Short answer

Your fertile window is the six days that end on the day you ovulate. LH urine tests detect the hormone surge that comes shortly before ovulation, and your basal body temperature (BBT) stays slightly higher after ovulation, confirming it has happened. Using both gives a clearer picture than calendar dates alone.

Key facts

  • In a study of 221 women trying to conceive, pregnancy happened only when sex took place in the six days ending on ovulation day.[1]
  • The chance of conception ranged from about 10% for sex five days before ovulation to 33% on the day of ovulation.[1]
  • Only about 30% of women had a fertile window entirely within cycle days 10 to 17.[2]
  • Apps that used cycle length alone predicted the day of ovulation correctly no more than 21% of the time.[3]

What is the fertile window?

The fertile window is the six-day stretch that ends on ovulation day: the five days before ovulation and the day itself.[1] Because ovulation day moves from cycle to cycle, the window moves too, even in people whose cycles are usually regular.[2]

How do LH ovulation tests work?

LH tests measure luteinizing hormone in your urine. A positive result means an LH surge, which happens shortly before ovulation.[4]

  1. Start testing a few days before your earliest expected ovulation. With long or irregular cycles, start earlier and test for longer.
  2. Test at about the same time each day, and follow the kit's instructions.
  3. Log each result, positive or negative, so you can compare cycles.

How does BBT charting work?

  1. Take your temperature with a basal thermometer right after waking, before you get up, at about the same time every day.
  2. Log it every day of your cycle.
  3. Look for a small rise that lasts for several days. After ovulation, progesterone keeps your temperature slightly higher until your next period.[5]

BBT confirms ovulation after it has happened, so on its own it can't warn you that the window is starting. Over several cycles it can show whether, and roughly when, you usually ovulate. Broken sleep, illness, alcohol and travel can all disturb readings.

Calendar, LH tests, BBT or cervical mucus: which helps most?

MethodWhat it showsTimingMain limit
Calendar estimateLikely window based on past cycle lengthsBefore ovulationOvulation day varies, so it is often wrong[3]
LH urine testsThe LH surge before ovulation[4]Before ovulationNeeds daily tests; doesn't confirm that ovulation happened
BBT chartingA temperature shift after ovulation[5]After ovulationOnly confirms afterwards; easily disturbed
Cervical mucusChanges in discharge around ovulation[5]Before ovulationTakes practice to read

Many people combine methods: LH tests to see ovulation coming, and BBT to confirm it afterwards.

Can the fertile window be used to avoid pregnancy?

Not with Preeo. Fertile-window dates in Preeo are estimates. Preeo is not a method of birth control and must not be used to prevent pregnancy.

When should you see a doctor if you're trying to conceive?

ACOG recommends an infertility evaluation if you haven't become pregnant after 1 year of regular sex without birth control, or after 6 months if you're older than 35. If you're older than 40, talk with your ob-gyn now.[6]