What pregnant woman hasn't heard the line, "You're carrying low. You must be having a boy." Or: "If you have morning sickness all day, it's definitely a girl."
Even stranger myths for predicting the baby's sex exist. One suggests that mommies-to-be hang their wedding ring from a strand of the father's hair over their belly. Another advises them to mix their urine with Drano; the color is supposedly a clue about the baby's sex.
Now that medical technology makes it possible to determine an unborn baby's sex with almost total certainty, why do these old wives' tales persist?
The Internet is at least partly to blame for the ongoing pregnancy rumor mill. Social media sites like Facebook and Twitter have turned anyone with a keyboard into a pregnancy "expert." Myths fly through cyberspace, bouncing from one email inbox to another at warp speed.
People reading those random comments can wrongly take them for medical facts. "Sometimes people take what's said online at higher value than what their physician says," says Sharon Mass, MD, a practicing ob/gyn in Morristown, N.J. and clinical assistant professor at the University of Medicine & Dentistry of New Jersey.
Believe It or Not?
Even though most wives' tales about guessing the baby's sex are harmless, "My medical caution to my patients is: Please make sure you know the source of what you're paying attention to or sharing," Mass says.
New moms -- especially those who are pregnant for the first time -- can be especially susceptible to pregnancy rumors, says Eileen Beard, CNM, FNP, MS, senior practice advisor for the American College of Nurse-Midwives. "You're so concerned about doing the right thing and getting all the information. I think it makes you very vulnerable in a way. Even people who wouldn't normally believe myths are more likely to."
Another reason why sex prediction myths persist is that sometimes they can appear to be right. When you're looking at 50/50 odds, predictions are bound to come true half the time. And surprisingly, at least a couple of these methods do have some evidence to back them up.
Here's what experts told WebMD about a few of the most rampant baby sex prediction rumors.
Pregnancy Myth #1: Carrying Low
Myth: If your belly hangs low (or in front), you're having a boy. If it's high (or wide in the middle), you're having a girl.
Reality: This one is pure myth. "How you carry simply has to do with the tone of your muscles and the position that the baby is in," Beard says. These factors, along with your body shape and how much weight you gain during pregnancy -- not the baby's gender -- will determine how low or high your belly sits.
Pregnancy Myth #2: Rhythm of the Heart
Myth: If the baby's heart beats faster than 140 beats per minute, it's a girl.
Reality: This is a myth Mass says her patients ask her about on a daily basis, and there might actually be a wee bit of truth to it.
A 2006 study showed no gender-related differences in fetal heart rate during the first trimester, but Mass says that's no surprise, considering that babies' hearts beat faster in general during the first 28 to 30 weeks of pregnancy. It's later in the pregnancy when the difference becomes apparent. A 1999 study showed that just before delivery, a female baby's heart does beat faster than a male's. Mass says she sees a similar trend in her own patients.
Pregnancy Myth #3: Swinging on a Hair
Myth: Hang your wedding ring from a strand of the father's hair over your belly. If the ring swings around in circles, it's a girl. If it sways back and forth, it's a boy. An alternate version of this myth recommends dangling a pin over the mother's wrist.
Reality: There's no real evidence to confirm or deny this one. Mass doesn't see any scientific basis for it, but she says people who follow traditional Chinese medicine might explain the dangling ring (or pin) as evidence of the body's natural forces at work.
Pregnancy Myth #4: The Drano Test
Myth: Stir some Drano into your urine. If the mixture turns green, it's a boy. Other color changes have been proposed for this myth, but green is one of the most common.
Reality: Mass doesn't know of any medical reason why a Drano-urine mixture could predict a baby's sex. "There's no change in the acidity or alkalinity of a boy or girl chromosome," she says.
The few studies that have been done on the subject also refute the claim. In the early 1980s, researchers at the University of Wyoming performed the test on 100 pregnant women and found it to be "roughly equivalent to flipping a coin" for predicting gender. A 1999 Canadian study yielded similar results.
Even if the technique did work, Drano is a caustic chemical -- not something you want to be playing around with or breathing in while you're pregnant, Beard says.
Pregnancy Myth #5: Sweet Tooth
Myth: A pregnant woman who craves sweets is having a boy. If she craves sour foods, she's having a girl.
Reality: Your baby boy might grow up to have a sweet tooth, but while he's in your womb he's not going to make you desperate for an ice cream cone or candy bar. If you're craving sweets (or any other food), it's probably because your shifting hormones have intensified your sense of smell.
Pregnancy Myth #6: Sick to Your Stomach
Myth: If you have morning sickness all day, it's a girl.
Reality: This myth might have some truth to it. Studies have found that women with a severe form of morning sickness called hyperemesis gravidarum are more likely to give birth to girls.
The reason? Levels of the pregnancy hormone hCG, which triggers morning sickness, tend to be higher in mothers who are pregnant with female babies.
But a pregnant woman can certainly have morning sickness, even bad morning sickness, when she's carrying a boy. So no, you can't count on it being a girl if you've got serious morning sickness.
Pregnancy Myth #7: Look at the Calendar
Myth: The Chinese Lunar Calendar can predict a baby's gender based on the mother's age when she conceived, and the month of conception.
Reality: The Chinese Lunar Calendar was discovered in a 700-year-old Royal tomb, and many pregnant women who've used it swear by it. Could this ancient artifact have some modern science behind it? Not necessarily. According to the same Canadian researchers who did the Drano test, the Lunar Calendar is no more accurate at predicting a baby's gender (50/50) than a random guess.
Learning Your Baby's Gender, For Real
One accurate way to predict whether you're having a boy or girl is to have an ultrasound, which is usually done between weeks 18-20 of pregnancy. Provided that your ultrasound technician gets a good view between your baby's legs, the imaging procedure should be able to tell you with about 80% to 90% accuracy whether you'll need to paint the nursery pink or blue.
Amniocentesis and chorionic villus sampling (CVS) also can determine your baby's sex with a high degree of accuracy, but these more invasive tests are usually reserved for situations in which the baby may have a genetic disorder or chromosomal abnormality, such as Down syndrome.
DNA tests of the mother's blood can also accurately detect the baby's sex, but because of their high cost, these tests are only used in specialized laboratories, not commercially.