Having a twin pregnancy can be an exciting yet anxious time for expectant parents. Unfortunately, the sad reality is that twin pregnancies do carry a higher risk of complications, including miscarriage. This article will examine how common it is for one twin to miscarry, the causes, risks and what to do if it happens.
Quick Facts
- Around 1 in 5 twin pregnancies will experience the loss of one twin, known as vanishing twin syndrome.
- Up to 30% of twin pregnancies diagnosed in the first trimester will go on to miscarry one twin.
- After the first trimester, the risk drops to around 8-10% chance of losing one twin.
- Vanishing twin syndrome is more common in identical (monozygotic) twin pregnancies.
- Certain maternal factors like age, smoking status and conception method can increase vanishing twin risk.
- In most cases, the remaining twin will go on to be born healthy if the pregnancy continues without other complications.
What is vanishing twin syndrome?
Vanishing twin syndrome is the loss of one twin during the first trimester of a multiple pregnancy. It occurs when a twin pregnancy is initially detected, but later in the pregnancy only one fetus can be found.
The ‘vanished’ twin is reabsorbed back into the mother’s body. This usually happens very early on, often before the mother even knew she was pregnant with twins. That’s why it’s estimated that vanishing twin syndrome happens in around 21-30% of multifetal pregnancies.
When does vanishing twin syndrome happen?
Vanishing twin syndrome most often occurs within the first trimester:
- Weeks 1-5: Around 30% of twin pregnancies miscarry one twin.
- Weeks 6-8: The risk drops to around 15%.
- Weeks 9-12: The risk further drops to 8%.
After the first trimester, the chances of miscarrying one twin are significantly lower at just 8-10%. The risk becomes even smaller in the second and third trimester.
This lines up with when miscarriages in general are most likely to occur. The further along in the pregnancy, the lower the chances of a vanishing twin.
Causes of Vanishing Twin Syndrome
There are a few possible causes of why one twin may miscarry in early pregnancy:
- Chromosomal abnormalities – Around 50% of miscarriages are due to genetic issues with the fetus. This may mean the twin is not developing properly and is reabsorbed.
- Problems with the placenta or umbilical cord – If there are issues with the developing placenta or umbilical cord, one twin may not receive enough nutrients and blood flow to survive.
- Birth defects – Major congenital abnormalities may mean a twin is nonviable and miscarried.
- Early fetal growth problems – One fetus may grow at a slower rate, causing it to vanish.
- Trauma – Physical trauma to the uterus in early pregnancy can potentially lead to loss of one twin.
In many cases, the cause is unknown. Vanishing twin syndrome seems more likely due to chance or natural reduction rather than external factors.
Risk Factors
Certain factors may potentially increase a woman’s risk of miscarrying one twin:
- Maternal age – Women over 30 have higher rates of vanishing twin syndrome.
- Fertility treatments – Medications used in IVF and other procedures can create twins or triplets, increasing vanishing twin likelihood.
- Early ultrasound – Getting an ultrasound very early at 4-5 weeks can sometimes detect a twin before one is miscarried.
- Being pregnant with multiples – Having triplets or quadruplets has increased risks.
- Smoking, alcohol use, poor diet – Unhealthy maternal lifestyle habits may contribute.
- Prior miscarriages – A history of pregnancy loss or complications puts a woman at higher risk.
- Uterine abnormalities – Problems with the structure of the uterus can increase chances.
However, a vanishing twin can happen to any woman. Many healthy women with normal twin pregnancies may also experience it by chance.
How is a Vanishing Twin Detected?
Vanishing twin syndrome is usually diagnosed during an early ultrasound scan:
- A very early scan may show two gestational sacs or fetal poles, confirming a twin pregnancy.
- A follow up scan some weeks later only shows evidence of one fetus, indicating twin loss.
- The mother may have also had hCG levels that indicated twins initially.
Sometimes, the remains of the miscarried twin are visible on an ultrasound. But often there are no signs of the vanished fetus. The placenta and amniotic membranes have reabsorbed it.
Rarely, a ‘vanishing’ twin is detected later at birth when a multiple birth was expected. This could be due to a very early scan missing the second twin or inaccurate hCG levels.
