Twins are a joyous occasion, but they also come with some confusion. You may have been told by your doctor that you’re having di di twins. But what does that mean? If you are having a difficult time understanding what type of twins you’re carrying, here is some information that will help clear up any doubts.
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The basic structure of a twin pregnancy
First, you need to understand the basic structure of a twin pregnancy. The placenta and umbilical cord connect the mother and each baby. The placenta is where nutrients and oxygen are passed to each baby from their mother. Each baby develops in their own sac that is connected by the umbilical cord to their mother’s placenta. Now that we’ve made that clear, let’s move on to discuss the different types of twin gestations.
What are di di twins, also known as dichorionic twins?
You are probably familiar with the terms identical and fraternal twins, but this is just the tip of the iceberg. The medical community breaks down types of twins into several different categories:
- di di twins (or dichorionic diamniotic twins), also known as fraternal twins = two placentas and two sacs
- mo di twins (or monochorionic diamniotic twins), also known as identical twins = one placenta and two sacs
- mono mono twins (or monochorionic monoamniotic twins), also known as identical twins = one placenta and one sac
Di di twins mean each baby has its own sac (called an amniotic sac), and they each have their own placentas. This is the most common and least risky type of twin pregnancy.
Are di/di twins identical or fraternal?
All fraternal twins are di di twins. Separate eggs are individually fertilized by two different sperm cells with a fraternal twin pregnancy, creating two individual zygotes and each zygote will have its own placenta and amniotic sac.
However, on rare occasions, di di twins could be identical if a single zygote splits early enough. An identical pregnancy begins with one egg and one sperm, and at some point, that single egg splits. Exactly when that happens affects the development of the placenta and amniotic sac. If the zygotes separate approximately 1 – 4 days after fertilization, it’s more likely to lead to di di twins.
The only way to know for sure if your di di fraternal twins may actually be identical twins is to do DNA testing after the twins are born.
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What is the meaning of dizygotic twins and monozygotic twins?
Dizygotic twins are also known as fraternal twins, which means they are two individuals who developed from separate eggs that were fertilized by two different sperm cells.
Monozygotic twins are also known as identical twins. This means they developed from a single fertilized egg that split into two fetuses. Identical twins (synonymous with monozygotic twins) have the same (or nearly the same) genetic code and many of the same physical characteristics, but they are not necessarily completely identical in appearance.
What are dichorionic twins and monochorionic twins?
Dichorionic twins have two placentas and two amniotic sacs. Because the twins have separate sacs, they each have their own cord, which means they have two umbilical cords and no shared placenta. This is the most common type of twin pregnancy.
Monochorionic twins have one placenta (chorionic refers to the placenta) and one or two amniotic sacs.
How are di di twins managed during pregnancy?
If you are pregnant with di di twins, you may be wondering what the doctors are doing to help you and your babies. Having twins of any kind will come with a higher chance of premature birth and pre-eclampsia than a singleton would experience.
However, the doctor will likely treat your pregnancy similarly to a singleton pregnancy but with extra monitoring and doctor visits. Your doctor will prescribe prenatal vitamins, check your blood, test your urine, give you an ultrasound, and monitor your baby’s heart rate with a Doppler at most visits.
In addition, you may be seeing a doctor in the maternal-fetal medicine unit at your hospital. This doctor will check your babies’ growth regularly to make sure they are developing as they should. You’ll probably have to come in for some additional testing as well, such as gestational diabetes screening and additional ultrasounds.
Depending on what your obstetrician and maternal-fetal medicine doctor have planned for you, it is likely that you will be visiting the hospital more regularly in the last trimester for monitoring and additional testing. The doctors are looking closely at where your placentas are implanted. It’s possible that one placenta could detach from your uterus, so they want to make sure that each baby has enough room and that you don’t have any complications.
How is twin chorionicity identified?
The definition of chorionicity is the number of chorionic (outer) membranes that surround babies in a multiple pregnancy. If there is only one membrane, the pregnancy is described as monochorionic; if there are two membranes, the pregnancy is described as dichorionic.
