An ultrasound performed between weeks 18 and 20 of a pregnancy can provide 80% to 90% certainty of a baby's sex, explains WebMD. Amniocentesis and chorionic villus sampling (CVS) are also highly-accurate, but more invasive and typically used only when there are concerns of genetic disorder or chromosomal abnormality.
Another accurate method of establishing the sex of a baby is a DNA test of the mother's blood, but this is prohibitively expensive for most laboratories, says WebMD.
One myth suggests that if the pregnant woman's belly is low, the baby is a boy, and female if she is carrying high. However, no scientific evidence supports this belief, states WebMD.