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
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.
(Video) Monochorionic vs Dichorionic Twin Pregnancy
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.
(Video) Twins, types causes and significance of twins
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!
Multiple birth babies have about twice the risk of congenital (present at birth) abnormalities including neural tube defects (such as spina bifida), gastrointestinal, and heart abnormalities.What are the highest risk type of twins? ›
One placenta and one amniotic sac.
This is the riskiest and rarest type of twin pregnancy. Fetal complications can arise due to tangling of the umbilical cords or an imbalance in nutrients, blood or other vital life supporting systems.
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.What gender is most common in di-di twins? ›
Are Di-Di Twins Usually the Same Gender? While identical twins will always be the same gender, di-di fraternal twins can go either way. You can end up with girl-girl twins, boy-boy twins, or boy-girl twins. In fact, science has found that the most common type of fraternal twins is boy-girl!What is the safest type of twins? ›
While there are risks associated with all twin pregnancies, dizygotic (fraternal) twin pregnancies usually bring about the least amount of complications, carrying the lowest risk of all types of twins. Worse complications are possible with monozygotic (identical) twins because the embryo is dividing.Are twins more likely to have health problems? ›
Multiples are about twice as likely as singleton babies to have birth defects, including neural tube defects (such as spina bifida), cerebral palsy, congenital heart defects and birth defects that affect the digestive system. Growth problems. Multiples are usually smaller than singleton babies.Are you more likely to miscarry twins? ›
Twin pregnancies have a higher rate of miscarriage. In some cases, one twin may miscarry or simply "vanish," leaving a surviving twin. This is also known as vanishing twin syndrome. Twins are at risk for intrauterine growth discordance, which is when one twin grows significantly slower than another.When should I stop working when pregnant with twins? ›
Mums-to-be of twins usually start their maternity leave at 26 weeks. But it will also depend on your health, how well your pregnancy is going, and the type of work you do. Talk to your line manager or HR department if you need to leave earlier than 26 weeks, if, for example, you have complications with your pregnancy.When does risk of miscarriage decrease with twins? ›
The risk is highest in the first trimester, although it drops significantly once a fetal heartbeat is established, Roshan says. A 2003 study shows that the risk for twin miscarriage once the babies' heart beats are detected is around 7 percent.How common are Didi twins? ›
Dichorionic twins are a form of multiple gestation in which each twin has a separate placenta (blood supply) and amniotic sac. Dichorionic twins are usually–but not always –fraternal (non-identical). Twins represent more than three percent of all U.S. live births, with the majority being dichorionic.
Do Identical Twins Run in Families? Identical twins are typically not hereditary like fraternal twins and occur in three to four births out of every 1,000 globally. A few families report a higher level of identical twins than expected, so there may be a genetic factor in rare cases.Are di di twins usually the same gender? ›
Fraternal or 'dizygotic' twins
Two separate eggs (ova) are fertilised by two separate sperm, resulting in fraternal or 'dizygotic' (two-cell) twins. These babies will be no more alike than siblings born at separate times. The babies can be either the same sex or different sexes, with the odds roughly equal for each.
Approximately 10% of Di/Di twins will be monozygotic, resulting from the early splitting (within the first 3 days) of a single embryo. Both fetuses will have arisen from the same egg and sperm, and therefore, will be genetically identical (and have the same sex as confirmed by ultrasound).Can Di Di twins have different dads? ›
It is possible for twins to have different fathers in a phenomenon called heteropaternal superfecundation, which occurs when two of a woman's eggs are fertilized by sperm from two different men.What week are most di di twins? ›
The most significant percentage of twins (49%) was observed to be delivered at 37-38 weeks, and 6% were preterm. About 60% of mothers were between 25 and 35 years of age, and in 57%, this was their first pregnancy.What not to do when pregnant with twins? ›
It's never advisable to drink alcohol excessively, smoke, or take drugs, whether you are pregnant or not. When you are pregnant, doing so exposes your unborn babies to toxic substances, raising their risk of birth defects and chronic illnesses.What age is best to have twins? ›
Age. According to the Office on Women's Health , women who are aged 30 years or older are more likely to conceive twins. The reason for this is that women of this age are more likely than younger women to release more than one egg during their reproductive cycle.Can fraternal twins have Down syndrome? ›
Results: Of 77,279 twin pregnancies, 182 (0.2%) had at least one fetus with Down syndrome confirmed by karyotype. The ratio of observed-to-expected Down syndrome incidence per pregnancy was 33.6%, 75.2%, and 70.0% for monozygotic, dizygotic, and all twins, respectively (P<.Do twins have shorter life expectancy? ›
Twins not only have a bestie from birth — they also live longer than singletons. And those two factors may be related, according to new University of Washington research.What are the disadvantages of being twins? ›
- Premature birth. Twins are born prematurely more than half the time. ...
