What mucus is for
The cervix continuously produces a secretion whose nature changes with the hormones. Outside the fertile phase it forms a dense plug that closes the cervix: sperm do not get through and die within hours in the acidic vagina.
As ovulation approaches, rising oestrogen transforms that secretion. It becomes abundant, highly hydrated, and its molecular structure aligns into channels. It stops being a barrier and becomes a transport and survival medium: it is mucus, and mucus alone, that lets sperm last up to five days. Without fertile mucus there is no five-day fertile window — mucus does not accompany fertility, it enables it.
How it changes across the cycle
| Phase | Appearance | Sensation |
|---|---|---|
| After the period | Nothing visible | Dry, or nothing in particular |
| Early rise | White, thick, sticky, scant | Damp at most |
| Approaching ovulation | More abundant, creamy, less opaque | Clearly wet |
| Peak fertility | Clear, elastic, stretching between two fingers, compared to raw egg white | Slippery, lubricated |
| After ovulation | An abrupt switch: thick again, or nothing at all | Dry or sticky |
What matters is not the appearance in itself but the change, and above all the abrupt switch that follows the peak. That switch marks the passage.
The peak day and its rule
The peak day is the last day of stretchy or clear mucus, or of the slippery sensation — so it can only be recognised the following day, once the switch is observed. The infertile phase is taken to open on the evening of the third day after that peak, the fourth in some schools.
A simpler approach asks one daily question instead: did I notice secretions today, or yesterday? If so, the day counts as fertile. This two-day variant loses in precision what it gains in accessibility.
How to observe
- Throughout the day, by the sensation at the vulva while walking: the most reliable sign, and the most often overlooked.
- On toilet paper, before and after using the toilet.
- Record once, in the evening, the most fertile observation of the day.
- No internal sampling in most teaching schools: you observe what presents itself.
Effectiveness
| Pregnancies per 100 women per year | |
|---|---|
| Perfect use | around 3, lower in some highly codified protocols |
| Typical use | very variable across studies, from around 11 to over 20 |
The considerable gap between those two columns is the real message: the method works when it is taught, followed and respected, and works poorly as soon as one of the three is missing.
What blurs the observation
Vaginal infection, thrush, semen from the previous day, lubricant, spermicide, douching, sexual arousal, certain medicines: in all of these the reading becomes doubtful. The postpartum period, breastfeeding and perimenopause produce patterns that are hard to interpret and call for specific support. Mucus gives no protection against sexually transmitted infections.