Gestational age at delivery of twins and perinatal outcomes: a cohort study in Aberdeen, Scotland.

Background: Twin pregnancy is associated with a threefold increase in
perinatal death compared to singletons. The objective of this study was to
determine the risk of perinatal death in twins by week of gestation and to
quantify the effect of known risk factors.
Methods: A cohort analysis was performed using data from the Aberdeen
Maternity and Neonatal Databank (AMND). The exposure was gestational
age at delivery and the primary outcome was perinatal death. Adjusted
hazard ratios (aHRs) for perinatal death according to gestational age at
delivery were determined by multivariate Cox proportional hazards
regression modelling with robust standard errors to account for clustering in
the twin infants. Confounders and risk factors quantified and adjusted for in
the model included maternal age, smoking, parity, marital status and year of
birth. Kaplan-Meier time to event analysis was used to determine the
differences in survival according to chorionicity and assisted reproduction
technologies (ART) conception status.
Results: The population comprised of 7,420 twin babies born between
1950 and 2013 in the Grampian area of Northern Scotland. There were 272
stillbirths in the cohort (3.67%) and 273 neonatal deaths (3.68%).
Compared to delivery at 37-38 weeks, delivery at or beyond 39 weeks was
associated with a significant increase in perinatal death (aHR 2.00 [95% CI
1.45-2.78]). Monochorionic twins had a 2-fold increase in perinatal death
compared to dichorionic twins (aHR 2.15, 95% CI 1.60-2.90). Twins
conceived by ART did not have a greater risk of perinatal death compared
to those naturally conceived (aHR 1.21, 95% CI 0.87-1.68)
Conclusion: This study suggests that delivery of twins at 37-38 weeks is
associated with the lowest risk of perinatal death.