What Retrospective Studies Reveal
Factors that may influence the timing of delivery include the duration of neonatal exposure to hyperglycemia/hyperinsulinemia (pregestational vs. gestational diabetes), the level of diabetes control, and comorbidities (e.g. maternal renal disease or chronic hypertension). However, research “investigating how these factors influence morbidity and the timing of delivery is limited,” said Dr. Grantz.
Overall, it has been difficult through retrospective studies, she said, to investigate neonatal morbidity in diabetic pregnancies and tease apart the relative effects of diabetes as a precursor for early delivery and prematurity itself. Among the studies suggesting an independent risk of diabetes is a retrospective study focusing on neonatal respiratory morbidity — “one of the most common adverse outcomes associated with diabetes.”
The study, an analysis of the Consortium on Safe Labor study (an electronic medical record study of more than 220,000 singleton pregnancies), stratified morbidity by the probability of delivering at term (≥ 37 weeks). GDM and pregestational diabetes complicated 5.1% and 1.5% of the pregnancies, respectively, and were found to be associated with increased risks of neonatal respiratory morbidity compared to women without diabetes — regardless of the probability of delivering at term.
However, these associations were stronger with a higher probability of delivering at term, which suggests that the neonatal respiratory morbidity associated with diabetes is not fully explained by a greater propensity for prematurity (Am J Perinatol. 2017;34[11]:1160-8).
In addition, the rates of all neonatal respiratory morbidities and mortality were higher for pregestational diabetes compared with gestational diabetes, said Dr. Grantz, a senior author of the study. (Morbidities included neonatal intensive care unit admission, transient tachypnea of newborn, apnea, respiratory distress syndrome, mechanical ventilation, and stillbirth.)
The pathophysiology of diabetes and neonatal respiratory morbidity is “not fully known,” she said. It is believed that fetal hyperinsulinemia may cause delayed pulmonary maturation and there is evidence from animal studies that insulin decreases the incorporation of glucose and fatty acids into phospholipid phosphatidylglycerol. Indirect effects stem from the physiologic immaturity of earlier delivery and a higher cesarean delivery rate in pregnancies complicated by diabetes, Dr. Grantz said.
Among other retrospective studies was a population-based study from Canada (2004-2014), published in 2020, of large numbers of women with all types of diabetes and a comparison group of over 2.5 million without diabetes. For maternal morbidity/mortality, there were no significant differences by gestational age between iatrogenic delivery and expectant management among any form of diabetes. But for neonatal morbidity and mortality, the study found differences.
In women with gestational diabetes, iatrogenic delivery was associated with increased risk of neonatal morbidity/mortality at 36 and 37 weeks’ gestation and with decreased risk at weeks 38-40. Increased risk with iatrogenic delivery was also found for women with type 1 and type 2 diabetes at weeks 36 and 37 (Acta Obstet Gynecol Scand. 2020;99[3]:341-9).
Another retrospective study using California vital statistics (1997-2006) examined rates of stillbirth and infant death in women with GDM by gestational age at delivery (Am J Obstet Gynecol. 2012;206[4]:309.e1-e7). The 190,000-plus women with GDM had elevated risk of stillbirth at each gestational age compared to those without GDM, but “the [excess] risk for GDM was lowest at 38 weeks and again at 40 weeks,” Dr. Grantz said. The investigators concluded, she said, “that the risk of expectant management exceeded that of delivery at 38 weeks and beyond.”
Dr. Grantz reported no disclosures.