We present the results of measurements and modeling of spectral behavior of phase and group modal birefringence and their sensitivities to temperature and pressure in one of the most popular ellipticalcore fiber available on the market, which is Corning PMF-38. The measurements were carried out in the spectral range from 630 nm to 850 nm using white-light and coherent interferometry. The modeling was fulfilled using the modified perturbation approach first proposed by Kumar. The modification consists in introducing the first order correction term associated with stress induced changes of refractive index and makes it possible to model the fiber response to different physical parameters. Data concerning the fiber geometry and composition, which are necessary in modeling, were obtained using an electron scanning microscope. Although the perturbation approach is a relatively simple method, the theoretical and experimental results show good agreement in the investigated spectral range.