Fertility window explained — why timing matters more than frequency

Quick frame: Pregnancy is most likely if intercourse happens in the 6-day fertile window ending on the day of ovulation. The egg lives ~24 hours after ovulation; sperm survive up to 5 days in the female reproductive tract — overlap is the window.

How the window is calculated

For a regular cycle:

• Ovulation typically happens 14 days before the next period (luteal phase is ~14 days)
• For a 28-day cycle: ovulation ≈ day 14
• For a 30-day cycle: ovulation ≈ day 16
• For a 26-day cycle: ovulation ≈ day 12

Fertile window = ovulation day − 5 to ovulation day + 1.

Use the Fertility Window Tool — enter last period date and cycle length.

What lowers accuracy

• Irregular cycles (varying by > 7 days) → calendar method unreliable
• Stress, illness, weight change, travel → can shift ovulation by days
• PCOS → may have anovulatory cycles (no ovulation at all)

For irregular cycles, layer in:

• Basal body temperature (BBT) — rises 0.3-0.5°C after ovulation
• Cervical mucus — clear, stretchy "egg-white" texture peaks 1-2 days before ovulation
• LH ovulation predictor kits — turn positive 24-36 hr before ovulation
• Ultrasound follicular monitoring — most accurate, doctor-supervised

How frequent should intercourse be?

Daily during the fertile window is fine — sperm count regenerates. Every-other-day works equally well for most couples. Beyond 6 days a week adds zero benefit.

When to see a fertility specialist

• Under 35 → after 12 months of trying
• 35-39 → after 6 months
• 40+ → after 3 months
• Any age with known issues (PCOS, endometriosis, thyroid, prior surgery) → see specialist sooner

FAQ

Q: Can I get pregnant during my period? A: Theoretically possible if you have a very short cycle (< 22 days) and bleed for 7+ days, since sperm can wait for ovulation. Practically rare.

Q: Does diet affect ovulation? A: Major weight loss, eating disorders, very low body fat (< 17%) suppress ovulation. Excess weight (BMI > 30) reduces ovulation regularity. Sweet spot: BMI 19-25.

Q: Are fertility apps accurate? A: For regular cycles, yes — same math. For irregular cycles or recent contraception cessation, layered tracking (BBT + LH + mucus) beats apps.