About

Standard fertility apps often assume ovulation occurs on Day 14, but many women experience ovulation on Day 18, 21, or later (Oligovulation). This delays the start of the next menstrual period, often leading to false hope regarding pregnancy or anxiety about missed cycles. This calculator adjusts the "Fertile Window" based on a custom ovulation day and Luteal Phase length. It is useful for women utilizing Basal Body Temperature (BBT) or OPK strips who know their ovulation is delayed and need to recalculate their actual period due date.

Formulas

The adjusted tracking relies on the fixity of the Luteal Phase. While the time before ovulation varies, the time after is relatively constant (12-16 days).

Date_Period = Date_LMP + Days_Ovulation + Days_Luteal

The Fertile Window is defined as the 5 days preceding ovulation plus the day of ovulation itself.

Reference Data

Phase	Standard Day	Late Example	Hormonal Event
Menstruation	Days 1-5	Days 1-5	Progesterone drop
Follicular	Days 1-13	Days 1-20	Estrogen rise
Ovulation	Day 14	Day 21	LH Surge
Luteal	Days 15-28	Days 22-35	Progesterone dominance
Next Period	Day 29	Day 36	Lining shedding
Implantation	Day 23	Day 30	Possible pregnancy

Frequently Asked Questions

Not necessarily. As long as you ovulate, pregnancy is possible. However, very late ovulation (after Day 21) can sometimes indicate "weak" ovulation or PCOS, and it makes the cycles longer, giving you fewer opportunities to conceive per year.

If you ovulated late, your period will be "late" relative to a standard 28-day count, but "on time" relative to your biological ovulation. The period always arrives 12-16 days after ovulation unless pregnancy occurs.

The egg survives for only 12-24 hours after release. Once this window closes, conception is not possible until the next cycle. This is why timing intercourse *before* the confirmed ovulation day is critical.

No. This tool runs entirely in your browser using JavaScript. No dates or health data are sent to any server.