- Low birth weight (LBW). More than half of twins are born with LBW, weighing less than 5 1/2 pounds. ...
- Twin-twin transfusion syndrome (TTTS).
The strongest risk factor for suicide is mental illness and studies show that mental illness is slightly more common among twins.Do all twin pregnancies have morning sickness? ›
Overall, 70% to 80% of women can experience some amount of nausea and vomiting with pregnancies, and up to 2% reportedly experience hyperemesis gravidarum, a form of severe morning sickness. At the same time, some moms of twins and triplets say they have no morning sickness.How many ultrasounds do you have during twin pregnancy? ›
In uncomplicated dichorionic twin pregnancy, ultrasound imaging should be performed in the first trimester, again at around 20 weeks' gestation (second-trimester anomaly scan), and every 4 weeks thereafter (unless a complication is detected which might require more frequent scans) (Figure 2)1.Are you a womb twin survivor? ›
A womb twin survivor is someone who lost their twin or multiple anytime through pregnancy or shortly after birth; thus miscarriage, stillbirth, failed abortion, or neonatal death. It has been estimated that 10–15% of all single births were originally a multiple, often with a twin being lost within the first trimester.Do you get extra benefits for twins? ›
You're not entitled to extra maternity or paternity benefits if you're expecting twins or multiple babies. Your entitlements to leave and pay are the same as if you were expecting one baby. But you can claim Child Benefit and Child Tax Credit for each of your children.At what week do twins not need NICU? ›
If your twins are born close to your due date or the average for twins, that's 36 weeks of the pregnancy, the odds are pretty good that they will not need any time in the NICU and they can go home with mom.How long do you stay in the hospital after having twins? ›
For twins, the average stay is nine to 25 days. For triplets it's 11 days to up to three months.What stage are you least likely to miscarry? ›
For women who know they're pregnant, about 10 to 15 in 100 pregnancies (10 to 15 percent) end in miscarriage. Most miscarriages happen in the first trimester before the 12th week of pregnancy. Miscarriage in the second trimester (between 13 and 19 weeks) happens in 1 to 5 in 100 (1 to 5 percent) pregnancies.Do Didi twins have separate placentas? ›
There are three basic types of twins.
In di/di twins, each twin has their own placenta and their own amniotic sac. Monochorionic/Diamniotic (mo-di) twins. Mo/di twins share a placenta, but each baby has it's own amniotic sac.
However, for a given pregnancy, only the mother's genetics matter. Fraternal twins happen when two eggs are simultaneously fertilized instead of just one. A father's genes can't make a woman release two eggs.
Can twins have two different blood types if they have the same father? Definitely yes if they are fraternal twins. And although much less common, it is also even possible if they are identical twins. In fact, a mom, dad, and twins could all end up with different blood types!How do you know if Di Di twins are identical? ›
Sometimes health care professionals identify same-sex twins as fraternal or identical based on ultrasound findings or by examining the membranes at the time of delivery. The best way to determine if twins are identical or fraternal is by examining each child's DNA.Do twins have higher risk of miscarriage? ›
What are the chances I'll miscarry if I'm pregnant with twins? If you're carrying more than one baby, your risk of having a miscarriage is higher than with a single pregnancy, particularly if your babies are identical.Are twins more likely to have fertility issues? ›
There are virtually no relevant differences between the fertility patterns of dizygotic and monozygotic twins.Do twins have a higher rate of miscarriage? ›
Twin pregnancies have a higher rate of miscarriage. In some cases, one twin may miscarry or simply "vanish," leaving a surviving twin. This is also known as vanishing twin syndrome. Twins are at risk for intrauterine growth discordance, which is when one twin grows significantly slower than another.How often do twins miscarry? ›
Likelihood of Vanishing Twin Syndrome
According to one study, about 36% of twin pregnancies experience vanishing twin syndrome. It also occurs in around half of multiple pregnancies, or pregnancies where a woman carries more than one baby.
How can I prevent vanishing twin syndrome? You can't prevent vanishing twin syndrome. Many people who experience a miscarriage worry over what they could've done differently to deliver a healthy baby. But the genetic abnormalities that cause an embryo to stop developing aren't curable or preventable.At what age do twins become more common? ›
Age. According to the Office on Women's Health , women who are aged 30 years or older are more likely to conceive twins. The reason for this is that women of this age are more likely than younger women to release more than one egg during their reproductive cycle.Are twins more likely to have developmental delays? ›
Since twins tend to be born early, it is common for them to have developmental delays since they just didn't have enough time to fully develop in the womb. Depending on when your twins were born, their development will vary.What type of twins is more common with fertility treatment? ›
The chance of a single embryo dividing and resulting in identical twins is higher after IVF than after natural conception. So it is possible to end up with identical twins from a single transferred embryo.