Ovulation predictor kits (OPK)

Conception happens only near the time of ovulation. The number of fertile days in each cycle is not known, but there are many signs of ovulation which can be used to know when ovulation is happening.

The most precise sign is detection of luteinizing hormone (LH). It is released by the pituitary gland when the egg is mature. It causes the release of the egg.

What are signs of ovulation?

  1. Positive LH test on OPK, showing the LH surge. This is a good predictor of ovulation.
  2. Change in vaginal discharge. You may have clear and stretchy cervical fluid before ovulation.
  3. Higher body temperature at rest (BBT) after ovulation.
  4. Progesterone hormone is made by corpus luteum after ovulation. It peaks one week after ovulation.
  5. Ultrasound of ovaries shows follicles large enough to ovulate.
  6. Menstrual period starts 2 weeks after ovulation.

The ovulatory cycle

Gonadotropin-releasing hormone (GnRH) is released in the hypothalamus at the base of the brain. GnRH is needed to start the cycle.
GnRH is carried to the pituitary where it stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH).  LH and FSH start the development of the ovarian follicle, which has the egg.
FSH acts on the ovary to keep the follicle growing and developing. FSH causes the follicle to make estrogen. Estrogen causes a mid-cycle LH surge to stimulate ovulation. The follicle ruptures and the egg is released.
The follicle becomes the corpus luteum and makes progesterone. This gets the endometrium ready for implantation of a fertilized egg (also known as an embryo).
Fallopian tubes
These are on the upper part of the uterus. The tubes pick up the egg after leaves the ovary. After fertilization, the tube moves the fertilized egg into the uterus.
The fertilized egg implants in the uterus and develops throughout the pregnancy until the baby is born.

How many days does the fertile time last?

The egg survives 12 to 24 hours after ovulation. Sperm lives in the body 2 to 4 days. So, you may have up to a 4 to 5-day window to conceive.

Do women ovulate on cycle day 14?

Day 1 of a cycle is the first day of your period.

Ovulation may change from cycle to cycle. Many fertile women will not ovulate on day 14. A woman with a 28 to 30-day cycle will ovulate day 14 to 16. If ovulation is later, the cycle will be longer.  A woman often gets her period 14 days after ovulation if she is not pregnant. So, in a 35-day cycle, ovulation likely happens around day 20 to 21.


Choose from these ovulation predictor kits
Kit Cost Benefits/Notes
Clearblue Digital Ovulation Tests (20 count) $$

This is the best test for IUI timing.

Ovulation within 36 hours of a "smiley face" is a positive result (LH detected).

This means you will have 1 to 2 fertile days. One kit can be used for more than 1 cycle.

Clearblue Easy or other brand $

Compare the color intensity of 2 lines. The LH surge is happening when the test line is as dark as the control. This can be hard to see.

This costs less. So it may help women with unpredictable ovulation who need to test for long amounts of time.

Clearblue Advanced Digital Ovulation Test $$$

Do not use this test for IUI timing. It is a detailed test that can be confusing.

A flashing symbol shows high fertility leading up to ovulation.

A solid symbol shows the LH surge (peak fertility). 10 out of 100 people do not see a peak reading. This gives a wide window of fertile time in the cycle.

You may choose to time intercourse every other day in the high fertility phase.

Clearblue Fertility Monitor $$$$

This test is not preferred.

Buy the digital monitor and test strips.

These are expensive and not needed for accurate timing.


OPK use and timing

OPKs test urine for LH. Ovulation will happen within 36 hours of the first hormone release. The purpose of OPK testing is to time intercourse or IUI procedures.

  1. Start testing day 11 of a cycle.
    • Day 1 is the first day of heavy bleeding.
  2. Do not test before day 11. Fertility drugs (like Clomid) may give a false positive result if you test early.
  3. Test 1 time each day at about the same time each day.
    • For the best IUI timing, do not test the first morning urine (ignore kit instructions).
    • Test during the day and call the clinic by 3:30 p.m. to schedule IUI for the next day if your test is positive.
    • If timing intercourse, have sex on the day of the positive result and the next day.
  4. Call the clinic if you have not had a positive result by day 22. Your progesterone blood level will be tested. This will help your care team know if you truly did not ovulate, or if the test just missed the LH surge and you did ovulate. Then, you and your care team can correct the problem the next month.
Last reviewed: 
January 2021

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