Ferning of cervical mucus (fern test) is due to:

Fern test- A specimen of cervical mucus obtained using a platinum loop or pipette is spread on a clean glass slide and allowed to dry. When viewed under low power microscope during the estrogenic phase, a characteristic pattern of fern formation is seen. Ferning is due to the presence of high NaCl and low protein content in the mucus secreted under high estrogen effect in the midmenstral phase prior to the ovlation. Shaw's textbook of gynaecology -16th edition-page no 256.