So, how can you determine the sex of your baby , and how soon can you find out? Keep reading for a breakdown of several options for when to find out your baby 's sex during each trimester of pregnancy. The age-old question: should you have sex during pregnancy? Let's bust some myths and get the facts straight. A child’s biological sex at birth may not be a 50-50 toss-up, according to a new study from Harvard T.H. Chan School of Public Health. The study, published July 18 in the journal Science Advances, found that birth sex appears to be associated with maternal age, certain genes, and the sexes of older siblings. If you’re wondering when a baby ’s sex develops in the womb, our experts explain the timeline of fetal development —how and when a baby ’s sex can be detected, the best methods to determine it and what factors might affect when parents find out. When and how a baby 's sex develops in the womb Here are the basics.