Diagnosis

Gestational dating

A Cochrane review of 11 trials including 37,505 women showed that early ultrasonography (in the first trimester) improves the early detection of multiple pregnancies (prior to 24 weeks' gestation) and improves gestational dating therefore leading to fewer inductions for post maturity (18).

More recently, the Clinical Standards Committee of the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) have recommended that twin pregnancies should ideally be dated when the crown-rump length (CRL) measurement is between 45 and 84 mm (i.e. 11⁺⁰ to 13⁺⁶ weeks of gestation). In pregnancies conceived spontaneously, the larger of the two CRLs should be used to estimate gestational age. Where the twin pregnancy was conceived via IVF, it should be dated using the oocyte retrieval date or the embryonic age from fertilization. If the woman presents after 14 weeks' gestation, the larger head circumference should be used (19). Some studies have recommended using a mean CRL or the smaller CRL for dating. However, a smaller CRL may reflect a fetal growth problem secondary to fetal abnormality. Indeed, the United Kingdom National Institute for Health and Care Excellence (NICE) guidelines on multiple pregnancy also recommend dating by the largest CRL to avoid estimating it from a fetus with early growth pathology, albeit that it is recognized that this may exaggerate this risk causing anxiety (11).

Labelling in twins

It is accepted as a good clinical practice to follow a clear consistent strategy for labelling of twins (3, 19). Various options include labelling according to their site, either left and right, or upper and lower; or mapping in the first trimester according to the insertion of their cords relative to the placental edges and membrane insertion (3). This is particularly important in laterally orientated twins (i.e.

left and right twins) where 8.5% change presenting order between the first and last scans, and 20.3% delivered by caesarean versus 5.9% delivered vaginally change birth order (i.e. the twin labelled ‘twin 2' delivers first) (20). Correct labelling according to orientation in relation to the mother as lateral maternal left and maternal right or vertical upper and lower, is better than assigning a fetus number as it enables consistency with longitudinal biometric assessment, accuracy when interpreting screening results and undertaking invasive diagnostic tests where necessary, and avoids misconception about birth order ensuring the parents and paediatric team are aware of the possibility of the ‘perinatal switch phenomenon' (i.e. possible change in birth order). This is particularly important if one fetus has an abnormality that is not outwardly obvious (e.g. cardiac abnormality). In such a case, it is important to perform an ultrasound scan prior to delivery to ascertain the fetal order and thereby plan any neonatal interventions for the appropriate fetus. It is, however, vital that the nomenclature assigned to twins from early pregnancy is clearly documented at each visit and remains consistent throughout the pregnancy (11).

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Source: Arulkumaran S., Ledger W., Denny L., Doumouchtsis S. (eds.). Oxford Textbook of Obstetrics and Gynaecology. Oxford University Press,2020. — 928 p.. 2020

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Diagnosis

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