Twin Types

Are you an expecting mom, dad, or family member of twins? Have you been told if you are having fraternal or identical twins? Do you wish you knew more about the research on all thing’s twins? Well, the first places to start is on the types of twins! Did you know that twins can be broken down into different categories and there’s more than just identical and fraternal twins.

Fraternal Twins: (di-zygotic twins)

Fraternal twins are the most common type of twins, which is around 2 in every 3 sets of twins born. Fraternal twins occur when two separate sperm fertilize two different eggs in one ovulation cycle. With fraternal twins each twin has their own placenta and their own amniotic sac. Fraternal twins can be the same sex or different sexes, and DNA wise their DNA is like that of siblings of different ages, fraternal twins DNA is not identical.

Identical Twins: (mono-zygotic twins)

Despite seemingly being extremely popular identical twins are relatively rare, with only 3 to 4 sets per 1,000 births. Identical twins occur when a single fertilized egg splits and develops into two fetuses. In the womb identical twins could possibly share a placenta and an amniotic sac, or the twins might share a placenta, and each have separate amniotic sacs. Genetically, the two babies will be 100% identical, they will be the same sex and share physical traits as well as characteristics.

Different Types of Identical Twins:

  • Dichorionic, Diamniotic Twins (commonly called “di-di” twins)”

    • Di-di twins make up the highest percentage of twin pregnancies, it is also a misconception that di-di twins cannot be identical. Research shows that a third of identical twins are di-di twins. Di-di twins split very early in the pregnancy, they have two placentas and two double layered water sacs. They are also the lowest risk for complications.

  • Monochoronic, Diamniotic Twins (commonly called “mono-di” or “modi” twins):

    • Mono-di twins are the most common type of identical twins accounting for around 70% of all identical twin births. This twin type has one placenta and has a single layered water sac dividing them. Mono-di fetuses are at risk for complications before birth, including Twin-Twin Transfusion Syndrome (TTTS) and Twin Reversed Arterial Perfusion (TRAP) sequence. This twin type requires additional assessment in pregnancy, including echocardiography and ultrasound assessments every 2 weeks starting in the 2 trimester. In rare cases, laser therapy can be needed to divide the circulation in the placenta.

  • Monochoronic, Monoamniotic Twins (“mono-mono” or “mo-mo” twins):

    • Mo-mo twins account for only 1% of all identical twin births. This twin type has one placenta and are also together in a single amniotic sac. They face the same complications and risks before birth as Mono-di twins including TTTS and TRAP. These fetuses are also at risk for becoming entangled in each other’s cords as they share one amniotic sac. This twin type also requires additional assessment in pregnancy and early admission to the hospital, early delivery will also likely be recommended.

Conjoined Twins:

Conjoined twins occur when an identical twin embryo does not fully divide. This means that the babies’ bodies are fused, most commonly at the chest or abdomen, the twins may even share vital organs, like the liver or heart. Conjoined twins are extremely rare and account for only 1 out of every 100,000 twin pregnancies.

Semi-identical Twins:

While this twin type is extremely rare it can happen, it is considered a “third” twin type called sesquizygotic twins or semi-identical twins. This twin type occurs when one egg is fertilized by two sperm. This leads to two babies that share the same placenta, they may also share anywhere from half to all of their DNA. Even though semi-identical twins form in the same amniotic sac they do not have to be the same gender, meaning it’s possible with semi-identical twins to have a boy and a girl when the fertilized egg splits into two. The fertilization of the egg by two sperms creates three sets of chromosomes, in turn the embryos don’t usually survive. Only a couple cases of semi-identical twins have been identified. These twin sets are extremely rare, so rare many consider them next to impossible.

Quaternary Twins:

Quaternary twins are not technically twins at all, but they are still an extremely interesting and rare phenomenon. Quaternary twins happen when one set of identical twins have children with another set of twins within a short period of time, usually within 9-10 months of each other. The children don’t share a parent or the same DNA makeup, they’re cousins but genetically they’re actually closer to siblings born to the same parents. Physically the children also might look very similar as well, as the two sets of parents share the same DNA. However, experts don’t scientifically consider the children to be twins.

