A pregnancy in which two fetuses (babies) develop in the womb at the same time. This can happen when two separate eggs are fertilized at or around the same time. It can also happen when a single fertilized egg divides in two.
This article is pending medical review.
Written by Carolin Becker
Reviewed by Lea Dörner, Alessandra Papitto and Sophie Oppelt
Edited by Juliëtte Gossens
Twin pregnancy is a pregnancy in which two fetuses (babies) develop in the womb at the same time. This can happen when two separate eggs are fertilized at or around the same time. It can also happen when a single fertilized egg divides in two. Twin pregnancies bring a higher risk of complications, such as preterm labor, gestational diabetes, and high blood pressure (hypertension). This requires close monitoring and care during pregnancy.
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Twin Pregnancy: How Does That Work?
Twin pregnancy is a unique and exciting experience, but it can also bring added challenges. It can increase the risk of complications. It’s important that the pregnant person is well taken care of and prepared. That’s to maintain the health of the babies, as well as that of the pregnant person.
When a sperm cell fertilizes an egg cell, the two cells combine and form one cell. This cell is called a zygote.
Twins can be dizygotic (fraternal), when two eggs are separately fertilized. Monozygotic (identical) twins happen when a single fertilized egg (the zygote) splits into two embryos (1).
We explain more about the different types of twins below.
Fraternal (Dizygotic) Twins
Dizygotic twins are the most common type of twins (around 90%). “Di” means two. Dizygotic twins occur when two separate eggs are fertilized at the same time (or around the same time). This leads to two zygotes, and two pregnancies, at the same time (1, 2). Each zygote implants into the uterus individually and develops its own placenta. Because fraternal twins come from different eggs and different sperm cells, they might not look the same. They have a different genetic makeup! Just like regular siblings.
Recently, the incidence of fraternal twin pregnancies has been increasing. This is due to mainly two reasons.
First, people are having children later in life. The likelihood of having twins rises as the pregnant person’s age rises. By the time you’re 35, your chances of conceiving with twins is twice as high compared to when you’re 25. The age of your partner doesn’t influence this chance.
Second, the increasing use of fertility treatments is contributing to the rise in twin pregnancies (2). Fertility treatments include assisted reproductive technology (ART), such as in vitro fertilization (IVF). You can read more about ART here.
Identical (Monozygotic) Twins
Monozygotic (identical) twins are quite rare. “Mono” means one. The twin embryos come from a single fertilized egg, the zygote. This egg can then split into two at various stages of development (1, 2). That means the twins share the same genes and will look very similar (identical!). Their development is not influenced by the above mentioned factors that lead to an increased risk of fraternal twins. That is, your age and whether you received fertility treatment don’t influence your risk of identical twins.
In most cases (99%) the embryo splits before the eighth day. Splitting after day 8 might bring more complications, as the twins share the same placenta and amniotic sac by then. Their umbilical cords might be entangled. They might even share a single umbilical cord, which is the case in conjoined twins (2).
There are different types of monozygotic pregnancies. We’ll explain a little bit about them below.
Dichorionic-diamniotic twins: 30% of all monozygotic twins split within 3 days after fertilization. Each embryo develops its own placenta and amniotic sac. They also have their own blood supply (2).
Monochorionic-diamniotic twins (most common): this type of monozygotic twin develops when the fertilized egg splits between 4 to 8 days after fertilization. The twins share one placenta but have their own amniotic sac. 69% of monozygotic twins belong to this category: the most common type! (2)
Monochorionic-monoamniotic twins: This is the rarest type of monozygotic twins (1% of cases). This occurs when the fertilized egg splits after the eighth day of fertilization. The twins share both the placenta and the amniotic sac, which can lead to complications during pregnancy (2). We talk about this more below.
Conjoined twins, also known as Siamese twins, are very rare. They form less than 1% of all monozygotic (identical) twins. Conjoined twins are two babies who are joined at some part of their bodies. This can range from being joined at just a small part of the body to being joined along the full length of their bodies. It’s also possible that they share organs. These organs then work for both of the twins.
In some cases, conjoined twins can be separated through surgery, but this is not always possible or recommended. The decision to separate conjoined twins is made on a case-by-case basis. It depends on how joined the twins are, and if separation would be risky. In places where medical technology is advanced, conjoined twins can often have a good quality of life. Many conjoined twins go on to lead normal, healthy lives with the help of supportive medical care and therapies (2).
What Complications Could Occur?
Twin pregnancies have a higher risk for complications such as preterm labor, gestational diabetes, and high blood pressure (hypertension). To make sure mom’s and baby’s health are OK, regular check-ups with your doctor are essential.
For Your Babies
The risk of death or illness around the time of birth is much higher in twin pregnancies than in singleton pregnancies. For example, twin pregnancies carry a 60% risk of preterm delivery. This often leads to a low birth weight, which means your baby can’t keep warm as well as full-term babies. It’s also harder for them to fight infections. Some organs, such as the lungs, might be immature. This can lead to complications, for example because the baby can’t breathe very well (7). This is what happens in respiratory distress syndrome, for example.
