It may seem in recent years like the possibility of twins is becoming more and more of an everyday reality. If you’ve found out you’re having twins (or just recently met a family with a pair of twins), you may wonder, how did this happen?
How did two babies come to be and how genetically identical are they? They may share the same birthday, but did they share the same placenta? Do they share the exact same genes?
Unless you are a twin or have given birth to twins, you may be confused about the different types of twins and what their genetic makeups are. (Completely understandable!) Perhaps your doctor or friend mentioned that the twins in question are di/di. What does this mean? Please allow us to explain…
When someone talks about twins they are referring to two babies that develop during the same pregnancy, but this can happen in many different ways. Believe it or not, the term twins is pretty broad!
Two major categories twins are typically divided into are identical and fraternal.
Identical twins are also sometimes called monozygotic twins, because they came from the same fertilized egg. (Identical twins are created when one egg and sperm meet like normal. The single egg splits in two shortly after fertilization.)
Because identical twins come from the same egg/sperm, their chromosomes will be identical, which means they will be the same sex and have the same genetic characteristics (e.g., hair and eye color).
On the other hand, fraternal twins only share about 50 percent of their chromosomes, as other siblings do. This means they can appear nearly identical or completely different. They can be different sexes with different hair and eye colors or the same sex and look very similar.
Fraternal twins are also called dizygotic twins, because they come from two different eggs. (The mother releases two eggs at around the same time, which are fertilized by two different sperm.)
Still with us? Well, there’s even more to it than whether they are identical and fraternal if you want to know how a set of twins developed. The type of twins (fraternal or identical) and the timing of their development affects whether the babies
We know what you’re thinking… what is a chorion? (And while we’re at it, what’s amnion/an amniotic sac?) And why do they matter?
The chorion is the outermost fetal membrane. The chorion connects the amnion, amniotic fluid, and the fetus to the placenta. It also contributes to placenta development.
Why is this important? Well, if two twins share a chorion, they’ll share one placenta. If twins have separate chorion, they will have two separate placentas.
The amnion on the other hand is the innermost fetal membrane. It protects the fetus and includes amniotic fluid. Like the chorion, twins can either share an amnion or have one all of their own.
Since this is the inner layer, the options here depend on the chorion. One chorion that is shared can contain one or two amnions. So even though two babies may share the same placenta, they can each still be swimming in their own amniotic fluid if they have their own amniotic sac. On the other hand, two chorions mean two separate amnions.
Got all that? Don’t worry, we know it’s a lot to take in, so here’s a quick view of the different ways this can all come together…
In a di/di pregnancy (more scientifically referred to as a dichorionic diamniotic pregnancy) the twins each have their own chorionic and amniotic sacs. Essentially, each of the babies is growing like they would as a singleton, but just a little more cramped, because they’re sharing the same womb.
In a mo/di pregnancy (more scientifically referred to as a monochorionic diamniotic pregnancy) the twins share a chorionic sac, but have different amniotic sacs. Simply put, the difference between a di/di and a mo/di pregnancy is that a mo/di pregnancy only involves one placenta.
In a mo/mo pregnancy (more scientifically referred to as a monochorionic monoamniotic pregnancy) the twins share both the chorionic and amniotic sacs. This means one placenta and one amniotic sac for both babies. Because the babies are sharing so much, mo/mo pregnancies can be tricky and require lots of monitoring!
While it may seem like an ultrasound should be able to offer all the answers, the truth is that it has its limitations when it comes to determining di/di, mo/di, and mo/mo pregnancies.
Ultrasounds are most accurate at determining chorionicity in the first trimester. Signs that may indicate a di/di pregnancy on an ultrasound done in the first trimester are:
- two gestational sacs with a thick echogenic chorion around each embryo
- a thick inter-twin membrane
- the twin peak sign (it looks like an upside-down letter Y)
- two yolk sacs (This won’t tell whether it’s a mo/di or a di/di pregnancy though!)
