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Pregnancy tests have come a long way. Historically, women didn’t have a reliable method of knowing if they were pregnant without going to the doctor. It wasn’t until the first at-home pregnancy test was invented in 1976 that women could confirm that they were expecting.

But despite technological advances that let women know they are pregnant, there’s still a lot of mystery about a woman’s menstrual cycle.

A woman may have a delayed or missed period, but still have a negative pregnancy test. In those situations, she has to wonder what’s going on. Is she pregnant? Is something wrong?

Here are a few reasons your period may be late, even if your pregnancy test is negative.

If you’re trying to get pregnant, there’s good news: You may still be pregnant. Sometimes, levels of the pregnancy hormone human chorionic gonadotropin (hCG) early in pregnancy aren’t yet high enough for a home pregnancy test to detect.

One study found that home pregnancy tests only have to detect hCG levels above 25 milli-international units per milliliter (mIU/mL) to achieve the commonly advertised 99 percent accuracy rate. The writers of a 1991 study calculated that to detect 95 percent of pregnancies, a test would have to detect levels as low as 12.4 mIU/mL. But not all home pregnancy tests were consistently sensitive enough to do so.

A woman’s cycle can vary widely, so if you conceived later in your cycle, your hormone levels may not be high enough at the time of your missed period.

There can be as much as a 13-day difference in when ovulation occurs, meaning that you may think you are 4 weeks pregnant when you’re only two weeks along. Pregnancy bleeding, recent hormonal contraceptive use, or breastfeeding can all interfere with accurately knowing your dates, as well.

If you think you may be pregnant after a missed period but got a negative result on your pregnancy test, wait a few days. Then retest. If you continue to miss your period, be sure to talk to your doctor to rule out any complications.

It’s rare, but sometimes an ectopic pregnancy can show up as negative on a pregnancy test. This happens in less than 3 percent of ectopic pregnancies.

Seek medical attention if your pregnancy test is negative and you have these symptoms:

Learn more: Ectopic pregnancy »

Several outside factors can wreak havoc on your menstrual cycle. Stress, for example, can delay your period. Malnutrition can affect it, too. Your cycle can fluctuate if you drink too much caffeine or don’t eat enough food.

Sudden lifestyle changes, such as intense exercise or working the overnight shift on your job, can also cause your period to be irregular.

Breastfeeding can cause some irregularities in your cycle. Even after your baby arrives and your period returns, it may take some time before your cycle goes back to normal.

Breastfeeding is also unpredictable month to month. As babies grow, their feedings may change. For example, if your baby goes through a growth spurt and suddenly increases the frequency of night feedings, it may interfere with your cycle.

Read more: Why women get irregular periods while breastfeeding »

Medical conditions such as polycystic ovary syndrome (PCOS) or thyroid problems may cause women to have highly irregular cycles and missed periods. Some women may have very light periods, some may have very heavy periods, and some may skip periods altogether.

Menopause for women typically begins around age 50. In some women, however, it can start prematurely, before the age of 40. It’s different for everyone. If you have missed your period for over 90 days and are not pregnant, talk to your doctor about getting tested for any underlying medical conditions.

Birth control may cause irregularities in your cycle. Other types of medications may lead to a missed period, as well. For example, blood pressure drugs or allergy medications can throw off your cycle.

There may be many different reasons for a negative pregnancy test after a missed period. You may be dealing with an undiagnosed medical condition, such as polycystic ovary syndrome, or it could be a lifestyle issue, such as extreme stress. After getting a negative test result, you should wait a few days to a week before taking another test. If you test negative a second time and still haven’t had your period, make plans to see your doctor immediately.