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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.


    Sounds easy peasy, right? Most nubs 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 gender 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 inside a toilet and looking up. Someone comes and sits on the toilet, whilst you stare upwards. What do 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.


    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











    Post Your Ultrasound Images Here!

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