Signs of Vanishing Twin Syndrome
Some mothers may experience symptoms or signs that one twin has been lost:
- Cramping and bleeding in the first trimester – This may indicate the miscarried twin is being passed.
- Reduction in pregnancy symptoms like nausea – As hCG levels drop, pregnancy symptoms may decrease.
- Lower abdominal size than expected – The uterus may be smaller if only one fetus remains.
- Reduced fetal movements – Less kicks may be noticeable later on if a twin is lost.
- HCG levels reduce or stop rising – Falling hCG can indicate a vanishing twin.
However, many women have no signs at all that one twin has vanished. They may continue to feel pregnant and notice no changes until a scan reveals a single fetus.
Does Vanishing Twin Syndrome Cause Complications?
The good news is that in most cases of vanishing twin syndrome, the remaining fetus is unaffected:
- Up to 97% of remaining twins will be born healthy at term with normal growth and development.
- The miscarried twin is reabsorbed without impacting the surviving fetus or mother’s health.
- Complications like premature delivery or low birth weight are no more common than in singleton pregnancies.
- Risks to the remaining baby are minimal as long as the rest of the pregnancy is uncomplicated.
However, there are some potential complications that may rarely occur:
- Hemorrhage from the miscarried twin if it occurs later in pregnancy.
- Severe blood loss or infection if remnants of the fetus remain in utero.
- The surviving twin has a marginally higher risk of cerebral palsy (1 in 75 chance).
- A small possibility of developmental or neurological issues if blood circulation was shared.
- Preterm birth may be more likely in very rare cases.
Talk to your doctor if you have any concerns about risks to your pregnancy after a vanishing twin occurs. But in most pregnancies, the one remaining fetus should not be adversely affected.
Vanishing Twin Syndrome vs Singleton Pregnancy Outcomes
Reassuring studies have found that continuing a pregnancy as a singleton after a vanishing twin has very similar outcomes to a singleton pregnancy:
| Outcome | Singleton Pregnancy | Vanishing Twin Pregnancy |
|---|---|---|
| Premature birth | 8.7% | 7.6% |
| Low birthweight | 7.2% | 4.7% |
| NICU admission | 8.1% | 7.3% |
| Developmental issues later on | 5% | 5% |
This shows overall rates of prematurity, low birth weight, intensive care admission and later development problems are very similar between vanishing twin and singleton pregnancies.
What to Do if You Miscarry One Twin
Losing one twin can be emotionally devastating for expectant parents. Here’s what to do if it happens:
- Seek urgent medical care if you have severe bleeding or cramping after a twin has vanished.
- Follow your OB’s guidance for managing the rest of the pregnancy and monitoring the surviving twin.
- Talk to a counselor about your grief. Your feelings of loss are valid.
- Join online support groups to connect with others who have experienced vanishing twin syndrome.
- Take care of yourself mentally and physically as your pregnancy continues.
- Discuss future pregnancies with your doctor to understand any potential risks if it happens again.
While emotionally difficult, try to focus on the thriving baby you still have growing. Their chance of a normal pregnancy remains very high.
Key Points
In summary, here are the key facts to understand about vanishing twin syndrome:
- Around 21-30% of twin pregnancies will miscarry one fetus, usually in the first trimester.
- After 12 weeks, the risk drops significantly to 8-10% chance.
- In most cases, the surviving twin is born healthy with no long-term complications.
- Women may notice changes like bleeding and cramping but sometimes have no symptoms.
- An early ultrasound that shows a twin followed by a later scan revealing one is the most common way vanishing twin is diagnosed.
- Talk to your doctor right away if you experience concerning symptoms after a twin vanishes.
- Join support groups and focus on your mental health as you grieve the loss but continue to care for the remaining fetus.
Conclusion
Vanishing twin syndrome is relatively common in twin pregnancies, especially very early on. But despite the sadness of losing one fetus, the outcome for the remaining baby is generally very positive if the rest of the pregnancy progresses smoothly.
While emotionally challenging, try to seek support and know your doctor will monitor you closely. For most women who experience this twin loss, they go on to deliver the surviving baby safely with no long term effects.
