Human saliva displaying ferning, light micrograph (LM). During the fertile phase of a women's menstrual cycle an oestrogen surge causes salt crystals to appear in the saliva. When viewed under a microscope the crystalline salt structure resembles fern leaves and can be used as an indication of a woman's increased chances of conceiving with ovulation commonly occurring within 24-48 hours of strong salivary ferning presence. Magnification: x67.5 when printed at 10cm wide