Overview

What is vanishing twin syndrome?

Vanishing twin syndrome is a phenomenon that occurs when one of two embryos(developing baby) is reabsorbed partially or fully by the uterus (womb) of the mother. Vanishing twin syndrome typically occurs within the first trimester of pregnancy. However, if the death of one unborn twin occurs later in the pregnancy, typically in the early second trimester, this can result in a fetus papyraceous. This is where the dead twin becomes mummified and compressed between the remaining twin and the wall of the womb.^1,2

In how many twin pregnancies does vanishing twin syndrome occur?

Vanishing twin syndrome is not extremely rare, and is estimated to occur in approximately 36% of twin pregnancies. Additionally, in other multiple pregnancies where there are 3 or more gestational sacs present, this phenomenon occurs in 50% of pregnancies.¹

Signs and symptoms of vanishing twin syndrome

What are the early signs?

Ultrasound findings

Due to the availability of ultrasounds, vanishing twin syndrome can be detected early in the first trimester of pregnancy, where more than one gestational sac and embryo are visualised early on an ultrasound scan. However, at later scans one of the embryos is no longer present.

Hormone levels

Beta-chorionic gonadotropin (β-hCG) hormone is a hormone that is used to detect pregnancy. This hormone is detected in a pregnancy test. β-hCG levels do not increase as quickly as they do in pregnancy with just one unborn child or twin pregnancies.  However, it is important to note that these increases are still within the clinically accepted normal limits, and having an abnormal trend in the rising and falling of your β-hCG levels does not necessarily mean that you are suffering from vanishing twin syndrome.³

What are the later signs?

Absorption of fetal tissue

When one of the twins dies in the womb the fetal tissue of the dead twin typically gets reabsorbed either partially or fully by the mother’s tissues.  However,  in some cases, the dead embryonic tissue is absorbed by the surviving twin, which can result in the surviving twin taking on some of the DNA of the dead twin.  This is known as chimerism where the DNA from two different zygotes (fertilised eggs) is merged into one embryo. If both embryos were the same gender chimerism is less likely to be detected, and very few cases have been reported in humans.  However, if the embryos are of different genders, chimerism can result in abnormalities in external genitalia, which is more likely to be identified by healthcare professionals.⁴

What symptoms may you experience?

There may not be any symptoms. However, you may experience symptoms of miscarriage within your first trimester of pregnancy, with the following being symptoms you should look out for:

• Bleeding
• Experiencing pelvic pain
• Having uterine cramps

Which conditions have a higher chance of occurring due to vanishing twin syndrome?

The following complications of pregnancy and health conditions have a higher chance of occurring if you have been diagnosed with vanishing twin syndrome. 

However, it is important to note that in one study the complications were higher when compared with pregnancy with one baby.⁵

Pregnancy complications

The following obstetric complications have an increased risk of occurring if you have been diagnosed as having vanishing twin syndrome:⁶

• Placental insufficiency: the placenta is very important during pregnancy as it aids in delivering oxygen and nutrients to your unborn baby. However, in placental insufficiency, there are issues in how the placenta works, which may lead to issues surrounding the growth of the unborn child, and further complications
• Placental abruption: placental abruption is a condition where the placenta prematurely detaches from the lining of the uterus.⁷
• Umbilical cord around the neck: this condition is known as the nuchal cord, where the unborn baby’s umbilical cord wraps itself 360 degrees around the neck of the unborn baby.⁸
• The unborn child having congenital abnormalities: congenital abnormalities are also known as birth defects, and are defined as structural or functional abnormalities
• True knot in the umbilical cord

Health conditions during pregnancy 

The following health conditions have a higher chance of occurring if you have been diagnosed with vanishing twin syndrome:⁶

• Gestational diabetes
• Gestational hypertension
• Preeclampsia

Causes of vanishing twin syndrome

It is important to note that the cause of vanishing twin syndrome, in most cases, cannot be pinpointed by a healthcare professional. However, the possible causes of vanishing twin syndrome are listed below.¹

Biological factors

Chromosomal abnormalities in one twin

Chromosomal abnormalities are when part of the chromosomal DNA (genetic material) is missing, there is an extra chromosome or a structural abnormality in the chromosomal DNA. Chromosomal abnormalities in the dead twin may cause vanishing twin syndrome.¹ 

Circulating free DNA from the vanishing twin can be detected in the mother’s circulatory system and can remain in the uterus, even after the death of the twin. Examination of this DNA has detected chromosomal abnormalities.⁴

Factors in the mother

Mothers age

Being an older mother is associated with an increased risk of having vanishing twin syndrome, with mothers over 30 having a higher risk.¹

Other factors

Other factors include if you have used assisted reproductive technologies to aid in getting pregnant, such as IVF(Invitro Fertilization).¹

Impact and considerations

Emotional impact on parents

Losing one twin whilst your other twin survives can be a complicated experience, as whilst you experience the joy of the birth of your child, you are also grieving for the loss of your other child. The experience of having a newborn may make it difficult for you to find both time and space to feel your emotions and work through your grief. Your grief may feel different, you may experience devastating loss and grief for your child, but also grief for not becoming a parent to twins.

 As a result of this experience, you may experience a range of emotions, including anger, anxiety, numbness, guilt, denial, disappointment and the feeling of failure. The vastness of emotions you may experience after this traumatic event may be conflicting, but it is important to be aware of what you are feeling and experiencing during this time of grief. Your local hospital may be able to refer you to either a bereavement midwife or counsellor to help you work through this period of grief. It is important to reach out for help if needed, and not feel shame after such a harrowing experience.

If you are looking to support parents who have suffered the loss of a twin it is important to acknowledge the twin that has sadly passed.

Potential health implications for the surviving twin

The death of one unborn twin in monochorionic pregnancies (where twins share a placenta) has been sadly associated with a higher risk of serious cerebral injury and neurodevelopmental delay. In addition, another study also reported the importance of diagnosing vanishing twin syndrome due to the surviving twin having a higher risk of suffering from mental health illnesses, which may affect their quality of life.⁹

Summary

Vanishing twin syndrome is a phenomenon where one twin dies within the womb and the dead twin is reabsorbed by the mother. Vanishing twin syndrome is not uncommon, occurring in approximately 36% of twin pregnancies

Vanishing twin syndrome may be observed on an ultrasound where two gestational sacs are present on an earlier scan, but only one embryo remains at a later scan

Reabsorption of foetal tissue may also indicate that vanishing twin syndrome has occurred

Symptoms of vanishing twin syndrome include bleeding, uterine cramps and pelvic pain

The causes of vanishing twin syndrome remain unknown in most cases

Vanishing twin syndrome may be due to advanced maternal age or chromosome abnormalities in the dead twin