Mirror Twins:

Mirror twins are essentially exactly what they sound like, they are mirror images of each other. This means that:

  • Their hair may naturally fall in opposite directions.

  • Their teeth may grow in opposite sides of their mouths.

  • They may have birthmarks on opposite sides of their bodies.

  • They also usually have different dominant hands.

Mirror twins are always identical, as in a usually identical twin pregnancy the egg splits with in the first week after fertilization, but with mirror twin pregnancies the egg splits 7-12 days after it’s been fertilized which is long enough for the egg to have developed a left and right side.

Parasitic Twins:

Parasitic twins are a twin type where one twin is fully developed, and the other ‘twin’ is underdeveloped. The underdeveloped fetus is nonfunctional, and it often doesn’t survive the pregnancy. The result may look like one person who has additional limbs or organs. Parasitic twins are known to develop in a similar manor to conjoined twins, where one zygote doesn’t fully separate. Or they may come from two separately fertilized eggs that fuse together. In that case, one zygote stops growing on its own and remains attached to the other twin. Parasitic twins are rare in occurrence and believed to affect fewer than 1 in 1 million births worldwide.

Superfetation:

Superfetation occurs when a person is already pregnant, and the body still ovulates resulting in a twin of a different gestational age. This twin type is rare because of the way reproduction works, as once a person is pregnant the chances of ovulating for a second time are extremely rare. Research suggests that superfetation occurs once out of a few million pregnancies. Additionally, there have been fewer than 10 cases in the world. Most specialists and experts speculate that it may occur when a person has a double uterus. Superfetation does not rely on maternal age, fertility treatments, genetic factors or environmental factors, it is a natural occurring condition.

Heteropaternal Superfecundation:

Also, part of superfetation, heteropaternal superfecundation occurs when the twins have different fathers. Heteropaternal Superfecundation is extremely rare and only a handful of cases have been recorded worldwide.

Listed below are possible complications to a twin pregnancy if you do not wish to continue reading please stop here.

Possible Complications:

Healthy multiples are born every day, still it’s important to be aware of the possible complications that come with carrying twins and higher order multiples.

  • Premature Birth – The more babies you are carrying, the less likely you are to carry your pregnancy to term. If you have signs of preterm labor, you might be given injections of steroid medication to speed up your baby’s lung development. Even then, your babies might experience complications, including breathing and digestive difficulties, vision problems and infection. Interventions to prolong pregnancy, such as bedrest, aren’t recommended because they haven’t been proven to decrease disease and death in newborns.

  • Gestational Diabetes – If your carrying multiples, you’re at an increased risk of gestational diabetes. This condition causes high blood sugar that can affect your pregnancy and your baby’s health. An endocrinologist, a registered dietitian or a diabetes educator can help you learn to manage your blood sugar levels during your pregnancy.

  • High Blood Pressure – If you’re carrying multiples, you’re at an increased risk of developing high blood pressure disorders during pregnancy.

  • Cesarean Birth – For twins a vaginal delivery is often possible if the first baby is in the head down position. If not, a cesarean birth might be recommended. In some cases, complications after the vaginal delivery of the first twin might require a cesarean birth for the second twin. For higher order multiples, the most common route of delivery is a cesarean birth.

  • Twin-twin Transfusion – With identical twins, it’s possible for a blood vessel in a shared placenta to result in one baby receiving to much blood and the other baby receiving too little. This is a serious complication for both babies that might result in heart complications and the need for fetal procedures while you are still pregnant.

  • Anemia – Anemia is when you do not have enough healthy red blood cells to carry oxygen to the rest of your body. A condition called iron-deficiency anemia is common in multiple pregnancies and can increase your chances of premature birth. Your provider may prescribe an iron supplement for you to make sure you’re getting enough iron.

  • Gestational Hypertension/Preeclampsia – Preeclampsia is a condition that can happen after the 20th week of pregnancy or right after pregnancy (4th trimester). It’s when a pregnant person has high blood pressure and signs that some of their organs, like the kidneys and liver may not be working properly.

This information is not to be taken as medical advice, please see your healthcare provider for professional advice and concerns.

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Allison RollanstwinsComment