Insecure attachment and internalizing behavior problems in growth discordant identical twins

Psychosocial development in monochorionic (MC) twins born after selective fetal growth restriction (sFGR) has been unreported to date, despite its importance for daily functioning and future relationships. To investigate psychosocial development, attachment and school functioning in MC twins with sFGR and compare outcomes with the general population and between smaller and larger twins. Observational cohort study. MC twins with sFGR (defined as a birth weight discordance ≥20 %) born between 2002 and 2017 and aged 3–17 years. Multiple parent report questionnaires: the Child Behavior Checklist (social-emotional development and behavior), the (Early) Childhood Behavior Questionnaire Very Short Form (temperament), the Attachment Insecurity Screening Inventory (attachment) and a school functioning questionnaire. Median age for the 48 twin pairs was 11 (interquartile range (IQR) 8–13) years. Attachment insecurity for both twins was higher than in the general population for ambivalence/resistance (34 % (21/62) vs. 16 %, p = 0.024) and total attachment insecurity (35 % (22/62) vs. 16 %, p = 0.016). Smaller twins had more internalizing behavioral problems, i.e. negative emotions and behaviors turned inwards (22 % (10/46) vs. 11 % (5/46), p = 0.021) and a higher negative affect, i.e. more likely to experience negative emotions (3.2 (2.9–3.7) vs. 2.9 (2.2–3.2), p = 0.009) than larger twins, as well as a lower secondary school level (p = 0.031). MC twins with sFGR have more ambivalent/resistant attachment insecurity following the complicated pregnancy course. Smaller twins have a tendency towards negative emotions and internalizing behaviors compared to larger twins, indicating an increased sensitivity for depression and anxiety. •Birth weight discordant monochorionic twins present with high attachment insecurity.•Smaller twins have a tendency towards negative emotions and behaviors.•Smaller twins attend a lower secondary school level than larger co-twins.•We recommend parent-child guidance throughout pregnancy and the first year.