Taking an Ovulation Test when Pregnant: Will the LH Surge Stay High?

by Sep 8, 2021

Trying to conceive (TTC) can feel like an exercise in test taking when it comes to predicting ovulation, checking to see if you’re pregnant, or even tracking hormones.  But when timing is everything, it can be important to get it right.  Ovulation test kits are a common tool when you’re TTC since they help you narrow down your fertile window and time intercourse accordingly.

Much like how home pregnancy tests detect a certain hormone in your urine, ovulation tests detect luteinizing hormone (LH) to predict ovulation.  A steep rise in this hormone, known as your LH surge, indicates the start of your most fertile time – or prime baby-making time.  But does your LH surge stay high if pregnant and can you use an ovulation test to detect pregnancy?  In this blog post we’ll help you understand more about how ovulation tests work and whether they can be used to detect pregnancy.

Will an ovulation test be positive if you’re pregnant?

It’s unlikely an ovulation test will be positive if you’re pregnant but it’s not unheard of.  Ovulation tests are not meant to detect pregnancy and work differently than home pregnancy tests.  They detect a completely separate hormone and using an OPK as a pregnancy test is not recommended.

You may have heard some women using ovulation tests as pregnancy tests, but this has more to do with the similarities between the molecular structures of the hormones than it does with accurate test results.  OPKs measure the hormone LH to predict ovulation, whereas pregnancy tests measure the hormone hCG (human chorionic gonadotropin) – a hormone not detectable until an embryo has been fertilized and implanted.  LH and hCG are similarly structured proteins and some tests have trouble telling the difference between the two.

Understanding ovulation tests

Ovulation, or the release of an egg from the ovary to the fallopian tube, is a short window of time during each menstrual cycle and when you are trying to get pregnant it can be important to know exactly when this time is.  Knowing when you ovulate can help you maximize your chances of getting pregnant by allowing you to take full advantage of your fertile window.  Ovulation tests can help by detecting the hormone that controls ovulation.

How do they work?

OPKs work by identifying LH hormone levels in urine.  Like a pregnancy test, OPKs consist of test strips or sticks that are inserted into a stream or a cup of urine.  A control line appears and when LH levels are above the threshold, a second darker line will appear to indicate an upcoming ovulation.

The second line should get progressively darker as you approach your LH peak and many people start testing as they approach their fertile window so they don’t miss it.  The results can sometimes be tricky to interpret but can generally identify when you are about to ovulate by pinpointing your LH surge.

What do they test?

Ovulation tests or OPKs work by measuring the hormone in your urine that controls ovulation – LH or luteinizing hormone.  LH levels remain steady throughout most of your menstrual cycle, but surge dramatically 24 to 36 hours before ovulation.

OPKs measure LH levels in your urine and compare them with a threshold so you can detect this surge and tell you if you are likely to ovulate soon.  A spike in this hormone means ovulation is on the horizon and you are in your fertile window.  Intercourse during this time will maximize your likelihood of becoming pregnant and ovulation test strips can help you plan your intercourse accordingly.

When should you take one?

Because ovulation tests are meant to detect the short window in your cycle where you are ovulating, it’s not necessary to take them all the time.  You can start taking tests a few days prior to your anticipated ovulation and use the results to predict your most fertile days.

When used correctly, studies suggest they can detect ovulation up to 97 percent of the time and you should start testing a few days before you are scheduled to ovulate so you don’t miss your most fertile days of the cycle.  Although many brands recommend to test with morning urine, it’s not necessary and you can test at any time of day as long as you have limited fluid intake for at least 2 hours before the test.

LH surges and pregnancy

LH surges before your ovulation but you may be wondering what happens if a pregnancy has started and how that affects your LH levels.  In a healthy woman with a regular cycle, ovulation usually takes place 24 to 36 hours after the LH surge.  After ovulation, LH levels decrease sharply.

What are my LH levels if I’m not pregnant?

LH levels do not look the same across all people and all cycles.  Every woman is different and research shows that there may actually be many different LH surge patterns.  Healthy LH levels fluctuate throughout both the life cycle and throughout the menstrual cycle.

In people who are menstruating, LH normally measures between 5 – 25 IU/L and higher during ovulation.  Once you’ve taken several tests, you should be able to identify a spike in your LH levels and see a pattern developing.

What are my LH levels if I’m pregnant?

Although the hormone spikes to kick start ovulation, LH levels do not remain high during pregnancy.  The amount of LH in your body will start to drop after ovulation and LH levels are typically very low during pregnancy (<1.5 IU/L).

What if you’re pregnant?

Some conditions can make detecting ovulation or an LH surge difficult.  Pregnancy is one of those conditions as the similarity of the hormone molecules means tests may be picking up either hormone and you won’t know the difference.

Pregnancy test vs ovulation test

These two tests measure very different things and cannot be used interchangeably.  Ovulation tests are meant to detect an upcoming ovulation whereas pregnancy tests are meant to detect the presence of hCG in your urine – which only happens if you’ve conceived and the embryo implants in the uterus.  You run the risk of inaccurate or false results when the tests are used for something other than what they were intended.

Should you use an ovulation test as a pregnancy test?

While it might be tempting to use an ovulation test as a pregnancy test, it’s not recommended.  Even though both tests measure hormones, their usefulness lies in how those hormones are measured and what the test is trying to detect.  In other words, they tell us different things at different times.

Ovulation tests are not meant to detect the hormone present during pregnancy (hCG) and are only meant to detect LH levels over a certain threshold.  Pregnancy testing after ovulation should be done with a pregnancy test rather than an ovulation test for the most accurate results.  Given the similarities between the LH and hCG molecules, and the limitations of OPKs, it would be impossible to tell if a positive result was from LH or hCG on an ovulation test.

What should you use instead?

If you think you might be pregnant, the most accurate way to confirm is through a blood test or an early detection pregnancy test.  It might be tempting to use an ovulation test strip, especially if you have extras, but they are not accurate at detecting pregnancy and you run the risk of getting a false negative.

✔️ Medically Reviewed by Katerina Shkodzik, M.D., OB-GYN

Dr. Katerina Shkodzik is a certified OB-GYN with a special focus on reproductive endocrinology and infertility issues. She has been practising since 2015.

Dr. Shkodzik completed her residency program in the Department of OB/GYN at the Belarusian State Medical University and fellowship program in the Department of Gynecological Surgery at the Medical University of Bialystok, Poland.

Dr. Shkodzik is extensively involved in digital health projects providing her medical expertise and integrating of cutting edge technologies in medical science and clinical practice since 2018.

Dr. Shkodzik has participated in several studies focused on PCOS, endometriosis, menstrual cycle characteristics and their abnormalities based on big data of digital health in collaboration with leading universities.

She believes that paying special attention to women's health is a crucial step to improving the world we live in.

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