At around 18 to 20 weeks, you will have an ultrasound to check your baby’s development, identify any problems, and possibly even identify the sex of your baby. It’s important to remember that it is not always possible to determine the sex of your baby at this stage and you won’t know for sure until your baby is born. To determine the sex of your baby, the ultrasound technician or obstetrician will look for the presence or absence of a penis on the ultrasound. Keep in mind that this method of determining the sex of your baby is not 100% accurate. A visual effect may create or obscure the image of a penis on an ultrasound.