Twin chorionicity is identified by ultrasound monitoring. During the scan, the doctor will be looking at:
- The number of placentas
- the Lambda (aka twin peak sign) or T‑sign
- the presence of amniotic membrane(s) and membrane thickness
Because twin chorionicity is most often identified during a routine ultrasound, it may not be necessary to do any additional testing. But an extra test, such as a saline ultrasound or a blood type/DNA marker test, may be ordered if there is any question about the chorionicity.
The prospect of dichorionic dizygotic twins does not mean that your pregnancy will be difficult. With diligent monitoring by your doctor and careful attention to nutrition, you absolutely can have a healthy twin pregnancy.
If you just heard that second heartbeat for the first time, or you know it’s been two for a while, you need to read our twin pregnancy week by week timeline to help you learn what happens week by week with twins. Click here to learn more… and while you’re at it, check out our expecting twins classes and twin parent coaching services.
Mo di twins
Mo di twins (aka mono di twins) mean that each baby has its own sac, but they share the same placenta (called a chorionic sac). This means that one amniotic sac is shared between two babies. Mo di twins may be underweight when they’re born because it’s harder for them to get enough nutrients from one shared placenta.
Before birth, both babies are sharing the same amniotic fluid and they share some of their blood supply. Mo di twins run the risk of their umbilical cords tangling, which poses a threat to the babies’ blood supply.
Doctors typically monitor monozygotic pregnancies closely for twin to twin transfusion syndrome, also known as TTTS. This means that one baby is getting more blood flow than the other, which can result in a deformity in one or both babies.
The doctor may choose to deliver your babies early for a C-section if there are signs of TTTS so they can perform an ultrasound and exam to see what’s going on with each baby.
In addition, you need to be monitored for TAPS, twin anemia polycythemia sequence. TAPS is a rare disease that affects twins sharing a placenta. It’s caused by tiny connections in the placenta that only allow red blood cells to move from one baby to another.
This causes one twin to become anemic, with blood like rosé wine, and the other with blood as thick as ketchup, known as polycythemia. There are also other signs of TAPS on ultrasound, which are essential to know about. A starry sky liver, bright-looking placenta, and enlarged heart are all warning signs.
Most doctors also monitor mo di twin pregnancies for intrauterine growth restriction and growth discordance.
Mo mo twins
Mo mo twins (or mono mono twins) mean that there’s a completely shared placenta and amniotic sac. This is the riskiest type of twin pregnancy because there isn’t as much room for each baby to grow comfortably in the uterus. It also means that one set of babies shares blood flow from their mother. As with mo di twins, the risk of complications is highest for both babies to be affected at the same time.
The biggest concern during monozygotic pregnancies is TTTS (twin-twin transfusion syndrome) and TAPS. You will have regular ultrasounds every two weeks to monitor your babies’ growth and well-being throughout your twin pregnancy. It’s important that you follow all of your doctor’s instructions to reduce the risk of complications.
If there are any signs of TTTS or TAPS, you and your babies will be monitored carefully until it can be determined if each baby is sharing blood flow. If this is confirmed, then you will be scheduled for a C-section. TTTS and TAPS are not just a concern during pregnancy; they can also be an issue after your babies are born. If you have any concerns, talk with your doctor right away so you can receive care for your twins.
Most doctors also monitor mo mo twin pregnancies for intrauterine growth restriction, fetal growth restriction, and growth discordance.
Do I need to worry with di di twins?
One of the most common pieces of advice that we offer at Twiniversity is this: You do not need to worry until your doctor tells you to worry.
Write this on a Post-it and look at it daily!
The vast majority of di di twin pregnancies are uncomplicated and carry the least amount of risks compared to the other types of twin pregnancies. If you are a mo di or a mo mo twin, your doctor will tell you what to watch out for during each visit.
It’s true that the risk for complications is higher with twins, but you’re most likely going to be fine. Di di twins have a lower risk than mo di or mo mo because they don’t share any amnions or other elements besides sharing space in the womb.
This means that each person has their own placenta and fetal membranes which makes it easier for them both to grow healthily during pregnancy.
If you are pregnant with twins we want to congratulate you! We hope this blog post was helpful as a resource when facing some of your worries about your twin pregnancy. Our team would love to help support you through these next few months (and years!)