Background: Differentiated thyroid cancer (DTC) is increasingly common in women of reproductive age. However, whether pregnancy increases the risk of DTC progression/recurrence after treatment remains controversial. The study aimed to assess the association of pregnancy with risk of progression in patients previously treated for DTC. Methods: This was a retrospective cohort study following 123 pregnant women and 1376 nonpregnant women at Peking University Third Hospital after initial treatment for DTC between January 2012 and December 2022. To control the effect of confounding, we carefully matched pregnancy (n = 107) and nonpregnancy groups (n = 298) in terms of baseline characteristics by using propensity score matching (PSM). Results: At baseline, the pregnancy and nonpregnancy groups were balanced in all matched variables. At follow-up, the percentage of DTC progression in the two groups was 12 (11.8%) and 47 (15.8%), respectively. Regression models showed no evidence of association of pregnancy with the risk of progression (odds ratio: 0.74 and 95% confidence interval: 0.37-1.50; p = 0.404), and remained consistent across long/short follow-up and other subgroup variables. We found that the shorter the time interval between treatment and pregnancy, the higher the risk of DTC progression (p_trend = 0.019). Conclusions: The risk of DTC progression in pregnant women was not higher than that in the well-matched, nonpregnant women. For young women previously treated for DTC, disease progression might not be a concern for their future pregnancy plan, but it seems safer to wait at least 1 year before pregnancy compared with immediate pregnancy.

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