For many expectant parents, an early pregnancy scan brings unforgettable news, not one heartbeat, but two. Yet for some, that excitement is followed by an unexpected discovery at a later appointment: one of the babies is no longer there.

The phenomenon, known as Vanishing Twin Syndrome (VTS), is a recognised pregnancy complication that occurs in twin or multiple pregnancies when one embryo or fetus stops developing in the early weeks of gestation. Although the diagnosis can be deeply upsetting, doctors say it is often the result of natural developmental processes and, when it happens early in pregnancy, usually does not jeopardise the health of the surviving baby.

What is Vanishing Twin Syndrome?

Unlike a miscarriage in which pregnancy tissue is expelled from the body, Vanishing Twin Syndrome takes a different course. The pregnancy continues, but only one fetus remains viable.

When the loss occurs during the first trimester, the embryo is still extremely small. Instead of being passed out of the body, the tissue is gradually absorbed within the uterus by the mother's body, the placenta or, in some cases, by the remaining twin. By the time of a later ultrasound, there may be little or no sign that the pregnancy initially involved two babies.

Because routine early scans are now common, the condition is detected more frequently than it was in the past. Without an ultrasound in the first few weeks, many parents would never know they had conceived twins.

Why does it happen?

Medical experts say Vanishing Twin Syndrome is usually linked to problems that arise during the embryo's earliest stages of development rather than anything the pregnant woman has done.

One of the leading causes is a severe chromosomal abnormality. If an embryo develops with an incorrect number of chromosomes, normal growth may not be possible, prompting development to stop naturally.

Other factors may also contribute. An embryo that fails to implant securely in the uterus may not receive the support it needs to continue growing. In some pregnancies, abnormalities involving the placenta can reduce the blood supply to one twin, while major developmental defects affecting vital organs may also prevent survival.

These are biological events that occur long before most women are aware of anything is wrong.

Separating myth from reality

The diagnosis often leaves parents searching for a reason, but everyday activities are not generally to blame.

Normal movement, travelling, climbing stairs, exercising, working, lifting light objects, sexual intercourse during an uncomplicated pregnancy or emotional stress are not considered typical causes of Vanishing Twin Syndrome.

For many families, understanding that the condition is largely beyond anyone's control is an important part of coping with the diagnosis.

Often discovered by chance

One of the striking aspects of Vanishing Twin Syndrome is that it may produce no obvious symptoms.

Many women continue to feel pregnant and learn about the condition only during a routine ultrasound scan. Others may experience light vaginal bleeding, mild spotting, slight cramping, pelvic discomfort or notice that early pregnancy symptoms such as nausea become less intense.

These signs are not specific to the condition, however, and can occur in many pregnancies. Any bleeding during pregnancy should be assessed by a healthcare professional.

What happens to the remaining baby?

For most families, the greatest concern is the wellbeing of the surviving fetus.

When the condition occurs during the first 12 weeks of pregnancy, the outlook is generally reassuring. The remaining baby usually continues to develop normally, and most pregnancies progress to the birth of a healthy infant.

The situation may require closer observation if one twin is lost later in pregnancy, when the risks of complications, including premature birth, can be higher. Regular antenatal care and follow-up scans allow doctors to monitor the pregnancy and respond to any concerns.

How doctors confirm the diagnosis

Ultrasound imaging is the main tool used to identify Vanishing Twin Syndrome.

An early examination may reveal two gestational sacs or two fetal heartbeats. At a subsequent scan, only one fetus is seen developing while the other is no longer present. Depending on the pregnancy, doctors may recommend additional scans or monitor hormone levels to ensure the surviving baby continues to grow as expected.

Can it happen again?

A diagnosis of Vanishing Twin Syndrome does not necessarily predict future pregnancy outcomes.

Most women who experience the condition later go on to have healthy pregnancies, and there is no clear evidence that one episode significantly increases the likelihood of recurrence.