1. Flo - ovulation calendar, period tracker, and pregnancy app
  2. Getting pregnant
  3. Trying to conceive
  4. Ovulation tracking

When to Take Ovulation Tests: Reasons for Positive and Negative Results

When do you ovulate? An ovulation kit will help you find out! And this simple guide by Flo will help you learn to use ovulation tests right. Let’s dive into it.

Ovulation test

With a positive ovulation test result, you now know when your next period will arrive, since ovulation occurs 12–16 days before your period begins. 

It is not recommended to use the test to track ovulation, in order to avoid pregnancy.

The test identifies the surge in LH up to 24 to 48 hours before ovulation, but sperm can survive in the body for 3 to 5 days.

Therefore, if you have intercourse before you discover the LH surge, the egg could still be fertilized.

One way to predict ovulation is to use home tests. They react to the luteinizing hormone in the urine, which is at its peak 12–36 hours before the egg is released.

Here are some ovulation test instructions:

  • You should start taking tests several days before the expected ovulation (with a regular 28-day cycle, it should be on day 11–12).
  • Continue taking the tests until the result is positive.
  • It is better to do tests twice a day (but do not use the first morning urine).
  • Before taking a test, do not drink a lot of water and do not urinate for about 4 hours.
  • Follow the instructions closely (collect urine in a clean container, put the test strip in it for no more than 10 seconds, check the result no later than 10 minutes).
  • If the second line is clear, ovulation will occur in 12–36 hours; a faint line means a negative result for ovulation.

To determine fertile days most accurately, you can additionally measure your basal temperature, monitor your cervical mucus during the cycle, and track ovulation by ultrasound.


The peak of the luteinizing hormone (LH), which is detected in the urine shortly before ovulation, varies in duration and level in different women.

Sometimes, the peak time of LH is short, i.e., it lasts only a couple of hours and, therefore, can be determined only during this period.

To not miss the surge of the hormone, it is better to do tests twice a day at the same time.

If the peak time is long (more than 24 hours), one test is enough to indicate an increased level of the hormone in the urine.

If the test is positive (i.e., the peak of LH is stated), ovulation will occur in 12–36 hours.

Therefore, it is recommended to plan sexual intercourse on the first day of the positive test and during the next 3 days.

Usually, ovulation tests that react to the luteinizing hormone (LH) in the urine are done a few days before the expected ovulation.

If the cycle is regular, then the source data will be approximately the following:

  • the menstrual cycle length: 28 days
  • the luteal phase (from ovulation to menstruation, fairly stable, lasts 12–14 days)
  • the beginning of testing: 3 days before ovulation.

That makes 28 – 14 – 3 = 11. Thus, it is necessary to start taking tests from the 11th day of the cycle (counting from the first day of your menstruation).

Irregular cycles make everything a bit more complicated.

The best option is to determine the shortest cycle in the last 6 months, and regard your current cycle as your shortest one.

For example, your shortest cycle was 21 days. Your luteal phase is stable and lasts for 14 days.

21 – 14 = 7

Hence, the expected ovulation is on the 7th day. Therefore, you should start doing tests 3 days before that, on the 4th day of the cycle.

Ovulation tests can help you specify the length of your luteal phase, if it lasts longer or less than 14 days.

Ovulation tests indicate the presence of the luteinizing hormone (LH) in the urine. Within 12–36 hours after its level rises, a follicle ruptures, an egg is released, and ovulation takes place.

However, this regulated physiological process can fail.

There are several reasons why ovulation tests are inaccurate:

  • In case of polycystic ovaries, a test marks the surge of LH, but ovulation may be absent.
  • With luteinization of an unruptured follicle, the level of LH is sufficient to be detected by a test, but the egg does not leave the ovary.
  • If there are abnormalities in the hypothalamus, LH production malfunctions; the hormone level may be high, but ovulation does not occur.

To determine approaching ovulation, it is better to apply a combination of methods: taking ovulation tests + measuring basal body temperature + monitoring changes in cervical mucus.

Nowadays, there is a great variety of ovulation tests. They mainly differ in price, accuracy, parameters for determining the approaching ovulation, and convenience.

The most common ones are test strips and digital devices that detect the rise in the level of the luteinizing hormone (LH) in the urine. They look like pregnancy tests.

The test strips differ in sensitivity, which is measured in international units per liter, or IU/L. This parameter indicates the minimum amount of LH that the test can detect in the urine (from 10 to 40 IU/L).

That is, high sensitivity tests (10 IU/L) are more likely to detect an increased level of LH even if its peak is low.

Some test systems additionally indicate the surge of estrogen, which precedes the rise of LH. This helps identify the beginning of the fertile window earlier than the LH test alone.

Most home ovulation tests are based on indicating a surge of the luteinizing hormone (LH) in the urine, which occurs 12 to 36 hours before an egg is released from its follicle.

This period and the first 24 hours after ovulation are known as the fertile window when the chances to conceive are the highest.

There are also test systems that can indicate increased levels of both LH and estrone-3-glucuronide (E-3-G) in the urine.

E-3-G is an estrogen decomposition product, and its level increases in the blood and urine before the peak of LH, which allows one to determine the favorable time for conception much earlier.

The fertile window determined by this test is about 6 days.

Five days before ovulation, a surge in E-3-G is detected, which means that ovulation is approaching.

From this moment, you can start having sex actively because sperm can survive in the female body up to 5 days, waiting for a mature egg.

A positive test result is indicative of an LH surge, and ovulation should occur in the next 24 to 36 hours.

However, this may not be the case for everyone. Intercourse should take place on the day you receive the positive result and during the 3 days following.

A positive ovulation test represents your best opportunity to get pregnant. If you are unsure of the result, read the booklet that is included with the ovulation kit.

Flo can provide you with more valuable information to help you discover the best time of the month to conceive.

A negative ovulation test means you are not having an LH surge, and ovulation has not occurred. If you are unsure about the results, consult the booklet included with the test.

Reasons for negative results:

  • use of first morning urine
  • the concentration of LH too low to detect
  • testing too early or too late in the menstrual cycle

Predicting the LH surge can be tricky, so don't worry if you get another negative ovulation test result. It may have come earlier than planned.

For the most accurate result, doctors suggest testing twice a day, 10 hours or more apart, for a few days prior to ovulation.

Log your ovulation test results and the app will use them to make even more accurate predictions.

Ovulation tests are a convenient way to determine fertile days.

It indicates an increased level of the luteinizing hormone (LH) in the urine. As a rule, LH is at its peak 12–36 hours before ovulation.

You can take an ovulation test at home; it is quite low in cost (as compared to the ultrasound method).

The ovulation test can be negative for the following reasons:

  • The peak time of LH is short, and you missed it (that is why you should do tests twice a day).
  • In some cycles, ovulation does not occur because of stress, intense physical activity, sudden weight changes, or unusual climate.
  • With long cycles, you may have taken a test too early. You should start taking them 17 days before the expected period or 3 days before the potential ovulation. Continue until the result is positive.
  • The test has been performed incorrectly. For example, your urine was diluted or you did not follow the instructions.

Be patient and record the test results for at least 3 cycles.

To get the most reliable result, you can apply other available methods: measure your basal temperature and monitor changes in your cervical mucus






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