10 Reasons You Might Not Know You're Pregnant (Until You're in Labor)

If a woman suspects she's pregnant, she might first take a home pregnancy test. These tests measure the level of a hormone called human chorionic gonadotropin (hCG) in urine. Elevated levels of this hormone suggest a woman is pregnant.

But home pregnancy tests can give false readings if used improperly, read wrong or taken too early. Women who don't know they're pregnant may have taken a pregnancy test and thought their results were negative when they were actually positive. This is also called a false negative pregnancy test. The tests themselves can also be confusing, so keep the instructions nearby just in case. Although these tests claim to be 99 percent accurate, they don't always pick up on all women's hCG levels in the first days of pregnancy.

Health experts suggest waiting at least one week after a missed period to take a home pregnancy test. Even then, it might be a good idea to use more than one test for reassurance.

Most medications don't influence the accuracy of pregnancy tests, but in some cases, fertility treatments that include hCG can produce a false positive test, or one that reads pregnant when the woman isn't [source: WomensHealth.gov].

Is it possible for a woman to not pack on pounds during pregnancy? Read on to find out.

A large, protruding belly and maternity clothes usually hint to others that a woman is pregnant. But not all women show motherhood this way.

Since each person carries weight differently, some women look quite large around the midsection during pregnancy whereas others barely show signs at all. A mother-to-be's body weight before becoming pregnant can determine whether she and others notice.

Generally speaking, body weight can affect how women show their pregnancies in two ways.

First, the extra pounds associated with pregnancy may not be as noticeable for women who are already overweight. Excess body fat, especially around the stomach area, can help hide the presence of a baby -- even from the mother. It's also true that not all pregnant women carry their unborn babies similarly, which influences how large and rotund a woman's belly may appear.

Second, women who start a diet or begin an exercising regimen soon after becoming pregnant may not gain a noticeable amount of weight. The weight they lose might negate the pregnancy weight they gain. Since healthy pregnancies require the mother to put on weight and eat a variety of healthy foods, it's important to not overlook any weight gain if you suspect -- even slightly -- that you might be pregnant.

Looking back, women who didn't know they were pregnant recall falsely blaming stress or poor eating habits for packing on a few extra pounds.

There's an entire list of side effects women experience during pregnancy. How can a woman miss these signs? Head over to the next page to learn about other side effects of pregnancy.

Women who don't know they're pregnant until they rush to the delivery room rarely recall hard-hitting side effects like fatigue, nausea (also known as "morning sickness"), tender breasts, headaches, food cravings, back pain, soreness and weight gain.

Since the raging hormones related to pregnancy affect different women in different ways, it's not surprising that some experience different pains and sensations. Women who don't know they're pregnant until they're in labor may confuse the side effects of pregnancy with other problems, including stress, food poisoning or indigestion.

Next, let's look at whether menstrual cycles influence whether a woman knows she's with child.

Perhaps one the earliest signs of pregnancy for women is the cessation of menstrual cycles, or the process through which a woman sheds the inner lining of her uterus.

Menstruation typically stops once a woman becomes pregnant because a fertilized egg -- at this point called a blastocyst -- has implanted itself on to the uterine wall. Keeping these layers allow for the blastocyst to establish itself and develop into a baby.

Occasionally, pregnant women continue to experience periodlike bleeding, which deceives them into thinking they're not pregnant. These false menstrual cycles are rare, and doctors are working to figure out what causes them in some women but not in others.

In other cases, women who don't know they're pregnant until they enter the delivery room may have a history of irregular menstruation cycles; for them, missing a period or randomly experiencing spotting could be the norm. Because of this, pregnant women who believe they're menstruating may still take oral contraceptives to prevent pregnancy.

Read about the role of stress in denied pregnancies on the following page.

Stress can negatively affect a woman's attitude toward pregnancy. Immense pressure and distress can push even the healthiest of women to deny the reality of pregnancy.

