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The risk of meconium aspiration syndrome (MAS) increases with gestational age at term.

Objective: To examine the risk of meconium aspiration syndrome (MAS) by the week of gestational age in pregnancies with meconium-stained amniotic fluid (MSAF).Methods: This is a retrospective cohort study derived from term pregnancies at a single tertiary institution over an 18-year period. Outcomes analyzed included MAS, acidemia, and Apgar scores. Univariate analysis was performed using chi-square and multivariable logistic regression was used to adjust for potential confounders.Results: A total of 34,303 deliveries ≥37 weeks were included; 23.7% were complicated by MSAF. Of the total study cohort, 272 (0.7%) neonates were diagnosed with MAS; this represented 3% of all deliveries complicated by MSAF. In the presence of MSAF, the risk of MAS increased with gestational age, from 1.3% at 38 weeks to 4.8% at 42 weeks (p < .001). Similarly, the risk of acidemia increased from 3.0% at 38 weeks to 7.0% at 42 weeks (p < .001). These findings were also demonstrated in patients with MAS in the absence of MSAF. The risk for both MAS and acidemia in the presence of MSAF persisted after controlling for potential confounders (aOR 1.31 [95% CI 1.18-1.46] and 1.20 [95% CI 1.05-1.37], respectively).Conclusion: In women with MSAF, as gestational age increases, the risk of meconium aspiration syndrome also increases. Other factors with late-term and post-term pregnancy besides the presence of meconium may contribute to the evolution of MAS.