Nub Theory Predictor:
Boy or Girl?

Upload your 12 to 14 week ultrasound image and predict your baby's gender using the nub theory

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Simply upload your ultrasound image and follow our step-by-step guide to accurately draw the nub lines.
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Our nub theory gender prediction test is a simple do-it-yourself tool, accessible to everyone.

Nub Theory Explained: Your Top Questions Answered

The nub theory is a method used to predict a baby's gender based on the angle of the genital tubercle (or 'nub') in relation to the baby's spine in early ultrasound scans, typically between 11-14 weeks of pregnancy.

At 10 weeks, it's too soon to use the nub theory to determine your baby's gender. At this point in time, the genital tubercle, or the 'nub', isn't formed enough yet to provide the stark angle differences required to tell whether the baby is a boy or a girl. The nub looks very much the same in both sexes, so predictions are speculative at best. For the nub theory to apply, it's advisable to wait until at least 12 to 14 weeks, when the nub is more well-defined. So guessing at 10 weeks might be fun, but it's not a case for accurate results. If you are chomping at the bit to find out your baby's gender, then patience and waiting — potentially an earlier ultrasound would clarify, clearer info.

Yes, this website offers the nub theory predictor free of charge. It's a do-it-yourself tool that will guide you through the process entirely free. However, keep in mind that these predictions are for entertainment purposes and should not be considered as medical advice.

The accuracy of the nub theory varies, with some studies suggesting it can be up to 90% accurate when performed by experienced technicians. However, the accuracy greatly depends on how many weeks pregnant you are. Studies show that if you are below 12 weeks, the accuracy is just above 50%. In the 12th week, it rises to 75-79%, and at 13 weeks or more, it approaches nearly 100%. Therefore, the further along you are, but still under 14 weeks, the better the prediction. It's important to note that this method should be considered for fun rather than as a medical diagnosis.

If you’re curious about what boy and girl nubs look like, check out our detailed article, Nub Theory Examples: Early Gender Prediction. It’s packed with ultrasound pictures showing clear girl and boy examples of nubs angled upward (over 30° for boys) and parallel or downward for girls, plus tips on spotting the differences yourself.

The nub theory is typically applied during early pregnancy ultrasounds, usually between 11 and 14 weeks of gestation. This is when the genital tubercle is most visible and before it has clearly differentiated into male or female genitalia.

According to the nub theory, if the angle of the nub to the spine is greater than 30 degrees (pointing upwards), it suggests a boy. If the angle is less than 30 degrees (more parallel to the spine), it suggests a girl. This simple distinction forms the basis of the nub theory's gender prediction method.

No, the nub theory should not replace medical gender determination methods. It's considered an informal method and is not as reliable as later ultrasounds or genetic testing. Always consult with your healthcare provider for accurate medical information.

Yes, several factors can affect accuracy, including the quality of the ultrasound image, the baby's position, the skill of the technician, and the exact gestational age. The theory is also less reliable before 11 weeks or after 14 weeks of pregnancy.

While some medical professionals may be aware of the nub theory, it is not widely recognized or used as a clinical method for gender determination. Most healthcare providers rely on more established and accurate methods for determining fetal sex.

The nub theory is considered more reliable than old wives' tales or myths about gender prediction. However, it is less accurate than medical methods like Non-Invasive Prenatal Testing (NIPT) or later ultrasounds that can directly visualize fetal genitalia.

No, you cannot always apply the nub theory. The nub is not always visible in ultrasound images, and in such cases, it is impossible to use the nub theory for gender prediction.

To measure the nub angle, look for a clear side-profile ultrasound image between 11-14 weeks. Identify the baby's spine as a straight line, then find the genital tubercle (nub) protruding from the torso. Draw a line along the spine and another along the nub's edge. The angle between these lines determines the prediction: over 30° suggests a boy, under 30° suggests a girl. Tools like NubCheck.com can help you draw and calculate this angle accurately.

Yes, nub theory can be wrong, especially if the ultrasound is taken before 11 weeks, after 14 weeks, or if the image is unclear. Studies show accuracy improves from 54% at 11 weeks to 91% at 13 weeks when the nub is visible, but factors like fetal position or technician error can lead to mistakes. It's a fun prediction tool, not a medical guarantee—later scans or NIPT are more reliable.

Nub Theory Week by Week: How it Affects Accuracy

The nub theory's accuracy varies at different stages of pregnancy. Below you'll find a detailed breakdown of what to expect and how accurate predictions can be at each week.

Nub Theory Accuracy by Week

100% 75% 50% 25% 0%
10w
11w
12w
13w
14w
Too early
Most accurate
*Based on clinical studies. Nub Theory Research

Nub Theory at 10 Weeks

At 10 weeks, it is too early to make any accurate predictions using the nub theory. The fetal nub is just starting to form and is extremely difficult to detect on an ultrasound. The nub is very small, and the genital tubercle hasn't developed enough to show any meaningful angles. Medical professionals agree that 10 weeks is always too early for nub theory predictions - any gender guesses at this stage should be considered purely for entertainment and not taken seriously.

Nub Theory at 11 Weeks

At 11 weeks, the nub is just beginning to appear, which makes predictions quite unreliable. In a study of 172 cases, 56% of male fetuses were misclassified as female, while only about 5% of female fetuses were misidentified. Remember, if you are expecting a boy, nub theory at this stage is especially unreliable — there is nearly a 50% chance of an incorrect prediction because the nub is not fully formed. Accuracy improves significantly by 12 to 13 weeks.

Nub Theory at 12 Weeks

The 12-week mark brings more promising results. By now, the genital tubercle is more distinct, and differences in alignment between male and female nubs begin to emerge. Research shows nub theory accuracy at 12 weeks could range from 70–80%, provided the ultrasound image is clear, and the baby is in a favorable position. Consulting someone experienced in nub theory can improve confidence in predictions. At this stage, nub theory offers moderate reliability for gender guesses.

Nub Theory at 13 Weeks

Nub theory accuracy improves significantly by 13 weeks. The angles of the nub are more pronounced, making it easier for experienced interpreters to distinguish between male and female. While factors like fetal position and ultrasound quality still play a role, predictions at this point can reach an accuracy of 80–90%. If you're excited to learn your baby's gender, 13 weeks is a great time to explore nub theory, but remember this method should only be used for entertainment purposes and is never a guarantee of your baby's gender.

Nub Theory at 14 Weeks

By 14 weeks, nub theory reaches its peak accuracy in the early stages of pregnancy. The genital tubercle is well-developed, and the angles—upward for boys and downward for girls—are much easier to identify. Nub theory accuracy at 14 weeks is the highest, often exceeding 90% when the ultrasound is clear, and the baby is in the right position. However, while it offers compelling insights, it's always wise to confirm your guess with the anatomy scan at 18–20 weeks for complete certainty.

Knowledge Base

Explore our in-depth articles on the nub theory and early gender prediction.