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Re: Is the third trimester repeat ultrasound scan for placental localisation needed if the placenta is low lying but clear of the os at the mid‐trimester morphology scan?

Dear Editor, The study by Kapoor et al. confirms previous similar studies, showing that mothers with a placenta identified on transabdominal abdominal ultrasound in the second trimester as within 20 mm of the internal cervical os are at risk of placenta praevia requiring abdominal delivery at term and thus require follow-up. Unfortunately in their introduction, the authors misquote national guidelines from the RCOG and SOGC as suggesting that such women do not require follow-up unless the placenta reaches or covers the internal cervical os. What both guidelines actually advise is that if the placenta is assessed on transvaginal ultrasound between 18 and 24 weeks, only those found to reach or cover the internal cervical os remain at risk of praevia at term and require follow-up. This is based on studies by Taiale et al., Becker et al., Smith et al. and Lauria et al. amongst others involving over 10 000 women in total. Whilst this (transvaginal assessment at 18–14 weeks gestation) is not currently routine practice in most centres, it may become so as recent evidence about the use of transvaginal cervical length measurement at 18– 22 weeks’ gestation, with progesterone treatment for those with a short cervix to reduce the risk of preterm birth, begins to be incorporated into practice. Thus, an added benefit of routine transvaginal ultrasound at the second trimester morphology scan should be to dramatically reduce the number of women requiring follow-up for a low-lying placenta in the third trimester, along with the restricted activities that many obstetricians continue to advise. FRANZCOG, FRCSC, FRCOG