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.