have GDM, 3) an equivocal result that requires more testing using a longer test.
More details can be found here.
Management of GDM
First, you will meet with a dietician and discuss lifestyle and nutritional changes that can help manage your blood sugar without medications. For example, your plan may include the ideal number of meals daily (usually three moderate-sized ones with two snacks) and an exercise schedule (usually 150 minutes of moderate-intensity aerobic exercise per week). You will also receive a glucometer (a device used to measure your blood sugar through a finger prick) and be asked to measure and document your blood sugars 4 times a day (in the morning when waking up, and 1-2 hours after each meal).
For some mothers, lifestyle changes won’t be enough to keep blood sugar levels within target ranges. In these cases, we may offer medications such as metformin or insulin.
It’s important to have a personalized lifestyle and meal plan, as well as an obstetrician following your GDM to make sure that you and your babies are receiving the best care possible.
How is GDM in twin pregnancies different than singleton ones?
Interestingly, research coming out of our group has found that GDM may be overdiagnosed in twin pregnancies, and that the risk of having too-large babies or other complications may not be as pronounced compared to singleton pregnancies. We have also found that mild GDM in twin pregnancies is less likely to be associated with complications than in singleton pregnancies, and may actually reduce the risk for having a too-small baby which is common in twin pregnancies. However, you should still be closely monitored for complications.
TTTS is a severe complication that affects monochorionic twins, as their shared placenta means that their circulatory systems are connected. Sometimes, the blood vessel connections in the placenta are imbalanced, causing one twin to persistently push blood to or receive extra blood from the other twin. The syndrome is characterized by a low levels of amniotic fluid (oligohydramnios) around one twin (the ‘donor twin’) and too much amniotic fluid (polyhydramnios) around the other twin (the ‘recipient twin’). If untreated, this condition poses considerable risks to both the donor and recipient twin.
Epidemiology
It is estimated that 9 to 15% of monochorionic diamniotic and 6% of monochorionic monoamniotic twin pregnancies will be complicated by TTTS, most commonly presenting between 16 and 26 weeks of gestation. In addition, there is a relatively rare and sudden type of TTTS (intrapartum TTTS) that affects 1.5 to 2.5% of monochorionic pregnancies during labour, regardless of whether the birth was vaginal or a c-section. This type of TTTS results in a rapid and large intertwin blood transfusion from donor to recipient, causing the donor to have too many red blood cells (polycythemia) and the recipient to have too little red blood cells (anemia). The donor twin may also go into shock from the sudden loss of fluid (hypovolemic shock).
Risks of TTTS
TTTS is a severe complication of pregnancy and can result in organ failure and the potential death of either twin. You may also be at higher risk of preterm birth due to the overstretching of the uterus (by the excessive amount of amniotic fluid around the recipient twin) and cervical shortening. Read more about preterm birth in the section above to learn about how that risk is managed!
Diagnosis
If you have a monochorionic twin pregnancy, you will have an ultrasound at least every two weeks throughout pregnancy to allow the detection of TTTS. The first sign of TTTS will be difference in the levels of amniotic fluid between the two babies. More advanced stages include include abnormalities in blood flow to the placenta (detected by ultrasound Doppler studies) and signs of fetal heart failure. In addition, the babies will have a fetal echocardiogram, which is a special ultrasound to detect congenital fetal heart malformations which are more common in monochorionic twins.
Maternal symptoms of TTTS include shortness of breath, premature contractions, and abdominal discomfort, caused from the rapid overstretching of the uterus due to the extra amniotic fluid.
TTTS management will depend on the stage of the syndrome, classified as stages I to V:
Management
The main procedure used to treat TTTS is fetoscopic laser ablation. It involves inserting a small camera called a fetoscope into the uterus through the mother’s abdomen along with a laser device that is used to destroy (or ablate) the placental blood vessels connecting the two twins. The procedure is done in the operating room under general anaesthesia. Risks of the procedure include preterm prelabor rupture of membranes (PPROM, where the amniotic sac breaks resulting in vaginal leakage of fluid), disruption of the membranes separating the two twins (which can result in entanglement of the umbilical cords of the two babies), and twin anemia polycythemia sequence (TAPS, discussed below).
