Objective: This review sought to examine the association of HbA1c levels <6.5% in early pregnancy with the subsequent development of gestational diabetes mellitus (GDM) and adverse pregnancy outcomes.
Methods: A search of Medline and EMBASE was conducted for the period of January 1, 2000 to July 9, 2019 and the terms: "gestational diabetes or pregnancy diabetes mellitus" and "glycosylated hemoglobin or glycated hemoglobin A" and "pregnancy trimester, first, or first-trimester pregnancy," "screening or prenatal screening," "prenatal diagnosis or early diagnosis or prediction," "retrospective studies or prospective studies." Quality of evidence was assessed using the Newcastle-Ottawa scale. Inclusion criteria were: measurement of HbA1c <20 weeks gestation, the absence of pre-gestational diabetes mellitus, and analysis of HbA1c levels below 6.5%. The primary outcome evaluated was the development of GDM. Secondary outcomes were adverse pregnancy outcomes, including large-for-gestational-age birth weight, macrosomia, preterm birth, neonatal and perinatal death, congenital anomaly, preeclampsia, shoulder dystocia, and cesarean section.
Results: We screened 121 relevant abstracts. Thirty-two studies qualified for a full review, of which 11 met the eligibility criteria. All studies were assessed as high quality and found an increased risk of GDM with HbA1c levels >5.7. Levels >6.0 identified all patients who developed GDM. Adverse pregnancy outcomes were associated with elevated HbA1c levels in 4 of 6 studies and included preeclampsia, induced labour, shoulder dystocia, cesarean section, large-for-gestational-age birth weight, macrosomia, congenital anomalies, and perinatal death. Two studies found no association with adverse events.
Conclusion: HbA1c levels between 5.7% and 6.4% in early pregnancy consistently identified patients who went on to develop GMD. The evidence that particular levels are associated with adverse outcomes is less robust.
Keywords: diabetes, gestational; glycated hemoglobin A; systematic review.
Copyright © 2020 The Society of Obstetricians and Gynaecologists of Canada/La Société des obstétriciens et gynécologues du Canada. Published by Elsevier Inc. All rights reserved.