Cervical Mucus Fertile Window Calculator

Enter 7–10 days of cervical mucus (type and quantity) to estimate your fertile window and a likely ovulation day. You’ll also see today’s fertility level, a next period window, and the earliest reliable test day.

Cervical mucus log (last 7–10 days)
Enter the most recent days you tracked. You can leave unused rows blank.
Methodology & Sources

What this estimates

This tool uses your recent cervical mucus (CM) pattern to place a fertile window, suggest a likely ovulation day with a two-day band, and offer the best two days to try next. If available, it considers OPK positives and a simple BBT “rise seen” toggle to refine timing. Outputs also show DPO today, a next period estimate, and the earliest reliable test day.

How inputs are mapped

  • CM type: egg white = 4 points, watery = 3, creamy = 2, sticky = 1, dry = 0.
  • Quantity bump: heavy +0.5, medium +0.2, light +0.0.
  • Fertile block: we look for a contiguous run near the end of your log where daily score is at least 2. The fertile window spans from the first day in that block to one day after the last day in the block.
  • Ovulation day: if egg white appears, we set ovulation as the day after the last egg white entry. If not, we place it at the end of the fertile window and mark confidence lower.
  • OPK positives: a positive nudges ovulation to the next day. If CM and OPK are within one day, confidence increases.
  • BBT “rise seen”: confirms that ovulation likely occurred. Confidence increases and the window may narrow around the chosen day.
  • Best two days: the two days before the ovulation day are recommended. If ovulation appears to have passed, we explain that recommendation is not available for this cycle.
  • DPO today: calendar days from ovulation day to today. If ovulation is in the future, we show “Not yet.”
  • Next period estimate: ovulation day plus luteal length (default 14 days). If ovulation is not available, we use last period plus your average cycle length if entered.
  • Earliest reliable test day: ten days past ovulation. If ovulation is unknown, we use four days before the estimated next period.

Confidence bands

  • High: CM peak present and OPK positive aligns within about one day, or CM peak plus BBT rise is reported.
  • Medium: CM peak present without OPK or BBT, or OPK present without CM peak.
  • Low: only moderate CM changes without peak-type, or sparse logging.

Insufficient data logic

If fewer than five CM entries are available, or there are no watery or egg white days, the tool cannot place a dependable window. It will suggest continuing daily logging and optionally adding OPKs or BBT.

Date format

All dates display as 01 Jan 2025 to be clear for an international audience.

Limitations

  • CM patterns vary across cycles, especially with illness, dehydration, antihistamines, fertility medications, and infections.
  • Irregular cycles, anovulation, postpartum changes, PCOS, and perimenopause can reduce accuracy.
  • Educational tool only. It does not diagnose conditions and it does not replace clinician guidance.

Sources

What each input means

  • CM log (7–10 days): Pick a type—dry, sticky, creamy, watery, egg-white—and a quantity—light, medium, heavy—for each day.
  • Last period date + average cycle length (optional): Helps anchor windows when recent CM is sparse.
  • OPK results (optional): Add dates with negative/positive. Positives often precede ovulation by 12–36 hours.
  • BBT “rise seen?” (optional): Toggle if you noticed a post-ovulation temperature rise this cycle.

How to read the results

  • Fertile window: Start and end dates based on the most fertile CM days. Shown as a clear range.
  • Today’s fertility level: Low/Medium/High, mapped from today’s CM type and quantity.
  • Likely ovulation day: A low-confidence single date with a 2-day band, refined by any OPK positive or BBT “rise seen”.
  • Next period estimate: Window based on your average cycle length if given, or typical timing after the ovulation estimate.
  • Earliest reliable test day: The day after your first period window ends.
  • Short guidance: Some medicines (e.g., antihistamines), dehydration, and low fluid intake can reduce CM. Combine CM with OPKs or BBT for better timing.

If all recent days are “dry/sticky”, the tool may show low confidence and suggest logging a few more days or adding OPKs/BBT.

Frequently asked questions

How many days of CM do I need?

Seven to ten recent days works well. The estimate improves if the log includes watery or egg-white days.

What does “egg-white” mucus mean?

Clear, stretchy CM that often appears near ovulation. Heavy amounts usually align with higher fertility.

Can I ovulate without seeing egg-white mucus?

Yes. Not everyone notices classic CM. Combining CM with OPKs or BBT can increase confidence.

Do medicines or dehydration change CM?

They can. Antihistamines and low fluid intake may reduce CM volume. Hydration and avoiding irritants can help observations.

How soon after the fertile window can I test?

Usually after the first expected period window ends, or about 12–14 days after the likely ovulation day when cycles are irregular.