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.
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.
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?
Patients with twin pregnancies are at an increased risk of giving birth early (called preterm birth or PTB). There has been much effort put into identifying effective strategies to prevent PTB in twin pregnancies. Research has shown that following patients with twins in a specialized Twins Clinic can result in improved pregnancy outcomes.1
Signs and Symptoms of PTB
Some of the symptoms of preterm birth include preterm contractions (which may be felt as abdominal cramping, tightening, or period-like lower back pain), pelvic pressure, leakage of fluid, vaginal bleeding or increased vaginal discharge. Although some of these symptoms may be non-specific, you should speak with your doctor or come to the hospital if you experience any of these symptoms.
Cervical Length Measurements
There are currently no effective interventions for the prevention of preterm birth in all twin pregnancies.2,3 However, there is evidence that in a subset of patients with twin pregnancies who are found to have a short or dilated cervix, certain intervention such as vaginal progesterone4 and cervical cerclage may be effective in decreasing the risk of preterm birth.5,6
Research from our group provided evidence that taking multiple measurements of cervical length over time (compared to a single measurement of cervical length at approximately 20 weeks) can improve the detection of patients at risk of PTB.7,8 Therefore, at the Twins Clinic we will measure your cervix via vaginal ultrasound every 2 weeks between 16 and 28 weeks to allow for early preventative interventions in the case of cervical shortening.
Ellings JM, Newman RB, Hulsey TC, Bivins HA, Keenan A. Reduction in very low birth weight deliveries and perinatal mortality in a specialized, multidisciplinary twin clinic. Obstet Gynecol. 1993;81(3):387-391.
Rehal A, Benkő Z, De Paco Matallana C, et al. Early vaginal progesterone versus placebo in twin pregnancies for the prevention of spontaneous preterm birth: a randomized, double-blind trial. Am J Obstet Gynecol. 2021;224(1):86.e1-86.e19. doi:10.1016/j.ajog.2020.06.050
Dang VQ, Nguyen LK, Pham TD, et al. Pessary Compared With Vaginal Progesterone for the Prevention of Preterm Birth in Women With Twin Pregnancies and Cervical Length Less Than 38 mm: A Randomized Controlled Trial. Obstet Gynecol. 2019;133(3):459-467. doi:10.1097/AOG.0000000000003136
Romero R, Nicolaides KH, Conde-Agudelo A, et al. Vaginal progesterone decreases preterm birth ≤ 34 weeks of gestation in women with a singleton pregnancy and a short cervix: an updated meta-analysis including data from the OPPTIMUM study. Ultrasound Obstet Gynecol. 2016;48(3):308-317. doi:10.1002/uog.15953
Abbasi N, Barrett J, Melamed N. Outcomes following rescue cerclage in twin pregnancies. J Matern Fetal Neonatal Med. 2018;31(16):2195-2201. doi:10.1080/14767058.2017.1338260
Li C, Shen J, Hua K. Cerclage for women with twin pregnancies: a systematic review and metaanalysis. Am J Obstet Gynecol. 2019;220(6):543-557.e1. doi:10.1016/j.ajog.2018.11.1105
Melamed N, Pittini A, Hiersch L, et al. Do Serial Measurements of Cervical Length Improve the Prediction of Preterm Birth in Asymptomatic Women with Twin Gestations? Am J Obstet Gynecol. 2016;215(5):616.e1-616.e14. doi:10.1016/j.ajog.2016.06.034
Melamed N, Pittini A, Hiersch L, et al. Serial cervical length determination in twin pregnancies reveals 4 distinct patterns with prognostic significance for preterm birth. Am J Obstet Gynecol. 2016;215(4):476.e1-476.e11. doi:10.1016/j.ajog.2016.05.018
Fetal growth in a twin pregnancy
Do twin fetuses grow differently than singletons?
This is a common question from parents expecting twins.
Twins grow like other babies until around weeks 26-28 of your pregnancy, when their growth does slow down a little.
The reason for the slower growth of twin fetuses is not entirely clear. In the past, researchers have understood this issue to be partly because of limited maternal resources available to support two fetuses.
However, recently, research has shown that the slower growth of twins may be the result of ‘fetal programming’ early in pregnancy. Slower growth may allow twins to gain maturity by decreasing the stretching of the uterus, and potentially decreasing the risk of preterm birth. In this situation, twin fetuses may ‘compromise’ their growth to increase survival.
This directly related to what growth charts providers should use. For example:
If the slower growth of twins is due to limited maternal resources to support two babies, it would be preferrable to use singleton growth charts to identify the small twin fetus that may be at an increased risk of complications.
If slower growth is the result of ‘fetal programming’, it may be preferable to use twin-based growth charts which would avoid unnecessary interventions because babies wouldn’t be classified as too small.
Our research team has found that the use of twin-based charts can result in a considerable reduction in the amount of twins identified as growth restricted, without compromising the detection of twins that are at risk of complications.
Therefore, Sunnybrook’s Twins Clinic uses both singleton- and twin-based growth charts to monitor your babies, along with other steps like keeping a close eye on your placenta. Please speak to your health care team if you have any questions or concerns about your babies’ growth while pregnant.
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.
To understand a bit more about this issue, we have compiled available evidence on this topic below.
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.
What are the possible effects of prostaglandins (hormones that play a role in induction of labour and uterine activity) in semen on preterm labour?
What are the possible effects of mechanicalstimulation (pressure during intercourse) of the cervix during intercourse on preterm labour?
What are the possible effects of uterine contractions during femaleorgasm and preterm labour?
Prostaglandins (PGs) have been shown to be present in the amniotic fluid, placenta, myometrium, and blood at the start of labour, and have also been shown to induce labour and stimulate uterine activity.1,2
There have been several studies in singletons on the topic of coitus and the onset of labour, all of which found no relationship between coitus at term and onset of labour.3–6 These studies had major limitations, thus there is no strong evidence about the effect of semen on labour onset.
No studies have been done on this topic in twin pregnancies.
Another concern related to coitus in pregnancy is mechanical stimulation of the cervix or lower-uterine segment, which may release prostaglandins.
There are no studies that examine endogenous prostaglandin levels after intercourse, nor are there studies comparing condom use to coitus without a condom (which may aid in separating mechanical stimulation causing release of prostaglandins, or prostaglandins from semen). In addition, the cervix and lower-uterine segment in low-risk pregnancies seems to be able to tolerate the impact of penetration during coitus.
No studies have been done on this topic in twin pregnancies.
Researchers have observed similarities between the uterine contractions and release of oxytocin during female orgasm and uterine contraction patterns of labour, raising concerns that female orgasm may elicit contractions and stimulate preterm labour.
All the research on this topic has been performed in singletons, and most of the studies were published in the 1970s. Despite this, one study performed in 2001 concluded that sexual activity was associated with a reduced risk for preterm birth.7 It is important to note that even strong uterine contractions due to orgasm are likely not as strong or long-lasting enough to induce labour.
No studies have been done on this topic in twin pregnancies.
Is there evidence from Twin Pregnancies?
There have only been two published, peer-reviewed research studies on the topic of sexual activity and twin pregnancies.8,9 Both studies did not find a relationship between sexual behaviour and complications such as preterm birth in twin pregnancies. In addition, a recent survey of 90 Canadian Maternal-Fetal Medicine specialists found that 97% (87/90) of specialists agree that sexual activity does not need to be avoided in an uncomplicated twin pregnancy.10
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.