Calculate your peak ovulation day, fertile window (6 days), and best days to conceive based on your last period and cycle length. Includes 3-cycle forecast, LH surge timing, cervical mucus guide, and OPK start dates.
Ovulation timing can shift by ±2–3 days each cycle. LH tests (OPKs) confirm the actual surge each month.
📊 Conception Probability by Day
Based on Wilcox et al. (NEJM 2000) — prospective study of 221 menstrual cycles. Probabilities represent average live birth rates.
Clinical Fertility Guidance
Fertility Assessment Log
Menstrual Cycle Phases — Fertility Guide
Phase
Days (28-day cycle)
What's Happening
Fertility Status
Menstruation
Days 1–5
Period — endometrial lining sheds. FSH begins rising to stimulate follicle development.
Not fertile
Follicular Phase
Days 1–13
Follicle matures in the ovary. Estrogen rises, causing endometrial thickening. Cervical mucus increases.
Low (increasing toward ovulation)
Fertile Window
Days 9–14*
Sperm can survive 5 days. Highest conception probability 2 days before and on the day of ovulation.
HIGH — best time to conceive
Ovulation
Day 14*
LH surge triggers egg release from dominant follicle. Egg viable for 12–24 hours only.
PEAK — single best day
Luteal Phase
Days 15–28*
Corpus luteum produces progesterone to prepare uterine lining for implantation. BBT rises 0.2–0.5°C.
Not fertile (unless fertilized)
Implantation Window
Days 19–23*
If fertilized, embryo implants in uterine lining ~6–12 days after ovulation. hCG starts rising.
Implantation period
*Days shown for standard 28-day cycle. Ovulation and luteal phase shift proportionally with cycle length.
Fertility Signs — How to Know You're Ovulating
Sign
Fertile Appearance
Non-Fertile
Timing
Accuracy
Cervical Mucus
Clear, slippery, stretchy — like raw egg white
Absent, sticky, cloudy, or thick
5 days before ovulation
High (85%)
Basal Body Temperature
Sustained rise of 0.2–0.5°C (retroactively confirms ovulation)
Low stable temperature
Confirms ovulation after it occurs
High for confirmation (BBT rises AFTER ovulation)
LH Surge (OPK)
Positive test = ovulation in 24–36 hours
Negative test
24–36 hours before ovulation
Very High (99%)
Mittelschmerz
One-sided lower pelvic twinge or cramp
No pain
During or just after ovulation
Low (only 20% of women experience this)
Cervical Position
High, soft, open (SHOW)
Low, firm, closed
5 days before ovulation
Moderate (requires daily self-exam practice)
Breast Tenderness
Mild tenderness begins after ovulation
No tenderness
Post-ovulation (luteal phase)
Low — many non-fertility causes
Ovulation Predictor Kit (OPK) — When to Start Testing
Cycle Length
Start OPK Testing
Expected Ovulation
21 days
Day 5
Day 7
24 days
Day 7
Day 10
26 days
Day 9
Day 12
28 days ⭐ Standard
Day 11
Day 14
30 days
Day 13
Day 16
32 days
Day 15
Day 18
35 days
Day 18
Day 21
Start OPK testing approximately 17 days before your next expected period (cycle length − 17). Test at the same time daily — best between 10 AM and 8 PM. A positive OPK (test line as dark or darker than control) means ovulation in 24–36 hours.
Ovulation Calculator — Understanding Your Fertile Window
Ovulation is the release of a mature egg from the dominant ovarian follicle — the moment of peak fertility in every menstrual cycle.
The egg is only viable for 12–24 hours after release. However, because sperm can survive in the female reproductive tract for up to 5 days, the fertile window spans approximately 6 days: the 5 days before ovulation plus the day of ovulation itself.
Conception probability is highest in the 2 days before and on the day of ovulation.
The Luteal Phase — Why It's the Key to Accurate Calculation
A key insight that most ovulation calculators miss: ovulation occurs a fixed number of days before your next period — not a fixed number of days after the start of your last period.
The luteal phase (post-ovulation) is remarkably consistent at approximately 14 days (range 12–16) in most women.
