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  • Nub Theory Explained

    What is a Nub? And what the heck is this Nub Theory?


    A nub is a small protuberance seen between the legs at as early as 9 weeks.  Both boys and girls start with a flat ambiguous nub (which is why most boys can be inaccurately guessed as girls during the 11th gestational week) 


    It can be used to assess gender using the angle of the nub between the 12th and 14th week.  30 degrees and up=Boy, Under that=Girl.


    The nub is a fact.  All babies have a nub and how they develop is also science and fact.  It is how we try and decipher the ultrasound image of the nub to correlate with this fact is what equals the nub theory.


    Sounds easy peasy, right? Most nubs in ultrasound images are not as cut and dry as that unfortunately, but that is where we can help.   Used with a perfect mid line image of baby the nub usually can image pretty perfect...but perfect is rare.  Some nubs are tricky -they are angled in "the grey zone" where it can go either way and that is usually due to the gestation of baby. 


    This "grey zone" is usually seen between 12w0d - 12w3d.  Also, since we are dealing with "parts" that are so tiny at this stage; if baby has their little bottoms turned even just a hair left or right, the sound waves can completely not reflect off certain parts of the nub. There are 2 parts of the nub theorists look for.  The underside (pubic bone region) and the end.  The end is usually the absolutely most important part of the nub.  This is the part that will be rising (developing penis) giving the nub an over 30 degree angle for boys as the scrotum starts to develop (the underside/pubic region) In girls, developing clitoris will stay flush against baby's bottom, even usually looking to be overhanging.  As such, staying usually flush against the pubic region and staying under the 30 degrees. 


    Gender guessing with the nub increases accuracy as you hit 13 wks, and is at its very lowest of accuracy in the 11th week. 


    Link to published medical study establishing the Nub Theory 1999


    Quick overview of other topics and help we can give in determining sex of your baby:


    My Nub has a Fork?

    A forked nub can be of either gender. Since ultrasound images in lines, especially anything round.


    Is 3D or 4D better for assessing Gender?

    No. And absolutely No. It adds lumps and bumps especially in early id'ing as baby has not much fat to them as yet.  A trained U/S will not attempt to id this early in 3D; only in 2D. If you have an elective scan and the tech says they can, please insist on 2D confirmation.


    Can you get 3 Lines on a Boy?

    Yes! Absolutely! If the tech takes the picture from too far back (ie behind the scrotum, near the bum), you can get 3 lines. Again, Many and most things will image as lines in ultrasound.


    Potty Shots for Gender?

    If you imagine yourself sitting on a copier machine and hit COPY (same position as you would take as sitting on the potty) what would you see?  Labia on girls, and scrotum/penis for boys.


    You can also identify and confirm gender in side profile shots well up until the 20th week as well.  If you have a tech that can confirm this way, please be rest assured the tech absolutely knows what they are doing!

    One quick explanation to have a better understanding of this is to imagine a loaf of sliced bread.  Ultrasound images in planes. Each slice of bread is a "slice" of image.


    Below is a very detailed synopsis of the developing genitals and how they correlate with what we try and help to do:



    "A scrotal bump kind of looks like a "stacked" nub with one part on top of another.  
    Later this will become the penis and scrotum.
    Scrotal bump, ie the beginnings of the scrotum, under the rising nub 
    We see a lot of pics with a bright white line that looks like a short girl nub turn out to be a boy. 
    It's either part of the groin or the folds on the side of the nub that are developing into a scrotum 
    (the aforementioned scrotal bump). 
    You can get that look easily if the whole nub isn't captured in the image. 
    The scrotal bump starts forming about the time that the nub starts rising. All fetuses start out with a flat genital 
    tubercle (i.e. the nub)  and labioscrotal folds. 
    If the baby is a boy, those folds will begin to fuse to form a scrotum around the same time the nub starts rising. 
    But that process doesn't happen overnight, so that "in-between" structure is referred to as a scrotal bump.
    When people here mention a bump on a nub, though, they're not usually talking about an actual scrotal bump. 
    Often boy nubs image as a straight white line with a little dot on top that represents the nub tip. That dot is most 
    commonly referred to as a bump on the nub because . . . 
    well, that's exactly what it looks like.  But "bump" is just a general descriptor in this case, not a technical term.
    The forked nubs can be tricky as it's all in how the image is captured (this is when video is most helpful) 
    but in general the fork on a girl nub is smoother and may even angle downward. With a boy nub is is a bit bulkier 
    and there is normally an upward tilt away from the body, rather than staying flush against it. I hope that makes sense. 
    Single still images can also be deceiving at times which doesn't help.
    We've seen many, many pictures like this with a flat white line but no visible tip to the nub that turn out to be boys. If the shot is just a tad off-center, 
    the most important bit can get cut off.
    I'm going to try to give a condensed version of nub theory, potty shots, and the "gray area" in between. 
    Nub theory has only been formally studied between 10-14 weeks (and only found to be of much accuracy between 12-14), 
    which is why the only hard numbers and statistics we have come from that age range. Because of that, 
    the 30-degree rule does not truly apply after 14 weeks. In fact, it's my (informal) observation that the 
    30-degree mark is most useful in the 12th week - once you get much beyond the 13-week point, 
    we typically see a much more obvious angle in boys, along with a scrotal bump. 
    Even a nub that's right on the 30-degree line in a 13.5- or 14-week baby seems to become a girl much more often 
    than a boy. Once you get to 16+ weeks, most babies have developed enough to get a clear potty shot showing either 
    a penis and scrotum, or labia and a clitoris.
    But obviously, that doesn't cover the period between 14-16 weeks. 
    For that we have to fill in the gaps with what we know about fetal development and with anecdotal observations. 
    The genitalia are differentiated by 14 weeks, but that doesn't mean that they suddenly go from looking very similar 
    to looking as well-developed and discrete as an older baby's as soon as you click over to 14w1d. 
    In a girl, the nub still has a long way to shrink down to the size of a normal clitoris, 
    and the outer labia have plenty of growing to do. For that reason, you will probably still see the clitoris 
    quite prominently, sometimes as late as 20+ weeks (since all babies develop at different rates). 
    What you'll not always see is 3, 4, or 5 lines, because that would imply more mature female genitalia.
    In a boy, the acute angle of the penis is not only still present, but tends to increase for a time, 
    often reaching 90+ degrees and staying that way through the 20-week mark. 
    A 30-degree angle is void at this time. There's no empirical research that I've seen to support the idea that a mere 
    30-degree angle is enough to indicate a boy after 14 weeks, nor do anecdotal observations support it. 
    Also, the absence of a scrotum on both the mid-sagittal shots and the toilet views would indicate that the baby 
    doesn't possess the requisite organs to be labeled a boy. Clitoris will be seen in mid-saggittal shots well up until the 20th week."
    -Credited to ColdWater from IG, long time friend and nub mentor


    Example boy nubs. Keep in mind these are perfect nub shots




    Great "stacked" nub

    one on top of the other (developing penis on top, scrotal line underneath)




    Girl nubs

    Again, perfect nub images











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