Twins Can Cause a False Negative Pregnancy Test Result—Here's What to Know

If you’re trying to conceive a baby, you’re probably intimately familiar with home pregnancy tests. In the vast majority of cases, these simple urine tests are incredibly accurate. They work 99% of the time if taken correctly, according to Planned Parenthood, and the results are most trustworthy after you've missed your period.

But it turns out there are rare instances where a home urine pregnancy test might produce a false negative result. Most cases involve a user error (more on that later), but studies have indicated that being pregnant with multiples can cause this phenomenon.

So how can twins cause a false negative pregnancy test, and do you need to worry about it? Here, we break down everything you need to know.

How Do Pregnancy Tests Work?

For starters, it helps to understand how home pregnancy tests work. You'll find many types of pregnancy tests in stores today, but every single one works by detecting levels of human chorionic gonadotropin (hCG) in the urine.

Your body starts producing this hormone after the embryo implants in the uterine wall—about six to 10 days after conception. The presence of hCG will lead to a positive pregnancy test, while a lack of hCG produces negative results.

"Most at-home pregnancy tests can detect relatively low levels of hCG, making some brands effective as early as the week before your missed period," says Catherine Caponero, DO, an OB-GYN in Beachwood, Ohio, who is affiliated with the Cleveland Clinic. Pregnancy tests are most accurate if taken after your missed period; this is when hCG has likely reached detectable levels.

Can You Have a False Negative Pregnancy Test With Twins? 

It's actually possible for twins (or triplets, quadruplets, etc.) to cause a false negative pregnancy test. That's because multiple pregnancies produce elevated levels of hCG, and if they're high enough, it can interfere with the results.

“When hCG is too high, an improper ratio of the hCG and the antibodies in the pregnancy test can occur,” says Dr. Caponero. “This overwhelms the pregnancy test and causes a false negative test.”

This phenomenon is called the "hook effect," and it's very uncommon. Research suggests that it occurs in less than 2% of negative pregnancy tests, according to Dr. Caponero. Levels of hCG need to be around 500,000 mIU/mL or higher to experience the hook effect, and in most pregnancies, they only reach about half that number.

Pregnancy with multiples isn't the only cause of the hook effect. A person might also receive a negative pregnancy test result, despite the presence of hCG in their body, for the following reasons.

• The pregnancy is much further along than expected (which means the person has higher hCG levels)
• The person has gestational trophoblastic disease, like a molar pregnancy. “Gestational trophoblastic disease is a rare complication that can happen when an egg and a sperm do not join correctly. This unfortunately will not result in a live birth and typically ends in a miscarriage or a procedure,” says Dr. Caponero. 
  

Other Reasons for False Negative Pregnancy Tests

It's important to note that the hook effect isn't very common. Most factors that lead to a false negative result fall into the realm of user error (or, in some cases, a faulty test).

Here, Dr. Caponero shares a few other reasons for false negative home pregnancy tests. 

• The pregnancy test is taken too early after conception
• The urine isn’t concentrated enough (most manufacturers recommend testing as soon as possible in the morning because urine becomes more diluted throughout the day)
• The pregnancy test is expired
• Directions aren't followed properly (for example, the user reads the results too early)

When to Consult a Health Care Provider

If your home pregnancy test is negative, Dr. Caponero typically recommends waiting a few days and then retesting. This is especially important if you take a pregnancy test before a missed period.

"If you think you could be pregnant, or if you continue to have negative pregnancy tests and your period doesn’t start, please contact your health care provider,” advises Dr. Caponero. They might conduct a blood test to see if you're actually expecting.

In the meantime, watch out for symptoms of early pregnancy, which might include the following:

• Sore breasts
• Excessive fatigue
• Cramping or backaches
• Nausea and/or vomiting
• Bloating
• Frequent urination
• Headaches
• Mood swings
• Food cravings or aversions