Endometriosis, a common gynecological disorder characterized by the growth of endometrial gland and stroma outside the uterus, causes several symptoms such as dysmenorrhea, hypermenorrhea, and chronic abdominal pain. 17β estradiol (E2) stimulates the growth of endometriotic lesions. Although estetrol (E4), produced by human fetal liver, is also a natural estrogen, it may have the opposite effects on endometriotic cells. We investigated different effects of E4 and E2 on the invasion and migration of immortalized human endometrial stromal cells (HESCs) and evaluated whether E4 affects the expression of Wiskott-Aldrich syndrome protein (WASP) family member 1 (WASF-1). We measured the invasion of HESCs by a Matrigel chamber assay. Cell migration was measured by wound healing assay and cell tracking analysis. The expression of WASF-1 was confirmed by independent real-time PCR analysis. Transfection of cells with siRNAs was carried out to knock down the expression of WASF-1 in HESCs. E4 significantly inhibited E2-induced invasion and migration of HESCs. WASF-1 was found to be a potential mediator based on metastasis PCR array. WASF-1 was upregulated by E2 and downregulated by E4. Knockdown of WASF-1 inhibited migration. Our results suggest that E4 may inhibit E2-induced growth of endometriotic lesions. Downregulation of WASF-1 is involved in the inhibitory effects of E4 on migration. The use of E4 combined with progestins as combined oral contraceptives may cause endometriotic lesions to regress in women with endometriosis.