First, how to define sex and gender

There is a difference between the words "sex" and the word "gender".

• Sex is usually what a person is assigned at birth, girl, boy, or non-assigned. Sex is based on anatomic characteristics. Girls usually have vaginas, a uterus, and ovaries, and boys have a penis and testes.
• Gender, on the other hand, is different and denotes a range of identities that do not necessarily correspond to established ideas of just male and female.

The most common medical tests to predict a baby's sex

The most common medical tests to predict the baby's gender are the cell-free fetal DNA blood test, CVS (chorionic villi sampling), amniocentesis, and ultrasound. 

What is a noninvasive prenatal test?

The NIPT or noninvasive prenatal testing is a blood test for mom-to-be and it can be done throughout pregnancy starting from as early as the 9th week of the pregnancy. The test checks for the presence or absence of the Y-chromosome. If the Y chromosome is present then the baby's sex is a boy, otherwise, it's a girl. However, the main reason to do this test is to check the baby's risk of chromosomal abnormalities including trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). The NIPT is a screening test, which means that if an abnormality is found, it must be confirmed with other tests such as CVS or amniocentesis. 

What is chorionic villus sampling (CVS)?

Chorionic villus sampling (CVS) is an invasive test done around 11-13 weeks of the pregnancy. A needle is inserted either through the abdomen or vaginally through the vagina with the help of ultrasound and a tiny portion of the placenta is removed and examined. The CVS reveals the baby's gender as well as chromosomal anomalies.

What is an amniocentesis?

Amniocentesis, like CVS, is an invasive test. The amniocentesis is usually done around 16-18 weeks of the pregnancy with a needle inserted through the abdomen with ultrasound guidance. A small amount of amniotic fluid, about 10cc or less than 5-10%, is removed and sent to the laboratory. The amniocentesis reveals the baby's gender as well as chromosomal anomalies. 

When can an ultrasound determine gender?

An ultrasound is a noninvasive test usually done around 18-20 weeks of the pregnancy. As part of the anatomy screen, the technician and doctor will check between the baby's legs to confirm if there is labia (girl) or a penis/scrotum (boy). The ultrasound around 18-20 weeks to reveal the baby's sex is close to 100% accurate, and it's less accurate before 15-16 weeks, but mistakes have been known to have been made.

NIPT, CVS, amniocentesis are very reliable methods to determine the baby's sex. There are other methods, most of which are less reliable.

Do baby-gender prediction kits work?

There are different kinds of products sold directly to the customer promising to reveal the baby's sex. Some kits sold on Amazon test your urine for as low as $14.95. These are not really accurate as the technology to reliably determine the baby's sex from the mother's urine does not exist yet. Another test sold online is a blood test that claims to be the same as the NIPT test. The test costs about $80 and can be done after 9 weeks of the pregnancy and claims to be 99% accurate.