Cycle Information
Normal range: 21-35 days (average 28 days)
Normal range: 3-7 days (average 5 days)
Methodology: fertile windows are calculated using the calendar method described by the American Society for Reproductive Medicine. The calendar displays the fertile window, predicted ovulation date, and suggested pregnancy test date. Adjust luteal phase length in settings if recommended by your doctor.
Plan around the moments that matter
Cycle-aware planning
Track average cycle length and luteal phase so the calculator refines future predictions as your data grows.
Medical conversations
Print the generated timeline and share it with your clinician to discuss hormone testing or fertility treatments.
Partner coordination
Export the fertile window calendar or sync it to a shared calendar so everyone understands the plan.
What to keep in mind
Cycle length range
We support cycles from 21 to 45 days. If your cycle regularly falls outside that window, consult a healthcare professional.
Data privacy
Cycle information is stored locally in your browser. Clear your history from the settings panel if you share a device.
Scientific references
Predictions are based on research published by the American College of Obstetricians and Gynecologists and adjusted for irregular cycles.
Frequently asked questions
How is the fertile window calculated?
We estimate ovulation by subtracting the luteal phase (default 14 days, adjustable in settings) from your projected next period. The fertile window spans the five days before ovulation plus ovulation day.
Can the calculator handle irregular cycles?
Yes. Enter at least three recent cycle lengths and the tool will calculate an average while highlighting months that deviate significantly.
Does the tool predict due dates?
After you log a positive pregnancy test, switch to due date mode to see the estimated delivery date based on your cycle information or ultrasound data.
Is this a substitute for medical advice?
No. The calculator provides educational estimates. Always discuss fertility concerns and treatment plans with a qualified healthcare provider.
For personalised guidance, speak with your healthcare provider or reach out to the NTN Tools support team.
Helpful resources
- American College of Obstetricians and Gynecologists Practice Bulletin on menstrual cycle variability.
- Centers for Disease Control and Prevention guidance on preconception health and prenatal vitamins.
- Resolve.org support groups for individuals and couples navigating fertility challenges.