Vanishing twin syndrome: early loss of one twin

Twins Trust describes vanishing twin syndrome as when one of the embryos or foetuses in a multiple pregnancy stops developing and is absorbed by the mother's body or the other twin, most commonly in the first twelve weeks. A hybrid literature review in a peer-reviewed journal notes that the definition is debated: some researchers apply it only to first-trimester events, others include similar losses later, and the review says the word "vanishing" can imply complete disappearance, which is not always the case.

A Scottish NHS guideline from NHS Greater Glasgow and Clyde defines a vanished twin as when one fetus in a twin pregnancy is non-viable and may be partially or completely reabsorbed.

Twins Trust says it happens when twins are detected at a very early scan but only one baby is found at the dating scan, and that you may not know because you may have had few or no symptoms. Scans may show an empty second pregnancy sac or a second sac containing a non-viable fetus, according to the Scottish guideline. The review adds that vaginal bleeding is commonly seen in some cases but is a non-specific symptom of early pregnancy that has several unrelated causes.

Many people only find out when a scan reveals it. Twins Trust notes that it can come as a huge shock and that it is not unusual to feel uncomfortable or ashamed if you did not realise.

Twins Trust says that when one twin dies in the first 12 weeks it does not usually affect the development of the remaining twin. Losing a twin in the second or third trimester carries a higher risk for the remaining baby, so you receive closer monitoring and doctors assess whether it is safer for the baby to remain in the womb or be born early, though many babies whose sibling has died later in pregnancy are born healthy.

The review cautions that clinical risks are not well defined. It notes limited evidence linking early loss of a co-twin with some outcomes in the surviving baby, mostly from small studies or case reports, and a possible effect on growth in some situations. It says that information is limited and mixed, so any individual risk should be discussed with your own obstetric team rather than assumed.

The Scottish guideline explains that the combined test for Down's, Edwards' and Patau's syndromes uses maternal age, blood markers (free beta hCG and PAPP-A) and ultrasound measurements (nuchal translucency and crown-rump length). With an empty second sac the combined test can be used to calculate the chance result. With a second sac containing a non-viable fetus, the combined test should not be used, and the quadruple test, which screens for Down's syndrome only and uses blood markers measured from 14+2 to 20+0 weeks, can be offered.

Because practice varies between areas and UK nations, tell your midwife or sonographer if you know you had twins at an early scan, so the right screening pathway is chosen. Screening is optional.

The literature review describes grief, disrupted identity and unmet support needs as prominent psychosocial effects, and says communication from professionals is often inadequate. Twins Trust offers bereavement support for parents who have lost one or more babies in a multiple pregnancy, including a Navigating Grief booklet. It is possible to grieve one baby while continuing with a pregnancy and hoping for the other. Ask your midwife, GP or the early pregnancy unit about counselling, which the NHS says is available after miscarriage.

If you are pregnant and you have heavy bleeding, severe tummy pain, shoulder pain, or feel faint or dizzy, the NHS says to call 999. For other bleeding or pain, contact your maternity unit, early pregnancy unit or NHS 111, as bleeding in early pregnancy can have several causes and should be checked. Local early pregnancy services vary across the UK nations.