Ovulation Calculator

The 2 days immediately before ovulation are the highest-probability conception days. According to the landmark Wilcox 1995 study published in the New England Journal of Medicine, conception probability is roughly 25–30% per day at peak, dropping to about 10% on ovulation day itself.

Six days total — the 5 days before ovulation plus the day of ovulation. Sperm can survive up to 5 days in the reproductive tract; the egg survives less than 24 hours after ovulation. After ovulation day, conception probability drops to nearly zero.

Very unlikely. Once the egg has been released and is past its 12–24 hour viability window, conception cannot occur until next cycle's ovulation. If you're unsure when you ovulated, the safest assumption is that any day in the previous 6 days could have been fertile.

In a 35-day cycle, ovulation typically occurs on day 21 (35 minus the 14-day luteal phase). Your fertile window would span days 16–21 from the start of your last period. Cycles longer than 35 days warrant evaluation by an OB-GYN, as they may indicate PCOS or other ovulatory issues.

LH (luteinizing hormone) urine tests detect the surge that triggers ovulation 12–36 hours later. They're ~99% accurate at detecting the surge in regular cycles, but they don't confirm that ovulation actually occurred. For confirmation, basal body temperature tracking shows the post-ovulation rise.

The follicular phase (period to ovulation) varies with stress, illness, travel, sleep changes, weight changes, and ageing. The luteal phase (ovulation to next period) is much more stable at ~14 days. So even within the same woman, ovulation timing can shift by 3–5 days month to month.

Knowing your fertile window helps with both pregnancy planning and avoidance. However, ACOG considers calendar-based "fertility awareness" methods less reliable for contraception (typical-use failure rate ~24% per year) than methods like IUDs (<1%) or hormonal contraception (~7%). Combine multiple signals (calendar + LH + temperature) for best accuracy.

ASRM recommends evaluation after 12 months of unprotected intercourse for women under 35, or 6 months for women 35 and older. Earlier evaluation is appropriate for irregular cycles, history of pelvic infection or surgery, known male-factor concerns, or recurrent pregnancy loss.