According to a study by INED (National Institute of Demographic Studies), 9 out of 10 couples want to know their baby’s gender before birth, mainly to pick a name and prepare for their arrival. But have you ever heard of the nub theory? The name might sound intriguing, but it’s all about early development, growth… and whether you’re having a boy or a girl.
Today, we’re diving into the first prenatal ultrasound—the one where the guessing game begins! So, boy or girl? It all starts with a tiny nub.
Table of Contents:
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The First-Trimester Ultrasound: The Moment You’ve Been Waiting For
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The Nub Theory: What Is It?
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Using the Nub Theory to Predict Baby’s Gender
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The Anatomy Scan: The Ultimate Confirmation
The First-Trimester Ultrasound: The Moment You’ve Been Waiting For
Scheduled around 12 weeks of pregnancy, the first-trimester ultrasound—also known as the dating scan—is often the very first time you meet your baby. At this point, your belly might just be starting to round out, and maybe you haven’t even shared the big news yet. But in that moment, parenthood suddenly feels very real. For the first time, you see your baby move. For the first time, you hear their heartbeat. And for the millionth time, you ask yourself:
“Is it a boy or a girl?”
Beyond this big question, the main purpose of the first ultrasound is to check that everything is progressing smoothly. Using imaging technology, the sonographer looks for the gestational sac inside the uterus and confirms the presence of an embryo. If no embryo is detected, it could be a blighted ovum. If there’s no fetal heartbeat, it may indicate a missed miscarriage. And if there are multiple embryos, congratulations—it’s twins (or more)!
This scan also gives you an estimated due date (EDD) based on the baby’s crown-rump length (CRL). Additionally, your healthcare provider will check for nuchal translucency (fluid at the back of the baby’s neck) to assess the risk of Down syndrome. If the space appears larger than usual, it may indicate a higher risk, which can also increase with maternal age or genetic history.
Finally, if you’re eager to know your baby’s gender and you specifically ask, your provider might discuss something called the genital tubercle, aka the nub.
The Nub Theory: What Is It?
The nub theory revolves around a small structure called the genital tubercle, which will eventually develop into a penis (for boys) or a clitoris (for girls).
At the moment of conception, the baby’s gender is already determined—XX for a girl, XY for a boy—but physical differences don’t start showing until around 5 weeks gestation (7 weeks since your last period). Around 9 weeks, those differences start becoming more pronounced.
By 12 weeks, when you go in for your first ultrasound, a boy’s penis will start taking shape, and his tiny testes will already be producing testosterone. A girl’s reproductive system (uterus, ovaries, and fallopian tubes) will be forming inside her body.
It’s at this stage that an experienced sonographer can start making an educated guess based on the angle of the nub.
Using the Nub Theory to Predict Baby’s Gender
Here’s how the nub theory works:
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Use a clear, side-profile ultrasound image of your baby taken at the 12-week scan.
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Locate the genital tubercle (nub)—it’s positioned between the legs and the lower spine.
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Observe the angle of the nub compared to the baby’s spine:
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If the nub is parallel to the spine (or angles slightly downward), it’s likely a girl.
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If the nub is angled upward (30 degrees or more), it’s probably a boy.
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Of course, if the image is blurry, baby is in the wrong position, or they move during the scan, the guessing game is over! You’ll have to wait for your 20-week anatomy scan for a more reliable answer.
Before 22 Weeks: Gender Predictions Are Never 100%
Under ideal conditions—with the right baby position and a clear ultrasound image—the nub theory is considered a fairly reliable (though never foolproof) way to predict baby’s gender.
That being said, nothing is guaranteed! We’ve all heard stories of parents expecting a baby boy, only to be surprised with a little girl—or vice versa. While the 20-week anatomy scan typically provides the most accurate gender reveal, some parents only get a final answer when they hold their baby in their arms.
The Anatomy Scan: The Ultimate Confirmation
The second-trimester anatomy scan is a mandatory ultrasound, performed between 21 and 24 weeks. It’s a much longer scan that takes a detailed look at baby’s development. By this point, your little one is about 10 inches long and weighs roughly 1 pound.
While the dating scan mainly checks for viability, the anatomy scan focuses on detecting any potential abnormalities. The ultrasound technician examines:
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The heart, brain, stomach, bladder, intestines
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The arms, legs, and bones
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Measurements like biparietal diameter (head width), abdominal circumference, and femur length
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The placenta and amniotic fluid levels
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The cervix
Once everything is checked, your doctor or sonographer will typically be able to confirm baby’s gender.
Important: If you don’t want to know your baby’s gender, make sure to let your provider know before the scan—and remind them during the exam! Some ultrasound techs are so used to excited parents that they might blurt it out before you even have a chance to stop them.
To Reveal or Not to Reveal?
Some parents go all out—using the nub theory, multiple ultrasounds, 3D scans, and even extra tests—just to find out baby’s gender as soon as possible.
Others, after failed nub theory attempts or a shy baby during the anatomy scan, decide to wait until birth and enjoy the surprise.
Some couples feel confident in their nub theory guess but still choose to wait for labor day to get the official answer
And then, there are parents who, whether due to superstition, personal beliefs, or simply having one of each already, opt to keep the mystery alive until delivery. No matter what, the choice is entirely up to you.
What About Old Wives’ Tales?
According to old-school wisdom, pregnancy symptoms can supposedly reveal whether you’re having a boy or a girl. Sound familiar?
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More morning sickness? You’re having a girl. (Fun fact: A 2020 study actually found that moms carrying girls had higher levels of hCG, the hormone linked to nausea.)
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Craving sweets? It’s a girl. Prefer salty foods? Boy.
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Carrying high and pointy? It’s a boy. Low and wide? Girl.
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Try the pendulum test! Swinging in circles? Girl. Front to back? Boy.
Ever tested one of these myths? Did your nub theory guess turn out to be right? Tell us your story!
