Blood tests that isolate fetal DNA from a mother’s blood to detect gender work after seven weeks of gestation and are better than ultrasound at ruling out some genetic abnormalities, according to a report. Ultrasound and urine tests are unreliable at determining fetal gender at that stage of development, according to a review of studies in the Journal of the American Medical Association. Amniocentesis, which involves using a needle to remove and test amniotic fluid, carries a risk of miscarriage and can’t be used until at least 15 weeks, after the end of the first trimester. The blood test can help women who are carriers of some hereditary diseases, such as hemophilia, avoid unnecessary invasive tests if the fetus is female. Males are more susceptible to diseases on the X chromosome because they only have one copy. Females have two, inherited from both parents, and can rely on a healthy one if the other is damaged.