For monozygotic (identical) twins who share a placenta, the twin-twin transfusion syndrome is also a risk. This is an abnormality of the placenta where one twin receives more blood than the other. The twin receiving more blood could develop complications such as heart failure or high blood pressure. The twin receiving less blood may not be able to grow very well. If it’s left untreated, the syndrome can result in preterm birth, stillbirth, or death of one or both twins.
To diagnose the syndrome, your doctor picks up signs on your regular ultrasounds. Depending on the severity of the syndrome, your doctor might advise not to treat it but to monitor your twins very regularly. That way, your doctor can intervene when necessary. That’s because any treatment also comes with risks that you’d want to avoid if possible (1-3). Which treatment options are available depends on your area.
Being pregnant with twins means the pregnant person’s body temporarily changes as well. In a singleton pregnancy (a pregnancy with only 1 baby), your blood volume usually increases by 40% (which is around 2 liters). This is necessary to make sure the fetus can grow and develop well. It’s achieved via hormonal regulation (5). With twins, your blood volume increases even more, by around 3 liters. This can lead to high blood pressure (hypertension) and an increased risk of blood clots (thrombosis) for the pregnant person. Your doctor might recommend that you drink a bit more than usual to “water down” your blood. It might even be necessary to start taking medications to thin your blood (2).
Your nutritional needs also change. You need more calories and nutrients to support the health of both mother and babies. Typically, it’s recommended to consume an additional 300 to 500 calories per day and focus on nutrient-dense foods. This comes down to a small extra meal per day, increasing the size of your regular meals, or having more snacks than you’d usually have. Try listening to your body’s hunger cues to know how much extra you need. Your doctor might also indicate you need to increase your intake of iron or folate, because the risk of deficiency is especially high in twin pregnancies (6). You might be able to do this through foods or supplements, depending on your doctor’s recommendations.
Twin pregnancies carry a higher risk of gestational diabetes. This is a disorder in which your blood sugar (glucose) increases and can’t be taken up into your cells. It can lead to complications for you and your babies. For example, gestational diabetes can lead to macrosomia (high birth weight). It can also give an increased risk of cesarean delivery. You can read more about gestational diabetes here.
Pre-eclampsia, or high blood pressure during pregnancy, is more common when you’re carrying twins. If left untreated, this can lead to serious complications, such as preterm delivery. In the worst case, it can even be fatal for the mother and the fetuses (1, 2).
How Are Twins Delivered?
There are several delivery options for twins. The best option depends on several factors, such as the position of the baby and the health of the pregnant person. You and your doctor can decide together on the best way to deliver your babies.
Vaginal delivery: in some cases, twin pregnancies can be delivered vaginally. That’s an option especially if the babies are in a good position and are of similar size. But this option is not always possible, depending on the presentation (position) of the babies (4). A cesarean section might sometimes be a safer option.
Assisted delivery may be necessary in some cases to help deliver one or both twins during a vaginal delivery (4). This could be the use of a forceps or a vacuum, both of which are tools your doctor can use to help pull out the head of your baby.
Cesarean delivery (C-section): In many cases, twin pregnancies are delivered via cesarean section (C-section). Especially if the babies are not in a good position for vaginal delivery or if there are other complicating factors (4). A C-section is a surgery in which your doctor makes a cut across your abdomen and into your womb. After they’ve pulled out your babies and their placenta(s), they can stitch you right back up.
Preterm delivery: In some cases, your twins may be delivered prematurely. This could be because you’re going into preterm labor or because you’re experiencing complications. The delivery could be scheduled, or it’s done as an emergency procedure. This depends different factors, such as your health and that of your babies (2).
In conclusion, twin pregnancy is a special journey filled with joy, but it is important to be informed about the risks and challenges that come with it. With proper prenatal care and preparation, most expecting parents can have a safe and successful twin pregnancy.
Sadler TW. Langman's Medical Embryology. 12th ed. Philadelphia, USA: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2012.
Hacker NF, Gambone JC, Hobel CJ (eds.). Hacker & Moore’s Essentials of Obstetrics & Gynecology. 6th ed. Philadelphia: Elsevier; 2016.
Santana DS, Surita FG, Cecatti JG. Multiple Pregnancy: Epidemiology and Association with Maternal and Perinatal Morbidity. Rev Bras Ginecol Obstet. 2018;40(9):554-562. DOI: 10.1055/s-0038-1668117
Christopher D, Robinson BK, Peaceman AM. An evidence-based approach to determining route of delivery for twin gestations. Reviews in Obstetrics & Gynecology. 2011;4(3-4):109-116.
Longo LD. Maternal blood volume and cardiac output during pregnancy: a hypothesis of endocrinologic control. Amercan Journal of Physiology. 1983;245(5 Pt 1):R720-R729. DOI: 10.1152/ajpregu.1983.245.5.R720
Luke B. Nutrition and multiple gestation. Seminars in Perinatology. 2005;29(5):349-354. DOI: 10.1053/j.semperi.2005.08.004
Romero R, Dey SK, Fisher SJ. Preterm labor: one syndrome, many causes. Science. 2014;345(6198):760-765. DOI: 10.1126/science.1251816
Please note: the information we provide to you here is for educational purposes only. If you’re experiencing any discomfort or have any complaints or questions about your health, please contact your doctor or other relevant health professional. We don’t provide medical advice.