Egg-white cervical mucus (EWCM) is peak fertile mucus: it means you are in the 2 to 3 most fertile days of the cycle, just before ovulation. Ovulation generally happens on its last day, or on the first day the mucus turns dry or sticky again.
It is called egg white because its consistency looks literally like raw egg white: clear, viscous and elastic. If you get a visible amount, try stretching it between two fingers. Egg-white mucus stretches 2 to 4 inches without breaking.
It is peak fertile mucus and, of all the signs your body gives, the most reliable one there is.
Under a microscope, fertile mucus forms parallel channels that work like rails, guiding sperm straight to the uterus. Infertile mucus forms a crosshatched mesh that blocks passage.
Fertile mucus also preserves sperm life for up to five days. That is what makes the days before ovulation count: sperm can already be there when the egg is released.
That timing is what makes egg white so valuable. The most fertile day of the cycle is the day before ovulation, not the day of it, and egg-white mucus shows up in the 2 to 3 days before ovulation: it marks the peak while there is still time to use it.
Ovulation generally happens on the last day of egg-white mucus, or on the first day the mucus turns dry or sticky again. You confirm it retroactively: the day after shows you which day was the last.
Watery mucus already means fertility is climbing fast. The book's advice is to start having sex when you see watery or egg-white mucus, and to keep going until it dries up again.
Egg-white mucus is part of Chapter 15 of Map to the Positive, in Station II (Attunement). The chapter also shows the other four types of mucus, how to observe step by step, how to tell fertile mucus from semen, arousal or infection, and what to do when it doesn't show up, with its common causes, from medications to hydration. Later in Station II you learn to confirm ovulation with basal temperature and ovulation tests, and to combine every sign into your personal map.
Take 30 days. If it does not give you more clarity than everything you have read for free so far, ask for a refund and you get every cent back. We cannot promise you a pregnancy. We can promise you will not be guessing.
References: Bigelow JL et al. Mucus observations in the fertile window: a better predictor of conception than timing of intercourse. Human Reproduction, 2004. · Billings Method Atlas, the classic reference on cervical observation.