Can your period cause a false positive pregnancy test

If you've missed your period but get a negative pregnancy test result, you may be feeling confused. Are you pregnant? If not, then why haven't you gotten your period?

First off, don't panic. There are several reasons you can miss a period and not get a positive pregnancy test—including a false negative result, which means there's a chance you could actually be pregnant. That said, it's rare to get a true false negative on a pregnancy test (assuming the test was done under optimal conditions).

Here are some of the more likely explanations:

• You are not pregnant, and your period is late for another reason (the most common explanation).
• You are pregnant, but the pregnancy hormones haven't built up enough yet to be detected. 
• You are pregnant, but the test is not working correctly.
• You are pregnant, but there is something wrong (less common).

Getting a negative result when you otherwise would expect a positive one can be stressful and emotionally challenging. Most of the time, the uncertainty will resolve in a few days. You'll either get your period or you'll take another test and discover that you are pregnant. Less often, something is wrong—but this is rare.

Here's an overview of some of the reasons for a negative pregnancy test when you've missed your period.

A Positive Pregnancy Test: Now What?

Reasons Your Period Might Be Late (That Don't Include Pregnancy)

The most common reason for a late period and a negative pregnancy test is that your period is simply delayed and you’re not pregnant. Having one or two irregular cycles a year is not unusual and does not mean there is something wrong.

Reasons your period might be late include:

• Breastfeeding
• High stress
• Hormonal birth control
• Illness
• Poor sleep
• Too much exercise
• Travel

Lifestyle Changes

If excess stress or illness comes just before ovulation, it can throw off your entire cycle. If you’re breastfeeding and your cycle has recently restarted, you can expect your periods to be irregular for while as your body adjusts. This can make it difficult to know exactly when your period is late. If you’re in your 40s and your period is late, it could be a sign that you are entering perimenopause. That said, don't assume you can’t be pregnant—test anyway!

If you recently stopped taking birth control pills, it can also affect your period. Birth control affects your natural cycle and impedes ovulation. After taking hormonal birth control, it can take a few months for your body's hormones to normalize. It’s not uncommon for the first few cycles to be slightly irregular.

Fertility Treatment

Another possible cause for an off-cycle is fertility treatment. If your cycles are typically short, fertility drugs like Clomid (clomiphene) may extend your cycle length. If you just went through an IVF, IUI, or injectable cycle, this can also throw off your expected period date.

You likely know when you ovulated if you were monitored during treatment. You can consider the “ovulation day” to be:

• The day of the insemination
• The day of your egg retrieval
• Within 24 to 36 hours after receiving a trigger shot

To determine if you are actually late, count 14 days from your “ovulation day." If 14 days haven’t passed, your period isn’t "late" yet.

Irregular Cycles and Amenorrhea

Pregnancy isn't the only reason you might not get your period for an extended length of time. While most people experience menstruation on a fairly regular schedule, for some, irregular timing is the norm. This can make it hard to know when a period is actually late. You might also miss a period (or more) altogether (amenorrhea).

What Is Amenorrhea?

The clinical definition of amenorrhea is not getting your period for three or more cycles in a row. This condition can come and go depending on multiple factors— most commonly, pregnancy and menopause.

Other possible reasons for a late period or for not menstruating for several months include:

• Being severely underweight or overweight
• Breastfeeding
• Chemotherapy and radiation treatments
• Excessive exercise
• Hormonal imbalance (like PCOS or thyroid issues)
• Medications such as some antidepressants and blood pressure treatments
• Problems with the hypothalamus or pituitary glands
• Scar tissue in the uterine lining
• Some forms of birth control (sometimes this is an intended result)

Don't assume that you can't get pregnant just because you are not getting your period. Depending on the cause of your lack of menstruation, you could ovulate again at any time. Consequently, you could get pregnant and not realize it since you won't miss a period.

A false negative pregnancy test is when you are pregnant but the test comes up negative. The most common reason for a false negative is that you took the test too early. Even if your period is late according to your typical cycle, you might have ovulated later in the month. It's not uncommon to occasionally have an off or irregular cycle. Additionally, your count of when your last period occurred could be a few days off.

No matter how sensitive the test, you won't get a positive home pregnancy test until enough days have passed since ovulation and conception and your body has had time to build up enough of the hormone the test detects.

If ovulation occurred later in the month, you will need to test later. There are many reasons you might ovulate later than you typically do. 

How Do Urine Pregnancy Tests Work?

Pregnancy tests look for the pregnancy hormone hCG, which increases as a pregnancy progresses. The normal range for hCG levels in pregnant people can vary widely. It's possible that your levels aren’t yet high enough to be detected (especially if you haven't waited for your period to be truly late).

