Down's Syndrome Screening With Twins

The test you are offered

NICE NG137 says women with a twin pregnancy should be offered information on, and screening for, Down's syndrome, Edwards' syndrome and Patau's syndrome as set out by the NHS fetal anomaly screening programme. The FASP handbook then says the test of choice for twin pregnancies is the combined test, and that every effort must be made to offer and complete it.

The combined test uses a nuchal translucency measurement from the scan alongside blood markers. The NHS says the two parts are usually done at the same time. With twins, the handbook adds a detail: the larger of the two crown-rump length measurements should be used in the chance calculation, and each baby has an individual nuchal translucency measurement.

Why chorionicity decides what result you get

This is the single most misunderstood point, and the handbook states it in two sentences. In a monochorionic twin pregnancy, the chance result is the same for each baby and one pregnancy chance result is reported. In a dichorionic twin pregnancy, the chance result is reported for each baby.

The logic follows from the biology. Monochorionic twins came from one egg and share the same genetic material, so a single result describes both. Dichorionic twins usually came from two eggs and can differ, so each gets a number. If you are given one result and you were expecting two, or the other way round, this is the reason.

The handbook also explains why multiple pregnancy complicates the maths at all: each feto-placental unit contributes to the levels of the biochemical markers used in the calculation.

What you are told, and what you can decline

The handbook says results can be reported, depending on your personal choice, as a term chance of Down's syndrome and a joint term chance of Edwards' and Patau's syndromes, or as a term chance of Down's syndrome only, or as a joint term chance of Edwards' and Patau's syndromes only. You can ask for some of it and not the rest.

When the combined test cannot be done

The quadruple test is the fallback, and the handbook restricts when it is used with twins: only when the nuchal translucency measurements cannot be obtained, or when the crown-rump length measurements are greater than 84.0 mm, which in practice means you have passed the window. The larger of the two head circumference measurements is used in the calculation.

The handbook is candid that this is a weaker option. It says the quadruple test in twin pregnancies is not as sensitive as the combined test and the decision-making process can be more difficult. It also splits performance by chorionicity: in monochorionic twins the performance is comparable to singleton pregnancies, while in dichorionic twins where one baby has the condition and the other does not, it is not as sensitive.

Crucially, quadruple test results in twins relate to the pregnancy rather than to individual babies, and the handbook says they are not interpreted in the usual way but used with a cut-off of 1 in 150 at term to define a higher-chance group. It also says an appropriately trained healthcare professional should interpret and explain these results because of the complexities.

Triplets: a different conversation entirely

NG137 says that before offering screening to women with a triplet pregnancy, they should be given information about the greater likelihood of these conditions in triplet pregnancy, the different screening options, the increased false positive rate, their greater likelihood of being offered invasive testing, their greater likelihood of complications from invasive testing, and the physical risks and psychological implications relating to selective fetal reduction.

For trichorionic triplets, NG137 says teams should map fetal positions, use nuchal translucency and maternal age, and calculate a chance for each fetus. Women with dichorionic or monochorionic triplet pregnancies who want screening should be referred to a tertiary level fetal medicine centre. Second trimester serum screening should not be used for Down's syndrome in triplets.

If the result is higher chance

NIPT is the next step on the NHS pathway for twins, offered after a result between 1 in 2 and 1 in 150. Beyond that, the diagnostic tests are chorionic villus sampling and amniocentesis. The handbook records that the risk of miscarriage is twice as high in twin pregnancies compared with singleton pregnancies, and that prenatal diagnosis is performed at a tertiary-level fetal medicine unit because of the specialised nature of the procedure and the increased risk.

NG137 also says teams should inform women about the complexity of the decisions they may face, including different options according to the chorionicity and amnionicity of the pregnancy. RCOG puts the practical version bluntly: if screening shows an increased chance, it can be difficult to know which baby is affected.

Counselling before and after, not just after

NG137 says a healthcare professional with experience of caring for women with twin and triplet pregnancies should offer information and counselling before and after every screening test. That wording is deliberate. With twins the difficult decisions start at the point of consent, not at the point of result, because what you would do with each possible answer differs depending on chorionicity.

Worth asking before the blood is taken: will I get one result or two, what would the next step be, and who would I be referred to.

The scan that is not screening for this

NG137 says screening for structural abnormalities should be offered in twin and triplet pregnancies as in routine antenatal care, and that 45 minutes should be allowed for the anomaly scan in a twin or triplet pregnancy, and 30 minutes for growth scans. If your appointments feel longer than a friend's, that is deliberate.