One of the chief challenges of wearable sensing is adapting electronic components and sensors to the wearable environment. Electronic components are often rigid, bulky, and impermeable: factors that usually detract from wearing comfort. Here, we present a novel stretch sensor fabricated using the top thread of a standard industrial coverstitch machine. The machine is common in apparel production and offers the ability to easily fabricate custom-placed stretch sensors on textile and apparel products. The sensing properties of the stitch are enabled by a conductive thread which increases its electric resistance as the fabric is stretched, due to the geometry of the stitches. Our empirical analysis shows a sensor response in the order of 10 ohms, with almost linear behavior prior to saturation (when the stitch is fully stretched) for low-frequency extensions of 119% of initial sample length. An equivalent electrical model is presented for theoretical modeling of the sensor behavior.