Prenatal sex determination is the process of determining the sex of an unborn baby during pregnancy. There are several methods that can be used to determine the sex of a fetus， including ultrasound， chorionic villus sampling (CVS)， amniocentesis， and noninvasive prenatal testing (NIPT).
1. Ultrasound: This is the most common method to determine fetal sex. It is a non-invasive and safe procedure that uses high-frequency sound waves to create images of the fetus in the uterus. The ultrasound technician can look for the presence or absence of genitalia to determine the baby's sex. The best time for an ultrasound to determine the sex of the baby is around 18-20 weeks of pregnancy.
2. Chorionic villus sampling (CVS): This is an invasive prenatal diagnostic test that is performed between 10-13 weeks of pregnancy. CVS involves removing a small sample of cells from the placenta for genetic testing. This test can detect chromosomal abnormalities and also determine the sex of the fetus.
3. Amniocentesis: This is a prenatal diagnostic test that is usually performed between 15-20 weeks of pregnancy. During this test， a needle is inserted into the uterus to remove a small amount of amniotic fluid for genetic testing. Amniocentesis can detect chromosomal abnormalities and also determine the sex of the fetus.
4. Noninvasive prenatal testing (NIPT): This is a relatively new and highly accurate method for determining fetal sex. It is a blood test that analyzes fetal DNA that is circulating in the mother's blood. NIPT can detect chromosomal abnormalities and also determine fetal sex as early as 9-10 weeks of pregnancy.
In conclusion， there are several methods for determining the sex of an unborn baby during pregnancy. Ultrasound is the most common and non-invasive method， while CVS， amniocentesis， and NIPT are more invasive and usually reserved for women who have a higher risk of chromosomal abnormalities or for medical reasons.
