Data collection
Data were collected using the United Kingdom Obstetric Surveillance System (UKOSS) which prospectively captures data on severe but rare complications of pregnancy and childbirth from all obstetric-led maternity units in the UK. Each unit in the UK has a designated UKOSS clinician data collector who completes monthly returns to UKOSS. The methodology is described elsewhere17 and is well established having been running since 2005. Additionally, the lead of each Fetal Medicine Centre in the UK was alerted to the study. Following case notification to UKOSS by clinicians, a piloted, anonymised data collection form was sent (see www.npeu.ox.ac.uk/ukoss/current-surveillance/stwin for the form) and completed from hospital records. Reports were cross-checked for duplication based on the woman’s year of birth and estimated delivery date. If forms were not returned, reminders were sent.
The surveillance period was between 1st July 2016 – 30th June 2017, defined as the time during which the sIUFD was diagnosed. There was a further 6 months of data collection to allow collection of delivery data. Case ascertainment was checked by comparison with MBRRACE-UK data from the same period. All deaths of twins were identified from MBRRACE-UK and compared with UKOSS data based on centre, month and year of reporting for case ascertainment. Where 1 potential additional case was identified, centres were contacted and asked to check whether the case met the case definition and if so to complete a UKOSS data collection form.