An early pregnancy test might pick up very low amounts of hCG. However, it’s possible that you don’t have enough circulating hCG to get a positive result even on a sensitive test.

This doesn’t mean anything is wrong. How much hCG you have isn't important; rather, it's how quickly your levels double and increase (and this can only be measured with a blood test).

Another common reason for getting a false negative is not having enough hCG in the urine you test. In early pregnancy, you can dilute the hormone concentration in your urine if you drink a lot of water before testing.

This is more likely to happen in very early pregnancy when you take the test later in the day. The hCG concentration is higher when you’ve held your urine for a while, which is why it's recommended that you take a pregnancy test in the morning.

Rare Causes of a False Negative

In rare cases, an ectopic pregnancy can cause a false-negative test result. An ectopic pregnancy is when the embryo implants somewhere other than the uterus. This usually occurs in the fallopian tubes, but it can also happen elsewhere.

An ectopic pregnancy does not develop properly and is not viable. The formation of the placenta is delayed which impedes the production of hCG. An ectopic pregnancy can be dangerous if it causes the fallopian tube to rupture.

Ectopic pregnancies are rare, occurring in around 1 in 40 pregnancies. However, they can be fatal. Around 9% of pregnancy-related deaths are caused by an ectopic pregnancy.

If your period is late and you are experiencing severe pain, call your provider right away or go to the emergency room.

Another rare cause of a false negative test is gestational trophoblastic disease (GTD), more commonly known as a molar pregnancy. GTD is a rare type of tumor that forms from a developing embryo. In less than 1 in 100 cases, a healthy baby can develop from a molar pregnancy. Usually, however, a molar pregnancy ends in miscarriage.

GTD causes extremely high levels of hCG. As mentioned above, elevated levels of hCG can throw off an at-home pregnancy test and give a negative result. While GTD is a tumor, it’s rarely cancerous. The treatment is usually having a D&C. If pregnancy hormone levels remain high, chemotherapy might be needed.

Pregnancy Test Errors

A false negative can also occur due to test errors. If you wait too long to read the results, you might get a false negative. That said, false positives are more common when reading a test too late. Make sure that you follow the directions of the particular pregnancy test you purchased. To avoid confusion, be sure to read the result within the time window recommended.

Another possible cause of a false negative is an expired test. Storing the test improperly (for example, in a damp, hot bathroom cabinet) can also cause it to malfunction.

Rare Causes of Test Malfunction

A rare, counterintuitive cause for a false negative is that you are too far along in your pregnancy. For example, if your period is weeks to months late, a pregnancy test might come up negative. This is called the variant hook effect.

Another unusual but possible reason for a false negative is that you’re expecting triplets or even twins. In this case, a false negative pregnancy test can be caused by what is known as the high dose hook effect. Ironically, unusually high levels of hCG can cause a pregnancy test to give a false negative result.

A very rare cause of a false negative pregnancy test is when the hCG hormone in your body does not react with the anti-hCG chemicals in the test. In this case, you might need to wait a few more days before you can get a positive result or need to have a blood test.

You might ultimately need to have a serum pregnancy blood test or ultrasound to confirm pregnancy.

If you still haven't gotten your period after a few more days, try the test again. If subsequent tests continue to show a negative pregnancy test result, you'll want to talk to your healthcare provider about further testing.

When to Call Your Provider

If your period is one to two weeks late, and you still are getting negative pregnancy tests, a visit to your gynecologist for a pregnancy blood test is recommended. If your periods are frequently irregular, talk to your provider about when they’d like you to call. Depending on your circumstances, many providers will want to induce a "period" if you go more than two or three months without menstruating.

If your cycle used to be regular but has become irregular, or your periods are irregular for more than three months after stopping birth control, you should see your provider. Irregular cycles can be a risk factor for infertility.

Call Your Provider Immediately

Contact your provider immediately and/or go to the emergency room if you have a late period along with any of these signs or symptoms, which could mean you have an ectopic pregnancy.

• Any of the below symptoms combined with sudden vaginal bleeding
• Fainting or extreme dizziness
• Nausea or vomiting
• Severe abdominal or pelvic pain
• Shoulder pain

A Word from Verywell

A missed period combined with a negative pregnancy result can be confusing but there are a number of explanations. Most often, if you aren't pregnant, your period is just a bit late. If you are pregnant, it's likely you didn't wait long enough before taking the test.

Regardless of the cause, don't ignore worrisome symptoms. The sooner you get things evaluated, the sooner you can know what’s going on and receive proper treatment, if any is needed.

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