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Conjoined twins are twins — two babies — that are physically connected to one another. They may be connected at the chest, abdomen, head, or some other part of the body and often share organs in these areas.

Conjoined twins are rare and face a number of health challenges before and after birth. If they survive to infancy, some conjoined twins may be surgically separated, depending on how they’re connected and what organs they share.

Here’s more about what causes conjoined twins, what symptoms you may experience with a conjoined twin pregnancy, and what options are available for treatment or separation after birth.

Conjoined twins are the result of a monozygotic twin pregnancy (identical twin pregnancy) that comes from the fertilization of just one egg.

During a typical identical twin pregnancy, the embryo divides at about 13 days after fertilization. With a conjoined twin pregnancy, this division is delayed. Sometime between day 13 and day 15, the embryo begins to divide but stays fused together in certain areas of the developing bodies.

Regardless of where they’re joined, the twins will be monochorionic monoamniotic, meaning that they’ll both share a single placenta and single amniotic sac.

The common types of conjoined twins are:

  • Thoraco-omphalopagus twins: These twins are joined at the chest and abdomen. This type makes up around 28% of conjoined twin pregnancies.
  • Thoracopagus twins: This type is joined at the chest. This type makes up around 18.5% of conjoined twin pregnancies.
  • Omphalopagus twins: These twins are joined at the abdomen. This type makes up around 10% of conjoined twin pregnancies.
  • Heteropagus twins: These are parasitic twins. This type makes up around 10% of conjoined twin pregnancies.
  • Craniopagus twins: These twins are joined at the head. This type makes up around 6% of conjoined twin pregnancies.

Less common types include:

  • Pygopagus twins: These twins are joined at the sacrum/perineum.
  • Rachipagus twins: These twins are joined at the spine.
  • Ischiopagus twins: These twins are joined at the lower abdomen/pelvis.
  • Cephalopagus: These twins are joined between the head to the belly button.

Depending on where the twins are joined, they may share organs to different degrees. Again, each case is unique and requires close monitoring by doctors to determine how much of the organs are shared.

There are no specific symptoms you may experience if you’re pregnant with conjoined twins. Instead, you may have symptoms similar to those in a typical pregnancy with multiples. These symptoms include:

  • rapid weight gain starting in the first trimester
  • severe morning sickness (nausea and vomiting)
  • extreme fatigue
  • severe breast pain and tenderness
  • large uterus for gestational age

While these symptoms are similar to those associated with pregnancy in general, it’s the intensity that’s different with multiples, including conjoined twins.

The reason? Research shows that higher levels of human chorionic gonadotropin (hCG) circulate in your body if you’re pregnant with multiples. More hormones may mean more intense pregnancy symptoms.

Many researchers believe that conjoined twins begin their journey just as other identical twins do.

An egg becomes fertilized by a sperm and travels down the fallopian tubes to implant into the uterus. Unlike typical identical twins, the embryo doesn’t fully separate into two separate babies. For some reason, the babies stay fused together and continue to develop that way until birth.

We don’t know why the embryo doesn’t fully separate. And in fact, some researchers believe that the embryos may separate normally but then fuse together early in their development. In either case, more research is needed to determine the exact cause of conjoined twins.

Higher hCG may give you an early heads up that you’re having multiples. But since other situations may lead to higher hormone levels, like molar pregnancy, it’s not a reliable way to diagnose conjoined twins.

Ultrasound allows doctors to visualize what’s going on inside the uterus by using high frequency sound waves to create a moving image. This test can show how many babies are in the uterus and whether they share an amniotic sac and fetal pole. Ultrasound can show whether twins are joined as soon as the first trimester.

Magnetic resonance imaging (MRI) can provide additional confirmation and detail. This test can show where the babies are connected and which organs they share. MRI doesn’t use radiation, so it’s safe during pregnancy.

Pregnancy with conjoined twins is considered high risk, so you’ll likely be referred to a maternal-fetal medicine (MFM) specialist or perinatologist for additional support.

Conjoined twins are usually delivered via cesarean delivery (C-section) near term to minimize risks. The average age at delivery for conjoined twins is reported to be between 34 and 35 weeks.

After delivery, the twins may need care in the neonatal intensive care unit (NICU). Experts share that around 25% of conjoined twins survive long enough to be considered for separation surgery.

That said, separation depends on where the twins are joined and which vital organs are shared. This means that some twins may be separated while others can’t be separated.

Separation surgery is undergone on a case-by-case basis when the twins are between 3 months old and 3 years old. No two surgeries are alike, and they bring their own individual risks and ethical questions. You’ll work closely with your doctor to determine the right choice in your case.

Complications of separation surgery may include:

There’s no standard treatment for conjoined twins. Instead, your doctor will take into consideration the babies’ health and any other special circumstances that are unique to their physical connection.

You’ll likely have a team made of many different medical doctors who can advise you on the next steps to help your twins with health issues, development, and other areas.

Options might include:

  • physical therapy
  • occupational therapy
  • specialized devices to help with mobility

There are no specific risk factors — genetic or environmental — for having conjoined twins.

They don’t run in families like fraternal twins sometimes do. Instead, conjoined twins occur when identical twins don’t fully separate. The chance of having identical twins is the same for all women — around 1 in 250 pregnancies.

Conjoined twins are 3 times more common in pregnancies with female twins than in male twins. And if you have one set of conjoined twins, your chance of having another set isn’t higher — you still have the same likelihood as everyone else.

Conjoined twins face various challenges throughout pregnancy and after birth. Around 28% of conjoined twins die during pregnancy or are stillborn. Another 54% may die soon after they are born. This leaves 18% of conjoined twins who live beyond the 24-hour mark after birth.

After the newborn period, the outlook depends on where the twins are connected. Researchers explain that the overall survival rate for conjoined twins is 7.5%. Around 60% of conjoined twins who are separated will survive.

How common are conjoined twins?

Conjoined twins are very rare. The rate ranges from 1 out of every 50,000 births to 1.5 out of every 100,000 births.

Can one conjoined twin be female and the other male?

No. Conjoined twins arise from what’s called a monozygotic twin pregnancy. This is where an embryo typically splits into two identical twins. Identical twins are always the same sex.

Why are conjoined twins sometimes called ‘Siamese twins’?

“Siamese twins” is an outdated term that originated in the 1800s. It originally referred to a set of conjoined twin brothers named Chang and Eng Bunker. They were born in Siam (now Thailand) and traveled throughout England as part of a circus exhibit called “The Siamese Twins.”

There are no set risk factors for having conjoined twins. Instead, they develop spontaneously much like typical identical twins do. There are a variety of different ways twins may be joined, and how they’re joined will determine whether separation is possible.

If you’re pregnant with conjoined twins or have other concerns about your babies’ development, your doctor can explain the risks and treatment options that apply to your specific case.