This is why the formula is: Ovulation Day = First day of LMP + (Cycle length − Luteal phase length).
For a 28-day cycle with a 14-day luteal phase: ovulation = Day 14. For a 35-day cycle: ovulation = Day 21.
The follicular phase (pre-ovulation) is what varies between women — the luteal phase stays consistent.
TTC Tips — Maximizing Conception Probability
Best timing: Have intercourse on the 2 days before ovulation and on ovulation day — the highest probability window (cumulative ~70–80% chance per cycle for fertile couples under 35)
Every other day is fine: Daily sex is not necessary and may reduce sperm quality in men with low counts. Every 1–2 days during the fertile window is optimal.
Use OPKs: LH strips (ovulation predictor kits) detect the LH surge 24–36 hours before ovulation — the most reliable at-home confirmation method
BBT charting: Basal body temperature rises 0.2–0.5°C after ovulation — confirms ovulation occurred but cannot predict it in advance
Watch cervical mucus: Egg white cervical mucus (clear, stretchy) is the most accessible real-time fertility sign — most abundant 1–2 days before ovulation
When to see a doctor: If trying to conceive for 12 months with regular intercourse under age 35, or 6 months if over 35, consult a reproductive endocrinologist
The probability of conception drops sharply after ovulation. The egg survives for only 12–24 hours. Intercourse on the day after ovulation has a pregnancy probability of approximately 5% or less, declining to essentially zero by 2 days post-ovulation. This is why the timing of intercourse relative to ovulation is so critical — after the egg is released, the window closes very quickly. Sperm deposited before ovulation (up to 5 days prior) can survive in the cervical mucus and fallopian tubes waiting for the egg. Sperm deposited after ovulation has no viable egg to fertilize. This asymmetry means it's better to have intercourse slightly before ovulation rather than slightly after.
Why do I keep missing my fertile window?
Common reasons couples miss the fertile window: (1) Assuming ovulation always occurs on Day 14 — this is only true for exact 28-day cycles. A 32-day cycle ovulates on Day 18; a 24-day cycle on Day 10; (2) Cycle irregularity — stress, illness, travel, significant weight changes, and hormonal conditions (PCOS, thyroid disorders) can shift ovulation by several days or prevent it entirely (anovulation); (3) Relying solely on calendar calculation without confirmation (use OPKs); (4) Not starting intercourse early enough — couples often wait until they think ovulation is happening, missing the high-probability days 1–2 before it; (5) Using lubricants that are spermicidal — most commercial lubricants damage sperm motility. Use sperm-safe lubricants (PreSeed, Conceive Plus) or none during the fertile window.
Can I ovulate twice in one cycle?
While multiple eggs can be released in a single cycle (fraternal twins occur when two eggs are fertilized), they are released within a 24-hour window during the same ovulation event — not days apart. Once ovulation occurs and the corpus luteum forms, progesterone rises and suppresses further ovulation for the rest of that cycle. So in practical terms: you ovulate once per cycle. However, some women experience a phenomenon called "delayed ovulation" — where the standard ovulation trigger fails and ovulation occurs later than expected. This can cause a positive OPK, then no ovulation, then another positive OPK days later. This is different from true double ovulation and is one reason cycle-to-cycle variation occurs.
What is PCOS and how does it affect ovulation?
Polycystic Ovary Syndrome (PCOS) is the most common cause of irregular menstrual cycles and anovulation (absent ovulation) in women of reproductive age, affecting approximately 8–13% of women. In PCOS, elevated LH and androgen levels prevent normal follicle maturation, often resulting in many small follicles that don't reach ovulatory size, irregular or absent periods (oligomenorrhea/amenorrhea), and unpredictable ovulation. Women with PCOS may ovulate irregularly — sometimes every 35 days, sometimes every 60+ days — making calendar-based tracking unreliable. Diagnosis requires 2 of 3 Rotterdam criteria: irregular cycles, polycystic ovaries on ultrasound, or elevated androgens. Treatment options for conception include lifestyle modification (weight loss in overweight PCOS significantly restores ovulation), letrozole (first-line ovulation induction), clomiphene citrate, metformin, and gonadotropin injections.