At the beginning and end of the cycle the mucus is sparse, sticky, non-transparent and white or yellowish in color. It seals the cervix. This plug of cervical mucus prevents sperm from entering the uterus and also protects against the entry of germs. Fertilization is unlikely.

In the course of the following days the mucus becomes thinner and more milky. As ovulation approaches (and thus the fertile phase) the mucus becomes increasingly transparent and watery. Fertilization can no longer be ruled out.

Three to four days before ovulation the amount of mucus increases considerably. The mucus is clear and smooth and can be stretched into a long thread between the fingers. This is referred to as spinnbarkeit or spin for short. This indicates that the woman is fertile.

Within one to two days after ovulation the mucus become thicker again. It again becomes sparse, thick, sticky and non-transparent, sometimes even crumbly. The following rule is applied: four days after the mucus has become thick, sticky and sparse again, fertilization is very unlikely.