TAPS is a severe complication that affects monochorionic twins, as their shared placenta means that their circulatory systems are connected. You can think of TAPS as a ‘mild form of TTTS’ where the placental connections between the two twins are very small and therefore do not result in differences in fluid (as seen in TTTS) but instead in a very gradual difference in blood hemoglobin (or red blood cells). Red blood cells carry oxygen throughout our bodies to keep our organs functioning via a molecule called hemoglobin. Consequently, this condition is characterized by anemia in one of the twins (less red blood cells and too little hemoglobin to carry oxygen to the organs) and polycythemia (too many red blood cells and too much hemoglobin) in the other.
Epidemiology
Spontaneous TAPS is reported in 3 to 6% of monochorionic diamniotic pregnancies. It can also very rarely occur in monozygotic dichorionic pregnancies if there are shared blood vessels between the two placentas. TAPS can also occur post-laser ablation for the treatment TTTS (discussed above) if the laser ablation leaves out small vessels that can allow for the exchange of red blood cells.
Risks of TAPS
The twin with polycythemia has lots of red blood cells which thickens the blood and makes it appear sludge-like. This can cause blood clots and damage the baby’s organs and brain if it stops blood from reaching them. The twin with anemia may develop heart failure. Both twins are at risk of passing away in-utero as a result of this condition. Long-term, there is also a higher risk of adverse neurodevelopmental outcomes.
Diagnosis
Middle cerebral artery-peak systolic velocity (MCV-PSV) is a Doppler measurement that can diagnose anemia and polycythemia in utero. It is based on the idea that, in cases of anemia, the blood flow is more watery (less thick) and will therefore flow faster, the same way water flows faster than oil. Other ultrasound findings include difference in thickness and color of placenta with a clear demarcation between the twins’ territories, a large heart (cardiomegaly) and fluid overload (ascites) in the donor twin, and a certain appearance on the recipient twins’ liver on ultrasound called a starry sky.
Sometimes the diagnosis is not made until after birth. At that time, TAPS may be suspected if one twin is very pale and the other is red (their respective side of the placenta will show the same colour imbalance). Blood testing can reveal that one twin is anemic and the other is polycythemic, and placental testing may show shared small blood vessels that allow unidirectional flow.
Management of TAPS depends on the stage:
Management
The treatment options in cases of TAPS involve fetoscopic laser ablation or intrauterine blood transfusion. Your obstetrician will help you make a decision on the best treatment option based on the stage of TAPS, your health, and the health of your babies.
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.
Check out this calculator from perinatology.com
Intimacy is an important part of your relationship with your partner. Sexual activity is not discouraged in twin pregnancies, unless there are complications that your healthcare provider has discussed with you.
Sunnybrook’s Twins Clinic recommends against sexual intercourse if you have both a short cervix and are pregnant with twins, especially in cases with severe or early-onset cervical shortening.
Twin and triplet pregnancies have been associated with an increased risk for complications, primarily preterm birth. The increased risk for preterm birth may have individuals pregnant with twins and their providers concerned that intercourse may prompt preterm labor or delivery. The anxiety may be even greater in women with twins who are thought to be an increased risk of preterm birth, such as those with a short or dilated cervix.
If you experience discomfort related to sex during your twin pregnancy, avoid lying on your back during intercourse after the fourth month of pregnancy. Try different positions to reduce pressure and use a personal lubricant to help with dryness.
If you notice pain, bleeding, abnormal discharge, or contractions after sex, we recommend you call your healthcare provider.
And remember, intimacy goes beyond sexual intercourse—it is not just sex. Intimacy is about closeness, about being together, and about creating and maintaining a relationship.
If you have any questions about sexual activity during your twin pregnancy, do not hesitate to ask your healthcare provider at your next prenatal visit.