Research has shown that stress can induce denied pregnancies in women who have no history of psychological problems [source: Brezinka et al.]. Among women who had no idea they were pregnant, many say stress partly distracted them from the realities of pregnancy -- sometimes until labor. Often, these women think chronic stress is responsible for symptoms such as weight gain, headaches, digestive issues and bodily pains.

Doctors also know that stress can affect the regularity of a woman's menstrual cycle. In this sense, a woman who misses a period because she's pregnant may falsely attribute her irregularity to stress and other upsetting factors in her life.

Fertility matters. Read how on the next page.

Fertility issues can also influence whether a woman knows she's pregnant.

Infertility can be a painful reality for couples that want children. But the ability to get pregnant may change over time. Women who are told that they or their partners are infertile may not consider pregnancy an option. Because of this, they are less likely to notice the first signs of pregnancy.

On the other hand, some couples want to eliminate the chance of becoming pregnant through surgical procedures such as vasectomies and tubal ligation. In rare cases, a man's vasectomy may not be entirely successful. For one, it may take longer than the couple realizes for the remaining live sperm to be discharged from the body after surgery. Also, if the man's two severed sperm ducts reconnect while healing, he can still produce live sperm [source: Haldar et al.]. Assuming their partners can no longer produce live sperm during intercourse, women may not use other contraceptive methods, which could lead to an unplanned pregnancy.

Even thinking you've already experienced menopause can get in the way of realizing you're pregnant. Both pregnancy and menopause have powerful hormonal impacts on the body, and sometimes, the two conditions share similar symptoms.

Where does mental health fit in? We'll explore this question next.

Women who don't know they're pregnant until they're in labor are sometimes accused of being mentally unstable. It's important to know that some -- not all -- women who experience denied pregnancy have underlying mental health problems that may get in the way of realizing they're pregnant.

Studies examining the mental health of women who didn't know they were pregnant reveal that some have been diagnosed with psychological conditions, including schizophrenia, depression and a range of personality disorders [source: Brezinka et al.]. Other women, meanwhile, felt their confusion was simply due to stress.

Scientists also know that denial, or ruling out pregnancy altogether, can play a significant role in denied pregnancy. Up next: the role of the baby.

Baby kicks are quite common for most pregnant women, but not for all.

Whether the baby rests in such a way that makes its movements hard to detect or it's simply less active than others babies, movement in the womb -- or lack thereof -- can shape a woman's perception of her pregnancy.

Women who didn't know they were pregnant sometimes put the pieces together in retrospect, noting instances when they confused their babies' movements with indigestion or an upset stomach.

In addition, the position of the baby matters as well. Sometimes, the baby may rest in a breeching position with its feet fixed toward the bottom of the womb. Since natural birth favors the baby exiting the mother's womb headfirst, doctors often try to reposition a breeched baby during labor.

Some women claim they became pregnant even after using contraceptives. Find out more next.

Women who don't know they're pregnant until they're in labor are often in shock over their situation. How can this be if they used a contraceptive while having sex?

Although contraceptives such as birth control pills and condoms are highly effective in preventing pregnancy, they aren't 100 percent successful. When contraceptives aren't used correctly, their effectiveness drops significantly. For example, missing a birth control pill or not taking it at the same time every day can increase your chances of getting pregnant if you have unprotected sex. Using expired condoms that are more likely to break can also increase this risk.

You should consult your doctor if you're unsure of whether switching birth control methods compromises your protection against pregnancy. Some women have claimed that switching from oral contraceptives to injected forms left them susceptible to an unexpected pregnancy.

Mistaking pregnancy with a medical condition is possible, too. Read on to learn more.

In some circumstances, a woman might not know she's pregnant because she believes her pregnancy symptoms are caused by some other health problem. Women with a history of ovarian complications such as tumors or cysts may attribute discomfort or pain to their previous condition.

Since not all women have medical insurance, this sometimes makes them reluctant to seek medical advice if they believe they have a health problem. Other women think their symptoms will go away, so they often skip out on seeing their local doctor.

Past health problems or not, a visit to the doctor can put your suspicions to rest or lead you to the treatment you need.

To learn more about pregnancy, browse the resources on the next page.