At 11 weeks the embryo already started to form sexual characteristics: boys growing scrotum and penis, the girls are developing labia. Before the rudiments of the genital organs is the same as a small bulge. But this time the ultrasound specialist can try to find out the sex of the fetus, although the likelihood of failure is very high, since the embryo has a small size. He can also prevent a large amount of amniotic fluid, the thickness of the abdominal wall, poor equipment, lack of experience.
Sometimes the genitals of boys is not visible, as sandwiched between the legs, and some doctors are taking a loop of umbilical cord or fingers in girls for the male organ. A few experts presume to clearly answer the question, it's a boy or a girl, but to assume anything is possible.
18 weeks in most cases we can give a more accurate answer if the hardware is good, and the child will take a comfortable position. Specialist can measure the approximate angle of formation of sexual tubercle: in boys it is more. But now there might be bugs: sometimes the child is lying so that the genitals on the screen is not visible, sometimes the liquid or fat hinder to consider in more detail the development of a fetus. More often girls take over the boys than Vice versa.
22 a week later
Only 22 weeks ultrasound specialists can with 80-90% certainty, who is born. The probability of error decreases with each week and especially low, if you conduct a study using 3D machines. As a rule, this period designate the second scheduled ultrasound, which is often misleading gender of the fetus. Since that time, the sex organs of the child are already clearly visible, and the little man is moving more actively, so you can wait to occupy a favorable position.
Starting with the third semester of the probability of error of sex determination on ultrasound increases as the child grows, occupies almost the entire area of the uterus and is moving less, so in the awkward position is by no means certain.
Invasive techniques of sex determination
There are invasive procedures of the study child during pregnancy, which can accurately determine gender from about 9 weeks, but their main goal is the detection of serious genetic disorders and developmental disabilities. These methods are quite dangerous, can lead to miscarriage, so no special indications is not necessary to use.