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Common Conditions with Twins

Common Conditions with Twins Q&A

Expecting multiples can be an exciting and challenging time for families.  While many twin pregnancies will progress without incident,  some families may face unexpected prenatal/fetal diagnoses.  

As the first center in Illinois to offer fetal surgery, the specialists at the Center for Fetal Care are fully equipped to handle high-risk pregnancies, offering the most innovative, conservative and comprehensive care available in the tristate area.  Whether your babies need nonsurgical care, open fetal surgery, or minimally invasive procedures, our highly trained experts at Advocate Children’s Hospital are here to guide you and your babies, from delivery into childhood.

How do twins develop?

Twins can be identical or fraternal, with each type forming differently:

Identical twins happen when a single sperm fertilizes a single egg, and the fertilized egg (zygote) then divides into two identical embryos. Identical twins share the same genes.

Fraternal twins occur when two separate sperm fertilize two separate eggs. Fraternal twins don’t share the same genes – they’re like siblings from different pregnancies.

What are the different types of twins?

Beyond identical and fraternal, twins have other characteristics that can affect how they develop before birth. A baby in a single pregnancy has one placenta (organ in the womb for nourishment) and one amniotic sac (protective, fluid-filled membrane).

In a twin pregnancy, the two babies often share a placenta and sometimes can share an amniotic sac:

Dichorionic Twins

Each twin has its own placenta and amniotic sac. All fraternal twins are dichorionic but only about 30% of identical twins are dichorionic.

Monochorionic Twins

The babies share a placenta, a feature in about 70% of pregnancies with identical twins. They may or may not share an amniotic sac.

Monochorionic-diamniotic twins

Identical twins who share a placenta, but baby has their own amniotic sac. This arrangement is by far the most common.

Monochorionic-monoamniotic twins

Only about 1% of identical twins share a placenta and an amniotic sac. How does sharing a placenta affect identical twins?  With monochorionic twins, each baby’s umbilical cord connects to the same placenta, which can affect the baby's share of nutrients and resources from their mother. 

Blood vessels also develop in the placenta to supply the babies with nutrients and oxygen. It’s natural for the twins to share some of the blood vessels, and blood can flow back and forth between them.

In most pregnancies with monochorionic-monoamniotic twins, the blood supply and placenta are roughly equal between babies; however, in some cases, an unequal share can increase the risk of complications for the twins.

What causes fetal conditions in twins?

The causes of fetal conditions in twins vary depending on the disorder, but not all causes are known or fully understood. Some possible causes and risk factors include:

  • Problems with cell division in the first few days after a sperm fertilizes an egg
  • Location of umbilical cord connection with the placenta
  • Problems with the placenta
  • Certain health problems in the mother, such as diabetes, high blood pressure or


What are the signs and symptoms of fetal conditions in twins and higher order multiples?

Our fetal specialists diagnose twin and higher order multiple pregnancies through ultrasound, and evaluate the babies for:

  • Differences between amniotic sacs, including their sizes and amounts of fluid
  • Differences in the babies’ sizes and estimated weights
  • Problems with blood flow in the babies, umbilical cords or placenta
  • Signs of strain on the babies’ hearts, such as fluid buildup


Sometimes, these conditions cause symptoms that you may experience, such as:

  • Enlarged uterus because of excess amniotic fluid
  • Feeling of rapid growth in the uterus
  • Less movement of the babies
  • Pain, pressure or contractions in the belly
  • Swelling in the hands, legs and feet during early pregnancy


How are fetal conditions in twins and higher order multiples diagnosed?

In routine checkups during your first trimester, your doctor can detect whether you’re carrying twins or a higher order multiple pregnancy. If so, further imaging is performed to see whether the babies are sharing a placenta, an amniotic sac, or both.

Knowing whether you have monochorionic twins helps guide care decisions for you and your babies through pregnancy and delivery.  For monochorionic twins, our fetal care specialists see you every two weeks to monitor the health of mother and babies. You may also be referred  to our genetics program for a consultation and evaluation.

We use the latest imaging to monitor your pregnancy and confirm a diagnosis. Our state-of-the- art scans include:

What types of fetal conditions affect twins?

Monochorionic twins can have a higher risk of certain conditions than dichorionic twins and single babies. These conditions can threaten the life of one or both twins when the placenta is shared, making the right care critical.

Conjoined twins

Some identical twins develop with connected body parts. They can join at the head, chest, abdomen (belly), and other areas. After delivery, we carefully evaluate conjoined twins and work closely with you to decide on the next steps. Separation surgery may be an option, depending on the twins' health, where they connect, and which organs and body parts they share. 

Discordant anomalies

In some identical twins, one baby develops congenital conditions, but the other does not. Discordant anomalies refer to structural problems that can affect many organs and body systems. These systems include the heart, lung, kidneys, intestines, and central nervous system (brain, spine, and nerves). 

Unequal placental sharing

Some monochorionic twins develop unevenly, with their estimated weights differing by 20% or more. This variation happens because of unequal placenta and blood sharing.

Intrauterine growth restriction (IUGR)

IUGR is a more severe form of unequal placenta sharing between monochorionic twins. The smaller twin's estimated fetal weight is below the 10th percentile, or the twins' weights may differ by 25% or more. 

Twin anemia polycythemia sequence (TAPS)

One twin has too few red blood cells (anemia), and the other twin has too many red blood cells, leading to thick blood. TAPS can prevent both babies from receiving enough oxygen. 

Twin reversed arterial perfusion (TRAP) sequence

TRAP occurs in monochorionic twins, when one twin doesn't fully develop. This twin, called an acardiac twin, may have a missing or poorly developed head, heart, or limbs, and will not survive after birth. The other twin, called the pump twin, supplies blood for both twins, straining its heart. The larger the acardiac twin grows, the higher the risk of heart failure in the pump twin.   See more on this top by clicking (insert hyperlink to surgical page)

Twin-to-twin transfusion syndrome (TTTS)

In some monochorionic twins and higher order multiples, abnormal vascular connections between the twins within the placenta cause differences in blood flow to each baby. 

The donor twin loses too much blood, which reduces its nourishment, growth, and urine production. Less fetal urine means less fluid in its amniotic sac. 

The recipient twin receives too much blood, which increases its growth and urine production, leading to excess amniotic fluid.

In about 10 to 15% of these cases, TTTS will develop. TTTS can affect both babies' heart function, putting them at risk of heart failure.  See more on this top by clicking (insert hyperlink to surgical page)

Umbilical cord entanglement or compression

Because they share one amniotic sac, monochorionic-monoamniotic twins are more likely to get their umbilical cords tangled. The umbilical cord is the lifeline for your babies, with blood vessels that carry oxygen and nutrients from the placenta. As the twins move in the womb, the cords can become tangled and compressed, potentially reducing or cutting off blood flow. 

Our experienced fetal care and high-risk pregnancy specialists offer comprehensive treatment for twins and higher order multiples, including surgical and nonsurgical options. They provide expert care to help your babies have a successful start in life.

Contact us to learn more about surgical and non surgical fetal support options.