13. Cycle Length, the Fertile Window and Timing Tests

First principle: count back from the next period

Because the luteal phase is fixed (~14 days) and the follicular phase varies, ovulation ≈ next menses minus 14 days.

The fertile window is ~5 days before to the day of ovulation (sperm survival + oocyte lifespan).

Cervical mucus as a bioassay

• Estrogen (peri‑ovulatory): thin, acellular, high‑spinnbarkeit, ferning — sperm‑permeable.
• Progesterone (luteal): thick, scant, impenetrable.

The classic timing error 🚩 (a pitfall worth its own line)

Discriminators

• Normal cycle 21–35 days, menses 2–7 days: usually ovulatory — routine ovulation confirmation is often unnecessary (Concept 31).

Pitfalls

• Rigid day‑21 testing; ignoring cycle length when interpreting a low progesterone.

Pearls

• 🎯 Ovulation = next period − 14 days. Time the mid‑luteal progesterone to her cycle.
• 🩺 "Egg‑white" mucus is a free, patient‑reportable ovulation predictor.

Next → What happens to bleeding when ovulation fails? (Concept 14).