One example of this is congenital adrenal hyperplasia, a genetic condition in which baby girls may have genitals that appear more masculine than feminine, according to the Mayo Clinic. Most of the women were happy even when the sex differed from that which they desired. The egg always contributes an X chromosome, and the sperm can contribute either an X or a Y chromosome, depending on the sperm cell. If the sperm is carrying a Y chromosome, the baby will be a boy. However, "more and more people are telling us they want to wait until the baby arrives to find out the sex," Carr said. What Is a Fetal Ultrasound? Although the test can be done at any point during pregnancy, women typically get one during the first trimester. In addition, ultrasound can identify certain birth defects, such as Down syndrome and spinal abnormalities, and investigate pregnancy complications, including miscarriage, according to the March of Dimes. There is no harm to the baby during the procedure, Carr said. The patients were also contacted by telephone and the sex at birth confirmed with relevant comments entered into the questionnaires. The objectives were to determine the accuracy of sonographic prenatal sex determination, perform binary classification test, and the impact it has, including mis-diagnosis. Folk wisdom has linked experiencing severe morning sickness with having a girl, and this idea may have some science to back it up. Unfortunately first trimester ultrasound sex determination has significant false negative rate. For this urban legend, a woman combines some of her first morning urine with the liquid drain cleaner. This article has been cited by other articles in PMC.