Purpose <p>This study aims to reveal the relationship between the nasal conjunctival origin of primary pterygium and the position of the medial rectus muscle.</p> Methods <p>The study involved 52 patients diagnosed with primary pterygium, among whom 4 experienced postoperative recurrence. Preoperatively, the distance from the limbus to the insertion point of the medial rectus muscle was assessed with anterior segment optical coherence tomography (AS-OCT), while intraoperatively, calipers were utilized for measurement. Hematoxylin and eosin was used to stain every tissue of the pterygium tissue after it had been removed. Using antibodies against α-smooth muscle actin, desmin, vimentin, and H-caldesmon, immunohistochemistry was carried out. The analysis of the results was conducted using paired <i>T</i> tests, Pearson correlation coefficients, one-sample <i>T</i> tests, and independent sample <i>T</i> tests to assess the differences.</p> Results <p>52 patients had their distance from the limbus to the medial rectus muscle insertion point measured; preoperative AS-OCT results were 5.74±1.40&#xa0;mm, and intraoperative caliper measurements were 5.72±1.44&#xa0;mm. A high Pearson correlation coefficient (<i>r</i> = 0.990) was seen in both measurements. ICC analysis showed very high consistency (ICC = 0.995). There was no notable difference in the position of the medial rectus muscle between patients with pterygium and healthy individuals; however, the position was found to be more anterior in patients with recurrent pterygium compared to those with primary pterygium. Fibrous tissue resembling bundles was found near the neck of the pterygium, which stained positively for α-SMA, vimentin, and desmin.</p> Conclusions <p>AS-OCT offers accurate measurements of the medial rectus muscle's position. There is no anterior shift in the position of the medial rectus muscle compared to normal individuals, but recurrent pterygium patients show a more anterior position. The fibrous tissue near the neck of the pterygium is composed of myofibroblasts. This could be attributed to the medial rectus muscle being situated more anteriorly than the other rectus muscles, which makes it more vulnerable to ultraviolet radiation. This vulnerability may lead to the migration of fibroblasts to the nasal conjunctival region and their activation into myofibroblasts, consequently resulting in a higher incidence of nasal conjunctival pterygium. The more anterior position of the medial rectus muscle in recurrent patients may also contribute to a higher likelihood of postoperative recurrence due to ultraviolet exposure.</p>