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Special FAQ on the gender of the fetus
And just how accurate is the diagnosis of fetal gender by ultrasound? Well, I have to say that it
should be very accurate (somewhere between 95 and 100% at 20 weeks), particularly after
about 20 weeks, or even sixteen. However I must say again that one must remember the sonologist may not always get a good view of the perineum (private parts) of the fetus for a variety of reasons and therefore may not furnish you with an "accurate" answer. One really needs to put this question to the person who performs the scan. He or she is the only person to be in the position to tell you just how accurate "that" particular diagnosis is at that particular scan.
Over the years the number one question that I have been asked was on the diagnosis of the gender (sex) of the fetus.
Although this is quite unbelievable, so many parents are so anxious about the diagnosis of fetal sex and it's accuracy. I have therefore put up this extra faq page to address this subject.
The following are some of the typical questions I receive from anxious parents, they are genuine and un-edited:
"I just went in and had an ultrasound at 11 weeks. We were looking for the
sex of the baby, among other things. It would appear we saw a penis, but
because it is so early...should I take this as fact?........"
"Hi I was curious as to how accurate are u/s at 20 wks for finding out the sex
of baby? I had one done at 20 wks and the tech told me it was a girl, and then she
showed me the labia which was two white lines between the legs. Should I
feel confident that this is a girl?....."
"Hi I wanted to know what is the percentage rate of a ultrasound. I
also wanted o know if the technician sees the testicles is it safe to
say it is a boy. I wanted to know a accurate way you can tell it is a
boy definite. thanks ..........."
"I am 11 weeks pregnant. When will my doctor be able to tell me what the sex
of my child is? I have a 2 year old son. Thank You........"
"If an ultrasound tech cannot see one or the other (penis/scrotum or
labia) Shouldn't they tell the patient they could not tell? My tech
always made sure that she seen the specific part before telling me, I
also ask to see it also. I had a scan at 15 weeks & was unable to
tell at the time but I will be having a high resolution sometime
within the next 2 weeks because I was born with a cleft palate & they
just want to check the baby. (My other 3 children were fine)............"
"I am now 18 weeks pregnant. At my next check-up, I should probably be
told the sex of the baby we are expecting. However, I prefer not to be
told! For our first child, I was told that it was going to be a baby boy
& that too in the 30th week scan! Anyway, I was a little disappointed
when our little girl arrived & surprised us! We love her very much
though!................"
"I fear being mislead this time around too. All that I need to know is
that when exactly can the doctor really 'see' what the sex of the child
is & if so how accurate or convincing can her conclusion be. To a
commoner like me, it is quite difficult to really make out by myself
what comes on the screen during the scan! & is it true that it is easier
to tell if it is a baby boy than if it is a girl?................"
"I had an ultrasound at 23 weeks and the technician said that she was 80% sure
the baby was a girl because of "three white lines" in the genital area. The
baby's feet were covering the area which made it hard to get a clear picture, and I left more confused than
when I arrived. Have you heard of these three lines before. She said you
dont "usually" get the three lines with a boy. Does this make sense or
should I ask for a re-scan? Please respond, I am very upset about this. I
wanted so much to "prepare" a room and buy things specific for a boy or girl......"
"We have taken scanning on April10th. This may be sixteenth week for
me.The result tells that the fetus may be a girl. I having doubt that it is
possible to tell the sex of the even in the sixteenth month .is it correct or
wrong?..........."
Sounds familiar?
Well, as I have mentioned before in the regular FAQ page, in
order to be certain the sonologists/ sonographer should reassure himself that he
actually sees the penis and scrotum in the case of a boy and the labia in
the case of a girl. The absence of the
penis must not be taken as sufficient evidence of the fetus being a
girl. The 3-lines sign which denotes the labia in the fetus is a very valid and accurate indication of the female gender. Most of the time one should be able to tell the sex of the baby by about 20 weeks and very often even at sixteen. With modern equipment, the diagnosis can be made even as early as 11 to 12 weeks (see below). 
On the
other hand, as the correct visualization of any fetal part depends of a
host of factors such as fetal position, amount of liquor and thickness of
the abdominal wall, I have had occasions when I cannot be certain about
the sex even at 28 weeks.
This study addresses the question of first trimester diagnosis of fetal gender by assessing the outcome of 447 women who had normal transabdominal ultrasound sometimes complemented with transvaginal ultrasound (26%) when the genitals were not clearly visible. Patients were scanned between 11 and 14 weeks and were referred for assessment of fetal anatomy. In families with X-linked disorders, fetal gender is determined by chorionic villus sampling (CVS) with 99% accuracy and ultrasound would also need to reach this level of accuracy. The discovery of a female fetus on ultrasound would avoid an invasive test with its risk of miscarriage.
Results showed that the overall success rate of gender identification between 12 and 14 weeks was 80% which is less than invasive karyotyping. In the transverse plane, male fetuses showed a dome shaped genital swelling with a cephalic-directed phallus and females had three or four parallel lines representing the labia. The gender of the fetus did not affect the operator’s ability to assign gender and when assignment was possible gender did not affect the accuracy. The ability to assign gender increased with increasing gestational age.
Another article, Determination of gender - 10 and 14 weeks by Z Efrat et al which appeared in the Journal: Ultrasound in Obstetrics and Gynecology 1999; 13:305–30, reported:
The 172 women were all scanned transabdominally. Using a midline sagittal scan with the fetus supine and the spine parallel to the transducer face (sound lines perpendicular) ‘without extension of the spine or limbs’, angles were drawn on an image of the genital area with a line through the genital tubercle and another through the lumbosacral skin line. Male fetuses were characterised by an angle >300 (angle increases with age) and a females by parallel or convergent lines (angle <300). In other words, the phallus was vertical/cranially directed in males and caudally directed in females. At 12 weeks the accuracy of gender assignment was 98.7% and at 13 weeks the accuracy was 100%. There was incorrect assignment of males as females in 3% at 12 weeks and no females assigned as males after 12 weeks. The authors therefore conclude that ‘a final decision on invasive testing for sex-linked conditions should be undertaken only after 12 weeks of gestation’.
The Genital tubercle angle has now been increasingly used as an accurate method for diagnosing fetal gender at around 12-13 weeks during a scan for the Nuchal translucency.
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