Even if some of these signs are spotted, further testing may be necessary to confirm.
Di/di pregnancies account for the majority of all twin pregnancies and can produce identical or fraternal twins.
All fraternal twins are di/di, but identical twins can also be di/di. How is this possible?
With a fraternal twin pregnancy, separate eggs are individually fertilized by two different sperm cells creating two individual zygotes. Each zygote will have its own placenta and amniotic sac.
On the other hand, an identical pregnancy begins with the combination of one egg and one sperm cell, but at some point that single egg splits. When that happens affects the development of the placenta and amniotic sac.
If there is a separation of the zygotes approximately 1 to 4 days after the fertilization it’s more likely to lead to a di/di twin set-up.
Since the combinations aren’t solely determined by whether the pregnancy is fraternal or identical, it can be difficult to determine via ultrasound if the twins you’re carrying are fraternal or identical.
Obviously, if ultrasound reveals you’re carrying one of each sex, it’s clear that the twins are fraternal. However, if the twins share the same sex, it’s less clear whether or not they’re identical.
A study from 2012 found that close to 15 percent of parents who were told their twins’ zygosity based on ultrasound were misinformed. For parents who want to know for sure, genetic testing performed after birth will give the clearest answers.
Being pregnant with twins does come with some additional risks. While a di/di pregnancy carries with it the lowest rate of complications among twin pregnancies, any woman carrying twins has an added risk of issues with the placenta and intrauterine growth restriction (i.e. babies outgrowing the real estate available!).
Some complications that may be more likely with a twin birth include:
- Placenta previa. A larger or heavier placenta from having twins may be more likely to hang low and cover the cervix making normal delivery dangerous.
- Placental abruption. A larger or heavier placenta from having twins may also be more likely to tear away from the uterine wall.
- Placenta accreta. This occurs when the placenta attaches itself too deeply into the uterine wall.
- Prematurity. Because of uterine space restrictions and the potential for other complications indicating an earlier birth would be beneficial, twins have a higher likelihood of being born premature (before 37 weeks gestation).
- Low birth weight. Since twin babies develop sharing space in the uterus and may need to be born a little earlier than what is considered full term, they are at a higher likelihood of having a low birth weight. (A baby is considered to have low birth weight if it is born weighing less than 5 pounds, 8 ounces.)
- Gestational diabetes. This can occur when blood sugar levels are too high.
- Gestational hypertension. This is high blood pressure during pregnancy.
- Postpartum hemorrhage. One potential reason for a hemorrhage after giving birth is that a large placenta from having twins has caused the uterus to stretch more than normal.
Of course, knowing which type of twin combination is present during pregnancy is important to understand the level of risk involved. As noted earlier, a di/di combination comes with a lower level of risk, since each baby is enclosed in their own amniotic bubble and they’re sharing space in the womb, but not other elements.
If the babies are sharing a placenta, there is a risk that one baby may receive more of the nutrients and oxygen needed for development. This issue occurs in approximately one-third of mo/di twin pregnancies.
This unequal division can lead to issues such as twin to twin transfusion syndrome (TTTS), twin anemia polycythemia sequence (TAPS), or selective intrauterine growth restriction (SIUGR). As such, mo/di pregnancies should be more closely monitored.
The highest risk occurs when twins are sharing both the amnion and chorion in a mo/mo pregnancy. Because there is nothing to separate the babies from one another in the womb there is a risk of cord entanglement.
These pregnancies are very rare — they account for less than 1 percent of twin pregnancies — but the risks mean you’ll be monitored very closely.
Whether inside or outside the womb, twin development can be confusing and offer some extra complications. (Don’t worry though, they can also bring double the joy!)
If you’re about to be a twin parent, you likely have a lot of questions about their genetics and how they came to be. Some questions can be answered via ultrasound, but others can only be discovered through genetic testing.
Armed with a little more background information, you’ll be better prepared to ask the questions you need to get the information you want!