Some identical twins look like exact carbon copies of each other. There's no mistaking that they're twins.
Others, however, look similar, but you might question whether they're fraternal or identical.
When identical twins look different, there could be a few different factors at play.
When identical twins look different, there could be a few different factors at play.
Sometimes, how similar a set of identical twins looks depends on when the egg splits—the later it splits, the more similar they'll look because they'll have more genetic similarities "locked in" before the division.
Other times, a pregnancy complication could be a contributing factor.
One such condition is Twin-to-Twin Transfusion Syndrome (TTTS).
What is TTTS?
Twin-to-Twin Transfusion Syndrome (TTTS) is a life-threatening complication that affects between nine and 15 percent of monochorionic (identical) pregnancies.
According to Johns Hopkins, with TTTS, "sometimes the vessel connections within the placenta are not evenly dispensed and there is an imbalance in the blood exchange between the twins."
This means that one twin, called the donor twin, gives more blood than it receives, can become malnourished, and runs the risk of organ failure.
Meanwhile, the other twin, called the recipient twin, receives more blood than it needs, which can cause the heart to work too hard and can lead to cardiac complications.
There are different forms of TTTS, including TAPS (twin anemia polycythemia sequence)), SIUGR (selective intrauterine growth restriction)), and TRAP (twin reversed arterial perfusion).
If you're pregnant with multiples, it's important to go for frequent appointments with your maternal-fetal medicine team, so they can conduct ultrasounds (including MCA doppler), monitor your babies' measurements, and either diagnose or rule out conditions like TTTS, TAPS, SIUGR, and TRAP.
The risks of sharing one placenta
There are many different types of twins, from fraternal (or Di/Di) twins with two placentas and two amniotic sacs, to identical twins, which can have either one or two placentas and either one or two sacs, depending on which type of identical twins you're carrying.
Monochorionic/Diamniotic (Mo/Di) twins are identical babies that share one placenta but each have their own individual amniotic sac. This type of twins occurs in about three or four of every 1,000 pregnancies.
Meanwhile, Monochorionic/Monoamniotic (Mo/Mo) twins share one placenta and one sac and are considered incredibly high-risk. This type of twins is one of the rarest types of twins, making up less than one percent of all births in the United States and about five percent of all monochorionic twin pregnancies.
Carrying Mo/Di or Mo/Mo twins is higher-risk than Di/Di twins because the placenta and its network of blood vessels that provide oxygen and nutrients for development in the womb is usually meant to nourish just a single baby—but, in this type of twin pregnancy, it's shared between both babies.
The shared blood supply in a Mo/Di or Mo/Mo pregnancy can lead to complications with your unborn babies. This is why people who are pregnant with these types of twins will go for screening more often than those who are pregnant with Di/Di babies, who each have their own placenta and sac (that is, unless they have other medical reasons for needing increased monitoring).
When growth restriction is selective
Selective intrauterine growth restriction (SIUGR) is a form of TTTS, and it occurs when there's an unequal share of the placenta between your babies.
This condition can be caused by problems with the placenta and umbilical cord(s), certain infections during pregnancy, or health conditions the mother lives with.
You may be diagnosed with this condition when one of your twins is under the 10th percentile for growth and there's more than a 20 percent size difference between your babies.
Intrauterine growth restriction is the reason our Mo/Di twins were born 6.5 weeks early, weighing only 4 lb., 13 oz. and 3 lb, 7 oz.
While such a big size difference between your babies makes it "easy" to tell your newborn preemies apart, fetal growth restriction is a serious issue that impacts about 10 percent of pregnancies.
When a fetus fails to receive enough oxygen or nutrients, it can weigh less than average for its stage of development.
The amount of oxygen and nutrients each baby receives depends on where their cords insert on the shared placenta after the embryo splits.
If both cords attach near the center of the placenta, each baby will receive a decent share of nutrients; if one cord attaches at the center and the other one attaches along the edge of the placenta (this is called a marginal cord insertion), the baby whose cord is along the edge will receive far less than their share of nutrients. This can then lead to selective intrauterine growth restriction.
SIUGR poses risks to the affected baby's immediate and long-term health. Babies with this condition can be born prematurely and may need to spend time in the neonatal intensive care unit before they're well and strong enough to come home.
They may also have to deal with challenges like delayed development or metabolic issues. This is precisely why early detection and prenatal monitoring throughout your twin pregnancy is vital.
Symptoms of SIUGR
Selective intrauterine growth restriction can be detected during routine ultrasounds, which is why it's so important to go for regular screening throughout your entire pregnancy.
- Possible signs of SIUGR can include:
- One of your babies' estimated fetal weight being below the 10th percentile for its gestational age.
- A more than 20 percent size difference between your babies.
- Poor circulation of blood flow from the placenta through the umbilical cord or blood vessels (this is done by reviewing the Doppler flow).
- Too little amniotic fluid surrounding the fetus.
Make prenatal monitoring a priority
When you're pregnant with twins, you run a higher risk of running into complications.
There are several conditions you need to be cognizant of, including Twin-to-Twin Transfusion Syndrome (TTTS). This dangerous condition can lead to challenges for your babies, and if it goes undiagnosed and unmonitored, it can lead to the loss of one or both of your babies.
Regular prenatal monitoring with your maternal-fetal medicine team throughout your entire pregnancy is crucial and can help diagnose or rule out conditions, like TTTS or SIUGR, so that you can decide on the best course of action for you and your twins.
Were you diagnosed with TTTS during your twin pregnancy? Which form of TTTS did you have to have monitored? Let us know in the Comments section below or connect with us on Instagram to share your thoughts!
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