The Natural Course of Low-Risk Papillary Thyroid Microcarcinoma During Pregnancy: A Prospective Active Surveillance Study

<b><i>Background:</i></b> Active surveillance (AS) has emerged as an established management strategy for patients with low-risk papillary thyroid microcarcinoma (PTMC). However, prior studies have suggested accelerated tumor growth during pregnancy, which raises concerns about the suitability of AS for reproductive-age patients. This study evaluates the longitudinal impact of pregnancy on tumor dynamics and validates the safety of AS management in this population. <b><i>Methods:</i></b> From February 2020 to October 2024, a single-institution prospective AS cohort enrolled 260 female patients diagnosed with low-risk PTMC. Eighteen female patients (21 pregnancy events) with complete series of ultrasound documentation underwent longitudinal analysis of changes in tumor size and growth velocity, with time-stratified comparisons before pregnancy, during pregnancy, and after delivery. Tumor doubling rate (TDR) was calculated to quantify tumor growth or shrinkage patterns across these periods. An increase in tumor size of 3 mm or more was defined as substantial enlargement. <b><i>Results:</i></b> Over a median follow-up of 59.0 (interquartile range 38.5, 72.0) months, tumor enlargement (>3 mm) was observed in 19.0% (4/21) of the PTMC cases by the last follow-up. During pregnancy, 76.2% (16/21) of tumors exhibited accelerated growth (TDR >0.5/year) or moderate growth (TDR 0.1-0.5/year), whereas postpartum stabilization (TDR -0.1 to 0.1/year) or regression (TDR <-0.1/year) occurred in 71.4% (15/21). The TDR peaked during pregnancy and decreased after delivery (0.39/year vs. -0.01/year, <i>p</i> = 0.006). Delayed surgery was required in only two patients and no instances of expanded surgical scope, T-stage progression, or tumor recurrence were observed. <b><i>Conclusion:</i></b> While pregnancy may transiently accelerate tumor growth in low-risk PTMC, most gestational changes are self-limited, with stabilization or regression commonly observed postpartum. AS remains a safe and effective strategy for reproductive-age patients, balancing oncologic safety with fertility preservation. Confirmatory studies incorporating extended follow-up and advanced imaging modalities are essential to further validate these findings and optimize clinical frameworks.