A second ultrasound is usually done between the 18th and 22nd weeks of pregnancy to make sure that the baby is growing and developing properly. Next, a probe called a transducer is moved over the woman's abdomen, which transmits sound waves that can produce images of the fetus as it develops inside the mother's womb. Here six important facts about ultrasound screenings and sex determination, according to Carr, a professor of obstetrics and gynecology at the Warren Alpert Medical School of Brown University. Gender determination is usually highly accurate Gender predictions made by ultrasound have an accuracy rate "north of 90 percent," Carr said. Expectant parents who want their child's sex to remain a secret until birth are in the minority, said Dr. Lately, as more couples turn to in-vitro fertilization procedures to conceive a child, the issue of selecting the sex of a baby for purposes of "family balancing" rather than for medical reasons is controversial and raises ethical concerns. Beyond ending the guessing game, there are medical reasons why mothers and fathers may want to learn the sex of their baby from an ultrasound. Ultrasound uses high-frequency sound waves to produce an image on a screen of the baby in the mother's uterus. One popular belief is that if a woman is carrying the baby high, she is supposedly having a girl, while carrying the baby low means it's a boy. Women may be asked to drink several glasses of water before an ultrasound because a filled bladder helps give clearer images. If the sperm is carrying a Y chromosome, the baby will be a boy. If it swings in a circle, the child is thought to be a girl. The scans are typically done twice during pregnancy, but the one done between 18 and 22 weeks is when the sonographer ultrasound technician might identify the gender of the baby, if parents want to know. Women carrying girls have higher levels of the pregnancy hormone hCG human chorionic gonadotropin , and these higher levels are associated with an increased risk of severe morning sickness, Carr said.