Cause of Fern Pattern in Cervical Mucus

The fern pattern of cervical mucus is mainly due to:

Correct Answer: High sodium chloride (NaCl) content

The fern pattern (arborization/ferning) of cervical mucus when dried on a glass slide is due to the high sodium chloride (NaCl) content of the mucus during estrus. When estrogen levels peak (during heat), the cervical mucus becomes watery and rich in NaCl, which crystallizes into a characteristic fern-leaf pattern upon drying.

Ferning and the Estrous Cycle

• During estrus: High estrogen → abundant, watery, acellular mucus with high NaCl → pronounced fern pattern
• During luteal phase (diestrus): High progesterone → thick, viscous mucus with low NaCl → no fern pattern (beaded or granular pattern)
• During pregnancy: No fern pattern (progesterone dominant)

Clinical Use

Ferning test is a simple, rapid, low-cost method to confirm estrus in cattle and other domestic animals before AI.