Since the late 1950’s ultrasonography has become a very useful diagnostic tool in Obstetrics. The technology has advanced greatly in its half-century lifespan. Today’s equipment is able to provide real-time continuous pictures of the moving fetus. These images are created from high frequency sound waves emitted from a transducer that is placed in contact with the abdomen and moved around like a flashlight all around the uterus. Ultrasound beams can the fetus in thin slices and are reflected back on the transducer. The data obtained from different reflections are put back together as a picture on the monitor. This allows the physician to view malformations in the fetus, take measurements to assess gestational age, size and growth, and assess movements such as heart beat.
Ultrasound scans are currently considered to be a safe, non-invasive, accurate and cost-effective way to investigate the fetus. It has progressively become an indispensable obstetric tool and can play an important role in the care of every pregnant woman.
In order to be certain the sonographer (person performing the ultrasound) should reassure herself that she 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. 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. On the other hand, as the correct visualization of any fetal part depends of a host of factors such as fetal position, amount of amniotic fluid and thickness of the abdominal wall, it may not be able to be certain about the sex even at 28 weeks.
Just how accurate is the diagnosis of fetal gender by ultrasound? In general, it should be very accurate (somewhere between 95 and a hundred percent), particularly after about 20 weeks, or even sixteen. With modern equipment and techniques it may be occasionally possible to determine gender as early as 11 or 12 weeks. However, one must remember the person operating the ultrasound may not always be able to get a good view of the private parts of the fetus for a variety of reasons and therefore may not furnish you with an "accurate" answer. You really need to press the sonographer as to the degree of certainty.