A baby’s gender is determined at the time of conception, long before more women even realize they are pregnant. However  the nub theory can tell you the sex of your baby as early as 12 weeks  if you go to a specialist in baby scanning.


Short Biology Lesson On Fertilisation!

The egg from the mother and sperm from the father carry the sex chromosomes. The egg always contributes the X chromosome and the sperm either an X or Y chromosome depending on the sperm cell. Shame that Henry VIII didn’t know this, it might have stopped him marrying 6 times to have a son!

So the sex of the fetus is determined by which sperm cell fertilizes the egg first. If it is an X chromosome, this will give a total chromosome result of XX – girl. If the sperm cell fertilizes with a Y chromosome, the total chromosome result will be XY – boy !


Fetal development follows a predictable course, measurements may differ slightly from fetus to fetus, depending on your personal genetics, but fetal structure and development continues through its pathway transforming cells to fetus.

The Nub Theory

The nub theory is the theory used to decide whether you are having a boy or a girl. Its the ‘angle of the dangle’ !!

The genital tubercle develops into either a penis or clitoris and this is possible to view from around 12 weeks by analysing the ‘nub’ in relation to the spine.  The consultant fetal medicine specialist will scan in a profile view allowing the position of the spine in relation to the nub and from this can tell you whether you are having a boy or a girl.

The male baby will have a nub that angles upwards at 30° or more. The female baby will have nub that is angled lower than 30° relative to the spine.  Your consultant can then tell you with degrees of certainty, depending on your stage in the pregnancy, which sex your baby is.  Occasionally, if viability is good, a reasonably accurate prediction can be given.

Scan Clinic

So it is important if you are wanting to know the gender of your baby, that you present yourself at Mr Penman’s fetal scanning clinic at the optimal time to achieve the highest accuracy in determining gender of your baby and hopefully get some gender scan pictures as a memory of that first moment you discovered your unborn baby’s gender. Early gender assessment is possible from 11 weeks but with less percentages of accuracy than at 14 or 15 weeks.  Mr Penman will tell you how accurate his prediction is when he scans you and can see what visibility he can get.

Visibility depends largely on the amount of body fat the mother is carrying, so the lower your BMI the better.


For a general scanning programme, please refer to the Baby Ultrasound Scan Programme section of the website.


When Can I Have My Baby Gender Ultrasound Scan?


A pregnancy gender scan cannot be carried out before 11 weeks. It is not possible to accurately scan and assess fetal gender before 11 weeks so gender assessment is not done at the Reassurance Scan (6-10 weeks).

Gender Scan and Nuchal Scan 11-13+ Weeks

From 11-13+  weeks range, the gender is assessed by the angle of the genital tubercle (nub) relative to the spine. Genitalia in boys and girls develop from the same embryonic body. Development of the external male genitalia is dependent upon the male hormones produced in the testes, which at this stage are located in the abdomen. The female external genitalia develop due to the absence of testosterone.

It is thus possible to assess the gender of your baby when you come for a Nuchal Scan between 11 and 13+ weeks if you wish. The accuracy rises rapidly from the bottom end of the 11th week (75%) to the upper end of the 13th week (95%).

At 11 weeks, typically at this gestation, the accuracy is 75%, which if you got this in an examination would be a pretty good percentage, but not when you want to know the gender of your baby! It means that on 25 occasions out of 100 scans, the gender of the baby will be wrong. So at this stage, patience is the key. The best time to have a Nuchal scan and gender assessment is between 12 ½ weeks and 13 ½ weeks.


At 12 weeks, the fetus is about 50mm (<2”) CRL (crown/ rump length – head to butt) and that is little more than the width of two 2pence coins! The nub at this gestation is about the size of a ‘pin head’! Take into consideration that Mr Penman has to scan through layers of skin, fat, womb and amniotic fluid to see this pinhead!


The fetal position and maternal obesity also play a part in allowing Mr Penman to get good images at 12 weeks to assess fetal gender.

Mr Penman’s accuracy at 12 weeks is 85% so out of 100 scans, 15 will be incorrect at 12 weeks, in the 13th week the accuracy in 95% so out of 100 scans, 5 will be incorrect.

Click Here to go to the Nuchal Scan With Early Gender Scan page

 Early Growth Scan and Gender Scan 14-18 Weeks

From 14- 16 weeks the external genitalia becomes fully formed with accuracy of detection rising from 95% at 14 weeks to 99% at 16 weeks and by 18 weeks the accuracy is 99.9%.

Mr Penman uses a combination of early and late methods of detecting the fetal gender to offer a detection of 95% at 14 weeks and 99.9% at 16 weeks.


Looking on ultrasound scan, Mr Penman is looking for two signs. One is a ‘snail’ sign, which will show him that your baby is a boy and the other is a ‘Big Mac’ sign which will indicated that your baby is a girl.  However the absence of a penis cannot be taken as evidence that the fetus is a girl. But the ‘Big Mac’ denotes the lines of the labia are a very valid and accurate indication the fetus is a girl. Click here to go to the Early Growth Scan page.


The accuracy of the scan even at this gestation is still dependent on fetal position, amniotic fluid volume and maternal obesity (BMI).

Anomaly Scan (22 weeks) and Fetal Growth/Wellbeing scan (24-42 weeks)

As the fetal external genitalia are fully formed by this stage of pregnancy the gender can be assessed with an accuracy of 99.9% and often, 3D picture can be obtained at either of these scans. Hop over to the Anomaly Scan page or Fetal Growth page for more information.


If you do not want to know the gender of your baby, please let Mr Penman know at the start of the scan. He is very good at ensuring that you will not see what gender your baby is .


But Mr Penman is one of the foremost Fetal Medicine Consultants in the UK, using his many years of fetal scanning experience in providing an accurate gender opinion.

Booking My Baby Gender Ultrasound Scan

Please remember that Mr Penman does not provide a scan just for fetal gender assessment but is willing an able to assess the gender of you baby as part of any scan done after 11 weeks. Your appointment will be with Mr Penman, Consultant Fetal Medicine Specialist.

 Ring: Spire Alexandra Hospital – (01634) 687166   or    Kent Institute of Medicine and Surgery – (01622) 237640 or go to the Baby Scan Booking Form and request an appointment there.

Length: 30 minutes


  • Full scan appropriate for the stage of pregnancy
  • Gender Assessment
  • 2D Ultrasound Black +White prints
  • 3D / 4D views of your baby
  • Still images on CD and the full scan on DVD
  • Opportunity to record your baby’s heartbeat


(3D ultrasound black+ white prints will also be included if the position of the baby is favourable, but should this not be possible; Mr Penman cannot offer a rescan)

Scan Preparation



As the scan requires Mr Penman to use a scan probe on your abdomen, two-piece clothing would be ideal.

A very full bladder is not usually necessary for this scan but try not to come with an empty bladder.

Space in the scan room is limited so please bring no more than 4 four guests with you for your scan at the Spire Alexandra Hospital.

If Mr Penman should detect something on the scans that requires further investigation, he will talk you through the next necessary steps.