Ferning of cervical mucus disappears after the following day of menstrual cycle:

Microscopically there is characteristic patterns seen in cervical mucus dependent on the stage of the ovarian cycle and the presence or absence of pregnancy. Mucus crystallization necessary for the production of the fern pattern is dependent on an increased sodium chloride concentration. Cervical mucus is relatively rich in sodium chloride when estrogen, but not progesterone, is being produced. Thus, from approximately the 7th to the 18th day of the cycle, a fernlike pattern is seen. After approximately the 21st day, a different pattern forms that gives a beaded or cellular appearance. Ref: Leveno K.J., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 8. Prenatal Care. In K.J. Leveno, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e