Vanishing Twin

What the term actually means

The NHS FASP handbook gives a working definition for screening purposes: a vanished twin is when one fetus in a twin pregnancy is non-viable at the dating scan. The ultrasound may show either an empty second pregnancy sac, or a second pregnancy sac containing a non-viable fetus.

The phrase is unfortunate. Nothing vanishes in the sense of never having been there. What happens is that one pregnancy continues and one does not, and the scan that finds this is often the first scan you have.

How often it happens

We are not going to give you a percentage. No UK source we could verify publishes a reliable incidence figure for this, and the numbers circulating online trace back to studies of early scans in selected populations rather than to routine NHS care. If someone quotes you a rate, ask where it came from.

What is documented is that it is common enough for the national screening programme to have written specific rules for it, and for those rules to have been updated. That tells you something about frequency without inventing a number.

The practical consequence: your screening changes

This is the part almost nobody explains at the appointment, and it is the reason the finding is recorded so carefully. The FASP handbook says that in both scenarios, an empty second sac or a sac containing a non-viable fetus, the biochemical markers can be affected, and therefore the standard NHS FASP combined and quadruple screening tests cannot be offered.

Instead, the handbook says that when a vanished twin pregnancy is identified, local guidelines for measuring crown-rump length and nuchal translucency should be followed and the woman should be referred to an appropriately trained healthcare professional, for example the local screening coordinator or a fetal medicine service, so that local testing options can be discussed as part of a clinical management pathway.

What the alternative tests are

The handbook lists the testing options available as part of that clinical management pathway: maternal age and nuchal translucency on their own; a modified combined test, either using maternal age, nuchal translucency and free beta hCG while excluding PAPP-A, or the same with adjusted PAPP-A; and a modified quadruple test in two versions.

It also publishes the modelled detection rates, which is unusually transparent. For Down's syndrome, maternal age and nuchal translucency alone is modelled at 77% detection with a 2.7% false positive rate. The modified combined test excluding PAPP-A is modelled at 82%. The modified combined test adjusting PAPP-A is modelled at 87% with a 2.3% false positive rate, which the handbook notes is the same as the current NHS combined test in singleton pregnancies.

These are modelled figures for vanished twin pregnancies published by the NHS screening programme, not observed rates from your hospital. But they show that the best available option recovers most of the performance, which is worth knowing if you are weighing up whether to have screening at all.

One timing detail that catches people out

The handbook says that when a modified combined test with adjusted PAPP-A is used, it is not possible to use a maternal blood sample taken before the ultrasound scan. The blood sample must be taken on the same day as the scan that identifies the vanished twin pregnancy. If you had bloods done in advance, they may need repeating.

The laboratory request form must also clearly indicate that there is a vanished twin, and if the adjusted PAPP-A version is used, the form must include the crown-rump length of the non-viable fetus if one is present.

NIPT is not available

The handbook is explicit that NIPT screening cannot be offered in vanished twin pregnancies. The reason given is that there is evidence to suggest the placenta can continue to shed cell-free fetal DNA even after the baby has died, so the sample would not reliably reflect the surviving baby.

This applies to the NHS pathway. If you are considering a private test, this is the specific question to ask, and to ask before paying.

Bleeding, and what to report

Some people bleed around the time this happens and some do not. The NHS advice on bleeding in early pregnancy is unchanged by the fact there were two sacs: contact your midwife, GP or early pregnancy unit rather than waiting for your next appointment. Bleeding in early pregnancy is common and often turns out to be nothing, but it is always worth getting checked when you are carrying an ongoing pregnancy.

What happens to the rest of the pregnancy

If one baby continues, the pregnancy is usually managed as a singleton pregnancy, with the screening exception above. Your notes should record what the dating scan showed, because that record is what triggers the correct screening pathway later. If you change hospital, check it has travelled with you.

Grief that does not fit the usual boxes

You are pregnant and bereaved at the same time, and the two do not take turns. Tommy's is straightforward that multiple pregnancies have a higher chance of ending in loss, and both Twins Trust and Tommy's provide support to people in exactly this position, including those whose pregnancy is continuing.

There is no correct amount of sadness here, and no requirement to feel relieved that one baby is well. Tell your midwife what happened rather than assuming it is in your notes, and ask what bereavement support your unit can offer, because it exists and is rarely offered unprompted.