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Standard, Population & Customised fetal size charts 17 – addendum for twins

September 29, 2019

Twins, especially identical ones,  tend to be smaller than singletons (click here). This has led to the suggestion that they should have special charts (click here, and here). The support group, the Twin and Multiple Birth Association, (TAMBA) apparently supports the idea (click here), but it’s a bad one.

The Perinatal Institute, normally an advocate of customisation,  explains why rather well (click here). To quote:

The [twin] charts […] are based on reference values derived from the whole population, not only from uncomplicated pregnancies. Therefore, they do not represent a normal growth standard but one that may have been affected by an unspecified number of pathological factors. This concept is particularly important in twin pregnancies as they have a substantially increased number of complications.”

“The pattern of slowed growth from 30-32 weeks in many (but not all) twin pregnancies may be pathological due to late onset fetal growth restriction associated with placental insufficiency,
which usually also becomes manifest from around 32 weeks. Adjusting the curves downwards may normalise pathology, reduce recognition of pregnancies at risk, and lead to false reassurance.”

Correct. Twin smallness is not healthy smallness. Twins also have higher rates of stillbirth, cerebral palsy and other problems. The reasons not to customise by ethnic group (click here), height (click here), weight (click here) and parity (click here), also apply to twin customisation. Special “small” charts for twins condemn them to being classified as “normal for twins”.

The Italian group (Ghi et al click here) recognise the problem; “adjusting for multiple pregnancy, thereby shifting the normal range of fetal growth downward, has the potential to mask truly growth restricted twins and increase perinatal morbidity from failure to recognize growth restriction”, but hope that because they excluded twins born before 36 weeks, or below the 5th centile on a singleton chart, that it will go away. They compare their twin chart with a local customised Italian one (click here) but provide no data of the detection rate of pathology.

The STORK study authors (here) compared detection rates for stillbirth, with a singleton chart. But their chosen comparison chart (Poon click here) which correctly excluded babies of smoking mothers, and those with major medical disease, has its own problems. Leona Poon not only based her chart on routinely collected data with all the error and bias that entails, but also used the weights of babies who were actually born, which seriously distorts the preterm part of the chart (click here).

Many twin fetuses fall below the Intergrowth-21 or WHO singleton growth standards. Some even below the 1st centile. These are reasons both to use charts with reasonably precise extreme centiles, and to consider delivery before term (click here).  Some experts also recommend basing decisions on estimated fetal weight discordance (EFW) a sign of selective growth restriction, is included as a factor in delivery timing. If so, it is preferable to use the most accurate and unbiased Intergrowth-21 EFW formula (click here).

TAMBA should think again.

Jim Thornton

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