Vanishing twin syndrome refers to a condition that can take place during early or later pregnancy. Vanishing twin syndrome is a type of miscarriage.

When more than one embryo appears to be developing in your uterus, you might be told that you’re carrying twins — or in some cases, triplets or more.

Later in the pregnancy, though, one of the embryos or fetuses may no longer be detected. The baby that doesn’t develop fully is called the vanishing twin.

Doctors didn’t know much about how often vanishing twin occurred until the development of ultrasound technologies. Now that mothers can view their developing babies from very early in pregnancy, this condition is diagnosed more often. After the developing twin disappears, its fetal tissue is absorbed by the surviving baby and its mother.

A vanishing twin can cause feelings of confusion, anxiety, and grief for people who have been told they’re carrying multiple pregnancies.

Vs. parasitic twin

A vanishing twin is related to but different than what’s called a parasitic twin. With a parasitic twin, two embryos begin developing together. They don’t fully separate in the initial phases of development, as is the case with conjoining twins. Then, one of the embryos stops developing, as with vanishing twin syndrome.

When these two situations happen, a baby might be born with tissue from the twin that didn’t develop — the “parasitic twin” — still attached to it.

Hard statistics about vanishing twin are limited in scope. Part of that is because ultrasound technology, which has given us insight into how common vanishing twin might be, is fairly new.

A vanishing twin can also occur before a person’s first ultrasound appointment, which typically happens at 12 weeks unless the pregnancy is considered high-risk. That means that in many cases of vanishing twin, parents and doctors never know.

At least one study suggests that vanishing twin happens more after natural conception of twins than in cases of in vitro fertilization. That same study estimates that 18.2 percent of multiples conceived without fertility treatments involve a vanishing twin. Some would put that number even higher — Seattle Children’s estimates that in pregnancies of multiples, a vanishing twin may occur up to 30 percent of the time.

Losing a developing fetus during the latter part of pregnancy isn’t defined as vanishing twin. This kind of loss is instead considered a late-term miscarriage. Causes and statistics for late term miscarriages vary widely.

There are some pregnancy symptoms that could indicate vanishing twin syndrome. Keep in mind that these symptoms don’t indicate that you’re certainly experiencing vanishing twin. Pregnancy symptoms feel different for everyone, and symptoms that appear to fluctuate or “disappear” aren’t typically a cause for concern.

Cramping and bleeding

Light spotting called implantation bleeding occurs in many healthy pregnancies. But if your doctor has confirmed that you’re carrying multiples and you then experience symptoms of cramping and some bleeding, it’s possible one of the embryos has stopped developing.

Abnormal hCG levels

Human chorionic gonadotropin (hCG) is a hormone that’s tested to detect if you’re pregnant or not. If you’re pregnant, especially with multiples, your doctor might want to monitor your hCG levels to make sure they’re rising the way they should. An hCG level that starts out high and then plateaus could indicate that one fetus has stopped developing.

Vanishing twin isn’t caused by any lifestyle choices made by the person who’s pregnant. From what we know about this condition, vanishing twin happens for the same reason most early miscarriages occur — something called chromosomal abnormality.

When an embryo implants in your uterus and begins to develop, the growing baby’s cells make infinite copies of its DNA every single second. During this process, chromosomes can be switched or left out of cells completely. As a result, a developing fetus can end up with DNA that can’t develop the way that it needs to. When this happens, a miscarriage occurs.

When you’re pregnant with twins or multiples, the multiple sets of DNA develop independently of each other. This means that one fetus can keep growing after its twin stops developing.

Vanishing twin syndrome is usually found during an ultrasound appointment. An ultrasound is typically first performed between 8 and 12 weeks of pregnancy, during which time you might see two or more heartbeats on the ultrasound screen. When vanishing twin happens, there’s one less embryo or fetal sac on the screen at your next appointment. If your ultrasound tech or doctor can’t find an additional heartbeat, you may be diagnosed with a vanishing twin.

In some cases, vanishing twin isn’t determined until you deliver your baby. Some fetal tissue from the twin that stopped growing may be visible in your placenta after delivery.

If you miscarry a twin during the first trimester, there’s usually little in the way of medical treatment. The twin that stops growing will be reabsorbed into your placenta and into the baby that you’re carrying.

Small indicators of the twin might remain in your placenta when you deliver your baby. In most cases, your pregnancy will continue as it would have if you were carrying one baby to begin with. There might be an increased risk of low birth weight or preterm birth for the remaining fetus, but that data on that is unclear.

If you lose a twin later in a pregnancy, your pregnancy may be deemed higher risk and require more testing and monitoring. Some studies suggest that losing a twin later in a pregnancy increases the risk of cerebral palsy for the fetus you’re still carrying.

Regardless of how early in a pregnancy it happens, vanishing twin syndrome can be emotional. The excitement, anxiety, and unknown of early pregnancy is confusing in and of itself. Finding out you’re carrying more than one child might have intimidated or thrilled you. Discovering one of the babies has stopped growing can cause feelings of grief.

Keep in mind that what you’re feeling is valid. Coping with miscarriage can look different for different people. Vanishing twin is especially confusing because you’ve lost a baby, but you’re still pregnant.

Make sure that you’re able to process your pregnancy experience with your partner or someone you trust with your emotions. Other ideas for coping with vanishing twin syndrome:

  • Join online support groups to talk about the grief you’re feeling. Support groups can be found on social media through hashtags or a group search function.
  • Talk through your feelings with someone who’s been through the same experience. Miscarriage is a lot more common than most of us acknowledge. If you’re candid about your experience, chances are you will find someone who has experienced a similar loss.
  • Give yourself extra self-care. Remember that you’re not just taking care of you — you’re still growing a baby inside of you. If at all possible, be extra gentle on yourself physically and emotionally in the days after you find out you’ve lost a twin.
  • Make a list of things that make you feel safe, comforted, and peaceful, and reserve time for things for the next week or two.

Vanishing twin syndrome is more common than many people realize. While it can be emotionally painful, the physical symptoms don’t often pose a threat to your continuing pregnancy. Give yourself time, space, and safe places to heal and grieve your loss.

If you’re experiencing spotting, cramping, or pelvic pain during pregnancy, you should always contact your pregnancy care provider. Only a medical professional can diagnose your symptoms and tell you if you need to be concerned.