[1] Univ Sheffield, Acad Unit Radiol, Sheffield, S Yorkshire, England
[2] St Marys Hosp, Fetal Med Unit, Manchester M13 0JH, Lancs, England
[3] Univ Birmingham, Coll Med & Dent Sci, Birmingham Ctr Womens & Childrens Hlth, Birmingham, W Midlands, England
[4] Birmingham Womens Fdn Trust, Fetal Med Ctr, Birmingham, W Midlands, England