Nub Theory Predictor:
Boy or Girl?
Load your 12 to 14 week ultrasound image and predict your baby's gender using the nub theory: free, fun, and 100% in your browser. Your photo never leaves your device.
No scan at hand?
How does the nub theory test work?
- Load your ultrasound photo. Pick a clear side-profile scan from 12 to 14 weeks of pregnancy. Everything happens on your own phone or computer, and nothing is uploaded.
- Draw a line along the spine. This blue line is the reference line for the measurement.
- Draw a line along the nub. The nub is the small protrusion near the baby's pelvic area. Our step-by-step guide and nub quiz help you find it.
- Get your prediction. The tool instantly measures the angle between the two lines. More than 30 degrees suggests a boy, less than 30 degrees suggests a girl.
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.
*Based on clinical studies.
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.
- Easy Upload & Guidance
- Simply load your ultrasound image and follow our step-by-step guide to accurately draw the nub lines.
- Instant Gender Prediction
- Receive an immediate boy or girl prediction based on the nub theory analysis of your ultrasound.
- Completely Free
- Our nub theory gender prediction test is a simple do-it-yourself tool, accessible to everyone.
- 100% Private
- Everything runs in your browser. Your ultrasound photo is never uploaded, stored, or shared. It never leaves your device.
Knowledge Base
Explore our in-depth articles on the nub theory and early gender prediction.
Nub Theory Examples: Early Gender Prediction
Dive into the world of nub theory with our comprehensive guide featuring real-life examples and explore how this early gender prediction method works.
Read the full guideNub Theory Explained: Your Top Questions Answered
What is the nub theory?
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.
Is the nub theory predictor free to use?
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.
How accurate is the nub theory?
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.
At what stage of pregnancy can the nub theory be applied?
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.
What angle suggests a boy or a girl according to the nub theory?
Generally, 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. However, these are not definitive indicators.
Can the nub theory replace medical gender determination methods?
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.
Are there factors that can affect the accuracy of the nub theory?
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.
Is the nub theory recognized by medical professionals?
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.
How does the nub theory compare to other early gender prediction methods?
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.
Can you always apply the nub theory?
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.
Is my ultrasound photo uploaded or stored anywhere?
No. This tool runs entirely in your browser. Your ultrasound image is never uploaded to a server, never stored, and never leaves your device. You can even use the tool offline once the page has loaded.