How Can Pregnancy Affect Ovulation Tests?

Pregnancy Hormone

Pregnancy tests measure the hormone human chorionic gonadotrophin, or hCG, to detect pregnancy. Once an egg has been fertilised, it travels down the Fallopian tube and implants in a woman's uterus. After implantation, the embryo begins producing hCG, which can be detected in the woman's urine. Home pregnancy tests detect this hormone at different sensitivity levels and turn positive when detected at those levels.

Ovulation Hormone

Ovulation is the release of an egg from an ovary each month. If this egg is fertilised, the woman becomes pregnant. Before ovulation, a woman's body releases luteinizing hormone, known as LH. This hormone is released in increasing amounts for several days before ovulation. Just before ovulation, LH surges, causing the release of an egg.

Ovulation Tests

An ovulation prediction kit or test detects an LH surge in a woman's urine. These tests work in a similar way as pregnancy tests, but are read in a different way. Both a pregnancy test and an ovulation test have a control line and a test line. With a pregnancy test, two lines indicates a positive result. However, with an ovulation test, two lines do not necessarily indicate a positive result. Because LH builds up before ovulation, you will see two lines on a test for several days before ovulation. The result is positive when the test line is darker than the control line and the LH surge has been detected. When the LH surge is detected, ovulation will occur in 12 to 36 hours.

Ovulation Tests as Pregnancy Tests

Because of the similarity of hCG and LH at the molecular level, ovulation tests can detect pregnancy as well. The difference between hCG and LH is a beta subunit attached to the hCG. The website peeonastick.com illustrates the difference by describing LH and hCG as identical twins. The difference between them is that hCG is wearing a hat and LH is not. So the ovulation test will turn positive when it detects either one of the identical twins. But because pregnancy tests are also looking for the hat on the twin, a pregnancy test will not turn positive during ovulation. So if you are pregnant, an ovulation test will turn positive.


The sensitivity levels of ovulation tests are generally lower than the sensitivity levels of home pregnancy tests. Because ovulation tests are detecting the surge of LH, they are made to have lower sensitivities. If they are too sensitive, they will turn positive at the presence of LH. Because LH is present for several days before ovulation, ovulation tests must be made to predict high levels, or the surge, of LH. Ovulation tests have an average sensitivity of 30 mIU. And early home pregnancy tests can detect pregnancy at levels as low as 12.5 mIU, according to peeonastick.com. So a home pregnancy test will often turn positive before an ovulation test will turn positive.

How to Use a Clearblue Ovulation Test

Consult the chart included with the Clearblue ovulation test to determine what day testing should begin. Women with longer menstrual cycles will begin testing later than other women, and women with shorter menstrual cycles will begin sooner. A woman with a 21 day cycle will begin testing for ovulation on cycle day 5, where a woman with a 40 day cycle will begin testing on cycle day 23. To determine cycle length, use the shortest cycle in the last six months as your guideline.

Test the amount of LH. Clearblue's instructions state that the ovulation tests can be used at any time during the day, but sources such as peeonastick.com state that LH is not synthesized until later in the day and the ideal test time is between 2 and 4 p.m. Open the test strip packet and hold the wick end of the test in the urine stream for 20 seconds. If using a non-digital test, a reference line will appear to show the test is working.

Read the results of the test after three minutes. On non-digital tests, a test line that is the same color or darker than the reference line indicates a positive result. For digital tests, a smile appears in the digital readout window when the test is positive. Positive results mean the LH surge has been detected, which is the signal for the start of ovulation. The follicle will rupture and the egg will be released usually within 12 to 24 hours of the LH surge. A woman's fertile period begins about 24 to 36 hours before ovulation and lasts until 24 hours after ovulation, after which the egg is no longer viable.

If the test results are negative, the line will be lighter than the reference line for non-digital tests. On digital tests, a circle will appear. Repeat testing with a new test. Tests may be repeated more than once per day, or at the same time every day on consecutive days.


Women who are taking fertility drugs containing luteinizing hormone or human chorionic gonadotropin should not use ovulation tests as the results may not be accurate.

Pregnant women may also receive a false positive result on a Clearblue ovulation test.

Can you be pregnant & have an LH surge?


Luteinizing hormone signals the follicle to release the mature egg into the Fallopian tube for fertilisation.


Understanding when and how the body ovulates is the key component in taking charge of fertility. Women who are trying to become pregnant may gauge their LH surge through the use of ovulation predictor kits.

The Facts

The complete menstrual cycle varies from 25 to 36 days and is made up of three distinct phases, all controlled by different hormones.

The first phase is the follicular phase, which begins on day 1 of menstrual bleeding. This is when the pituitary gland starts upping the production of follicle-stimulating hormone, which may produce up to 30 follicles at a time. Each follicle produces an egg, but only one egg will fully mature and produce oestrogen. The follicular phase averages 14 days but may vary from cycle to cycle.

Next comes the ovulatory phase, which begins on the day of the LH surge. The increase in luteinizing hormone sends a message to the follicle to release the mature egg into the Fallopian tube.

And last starts the luteal phase. After release of the egg, the follicle closes and creates a 2- to 5-cm structure called the corpus luteum, which is essential in maintaining pregnancy. The corpus luteum secretes the hormones progesterone and oestrogen, which prepare the uterus to nourish a foetus. The high levels of oestrogen and progesterone also inhibit the production of luteinizing hormone, so an LH surge during pregnancy is not possible.


Not all menstrual cycles are the same. It is possible to have an LH surge and not release an egg, as well as absence of the LH surge altogether. It may also be possible to release more than one mature egg during the ovulatory phase, but this is an extreme exception to the rule.


Some women may feel symptoms of a second LH surge and test after already becoming pregnant. It is possible to get a positive result as a result of pregnancy and not an LH surge. The test will indicate a positive result if the hormone human chorionic gonadotrophin is present in the urine. This is important to note because early detection of pregnancy leads to prompt prenatal care, a proven benefit for a healthy pregnancy.