RT Journal Article SR Electronic T1 Sequential co-enrolment in randomised trials in neonatal intensive care medicine JF Archives of Disease in Childhood - Fetal and Neonatal Edition JO Arch Dis Child Fetal Neonatal Ed FD BMJ Publishing Group Ltd and Royal College of Paediatrics and Child Health SP 128 OP 131 DO 10.1136/archdischild-2019-316818 VO 105 IS 2 A1 Yoder, Whitney A1 Groenendaal, Floris A1 Onland, Wes A1 van Oploo, Anna A1 Rietbergen, Charlotte A1 Groenwold, Rolf YR 2020 UL http://fn.bmj.com/content/105/2/128.abstract AB In many medical research settings, such as the neonatal intensive care unit, the number of patients who are eligible for a randomised clinical trial is relatively small and recruiting a sufficient number of patients into trials is often difficult. Furthermore, some infants may have already been enrolled into a trial as a fetus. Sequential co-enrolment of patients into more than one trial may offer a solution, yet runs the risk of contaminated results. We consider the situation of two sequential trials and describe requirements for different possible treatments effects (‘estimands’) to be estimated in such situations. These estimands differ regarding the extent to which participation status and treatment status in the previous trial is accounted for. Because of differences in available information about previous trials, analyses may result in estimated effects which differ in terms of interpretation and generalisability, except when in the absence of an interaction between the studied treatments. If co-enrolment cannot be ruled out, researchers should collect information about co-enrolment and treatment status in a previous or concurrent trial and mitigate the trial analysis plan in order to estimate meaningful effects.