1. Department of Obstetrics and Gynaecology (G.C.S.S.), Cambridge University, CB2 2QQ, United Kingdom
2. Department of Fetal Medicine (E.J.S., A.D.C.), The Queen Mother’s Hospital, Glasgow, G3 8SJ, United Kingdom
3. Institute of Medical Genetics (J.A.C., D.A.A., J.M.C.), Yorkhill National Health Service Trust, Glasgow, G3 8SJ, United Kingdom