Clues about ovulation can be gathered from the appearance and quality of the cervical mucus. As ovulation approaches, the cervical mucus thins and becomes increasingly elastic and transparent. By monitoring cervical-mucus changes you can predict when ovulation is likely to happen so you know when to avoid intercourse. Sometimes this form of birth control is referred to as the Billings method.
You should observe your cervical mucus every day, beginning the first day after your last period. Cervical mucus may be found by wiping your vagina before urinating, inserting your clean finger into your vagina, or simply noting its presence on underwear. Regardless of which technique you choose, there is almost no mucus found immediately after your period stops.
It's OK to have intercourse in these early days after your period, but sometimes the semen from intercourse can be confused with mucus because seminal fluid can have a whitish, thick appearance. When you see the cervical mucus first appear, you should refrain from sexual intercourse especially when the mucus becomes more abundant, thin, transparent and slippery, like egg whites, as ovulation approaches.
You shouldn't resume sex for at least three days after the disappearance of the slippery mucus, when it becomes more opaque and sticky.