It might be difficult to understand, but a small minority of people don't know they're expecting until they go into labor. Here's what causes these rare "I didn't know I was pregnant" situations.

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If you're pregnant right now, or ever have been, it's probably difficult to understand how someone could possibly not know they're expecting. After all, pregnancy comes with lots of different symptoms, and some start even before those two pink lines show up (super sore boobs, anyone?). But the fact is, there's a small number of people who manage to progress several months without ever realizing there's a baby on the way—a phenomenon known as cryptic pregnancy or stealth pregnancy.

"This type of pregnancy is a rare condition," says Daniel Boyer, M.D., a practicing doctor of medicine with a focus on medical research at the Farr Institute. "But recent scientific research shows that 1 in 475 pregnancies can classify as a cryptic pregnancy," meaning the pregnancy isn't discovered until at least 20 weeks. Even more rare, about 1 in 2,500 people don't know they're expecting until delivery.

Here's how cryptic pregnancies can happen, as well as the common causes of "I didn't know I was pregnant" situations.

How Does a Cryptic Pregnancy Happen?

Generally speaking, a pregnancy is defined as cryptic when someone goes 20 weeks or more without knowing they're expecting. Here, we've listed some factors that can make a cryptic pregnancy more likely to occur.

Lack of Pregnancy Symptoms

Oftentimes, cryptic pregnancy occurs because the person has either very mild or no symptoms of pregnancy. "A woman may not be aware she is pregnant when she has this pregnancy condition because there are no typical pregnancy signs like fatigue, nausea and vomiting, missed period, or abdominal pain," says Rebekah Diamond, M.D., a pediatric hospitalist in New York City and Assistant Professor of Pediatrics at Columbia University

Mistaken Pregnancy Symptoms

Cryptic pregnancy can also happen when someone chalks up their symptoms to something else entirely. For instance, early pregnancy fatigue could be mistaken for not sleeping well or being under too much stress. Vomiting, heartburn and bloating? Digestive issues! Missed period? Just another irregular cycle. "A pregnant person can continue to have bleeding that resembles menstrual periods, can have irregular bleeding, or can not have periods but not think of this as a sign they might be pregnant due to having underlying conditions that make their periods very irregular already," says Dr. Diamond.

Small Baby Bump

What about the unmistakable baby bump that usually appears in the middle of pregnancy? As it turns out, bumps aren't always super noticeable, which can make cryptic pregnancy more likely. "Depending on how the fetus is positioned in the womb, its growth rate, and body weight distributions, women experiencing cryptic pregnancies may not show prominent baby bumps," says Dr. Boyer. "This is especially common in younger women who are very athletic and are getting pregnant for the first time, because their abdominal muscles have never stretched before as a result of pregnancy, making them even tighter." Pregnancy bumps may also be hidden in overweight individuals.

No Positive Pregnancy Test

False negative pregnancy tests definitely aren't common, but they can happen, especially if the test was taken too early or improperly. This could certainly cause someone to have a false sense of assurance that they aren't pregnant when, in fact, they're actually expecting a baby. "The hormone that is positive in a pregnancy test (hCG) is also the hormone that causes a lot of pregnancy symptoms, including nausea," says Dr. Diamond. "So there may be a relationship with people who have false negative pregnancy tests (especially early on) and people who have few or no pregnancy symptoms, as both could be caused by low hCG levels." Further, she says, "Some pregnant people don't think to take a test at all because they do not have signs or symptoms."

An image of a negative pregnancy test on a blue background.
Credit: Getty Images.

Causes of Cryptic Pregnancy

While cryptic pregnancy can happen to anyone, it sometimes has a logical explantation. This can range from hormonal conditions to contraceptive methods to life changes. Dr. Boyer shares some possible causes of cryptic pregnancy.

Polycystic Ovarian Syndrome (PCOS): PCOS may limit your fertility, and it often causes irregular periods, weight gain, excessive oiliness of the skin, and digestive issues. Someone with PCOS may dismiss pregnancy symptoms as a complication of PCOS-induced hormonal imbalances.

Perimenopause: Pregnancy and perimenopause (a transition into menopause) have some overlapping symptoms, such as weight gain and mood changes. Women may think they're beginning a new stage of life instead of expecting a child. It's important to realize that you can get pregnant during perimenopause, even if you have an irregular cycle.

Birth Control Use: IUDs and birth control pills are very effective, but it's possible to conceive while using these contraceptive methods. If that happens, you may suffer from a false sense of security, not realizing you're actually pregnant until you're far into the gestation.

Intense Sports: Highly intensive activities and sports may also bring about hormonal fluctuations that can make it harder to detect a pregnancy. In addition, people in excellent physical condition are less likely to sport an obvious baby bump.

Thinking You Can't Get Pregnancy: If someone has an infertility diagnosis, they might not think it's possible to conceive. Similarly, women who are breastfeeding or recently gave birth might dismiss pregnancy symptoms if they didn't know ovulation returned yet.

One thing to note: In some cases, a cryptic pregnancy occurs because the pregnant person is in denial, or is experiencing a mental issue. "Some cases of cryptic pregnancy occur in the setting of mental illness (and denying that one is pregnant despite signs of pregnancy is a more prominent feature), but this is not the case by any means for most cryptic pregnancies," says Dr. Diamond.

Possible Complications of Cryptic Pregnancies

Because cryptic pregnancies progress without proper medical care, they increase the risk of complications for Mom and Baby. Fetal abnormalities and maternal health conditions (like preeclampsia or gestational diabetes) might go unnoticed. Parents-to-be might also use alcohol or drugs, which isn't considered safe while expecting. Unnoticed pregnancies have been associated with a higher risk of prematurity, low birth rate, hospitalization, unattended delivery, or death.

A new parent's mental health might also be affected by cryptic pregnancy, especially if they aren't prepared for a baby. This could lead to child abuse or neglect, disassociation from the baby, depression, and other signs of psychological distress.

The Bottom Line

While cryptic pregnancies may be hard to comprehend, they do happen on rare occasions. It's important to seek proper medical care if you experience any unexplained pregnancy-related symptom, even if you received a negative home pregnancy test (and you're sexually active). Proper diagnosis and care increases the odds of a healthy parent, baby, and pregnancy.