Human saliva displaying ferning. During the fertile phase of a woman's menstrual cycle an oestrogen surge causes salt crystals to appear in the saliva. When viewed under the microscope the crystalline salt structure resembles fern leaves and can be used as an indication of a woman's increased chances of conceiving at this time.

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Credit: Human saliva displaying ferning. During the fertile phase of a woman's menstrual cycle an oestrogen surge causes salt crystals to appear in the saliva. When viewed under the microscope the crystalline salt structure resembles fern leaves and can be used as an indication of a woman's increased chances of conceiving at this time. Anne Weston, Francis Crick Institute. Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)