When must girls get their first dental x-ray?

Same As Boys. The rule of thumb here is: first exam by first birthday.
3 years old. I recommend the first dental exam for children around 3 y/o. Unless the parent sees something before then that needs attention. Whether dental x-rays are needed at that age is up to the dental professional. Usually if a child has other decay at that time, then x-rays are indicated. If the child has no decay and the baby teeth have good spaces between them, then no x-rays are needed.
Only when necessary. It is generally best to have all children be seen by a pedodontist around the age of one year. Not only is this important to check on their dental health, but to help guide you as a parent how to best prevent future dental problems. X-rays will only be taken when necessary and not usually before the age of 5, and probably much later than that.
When necessary. Children are usually seen after all 20 baby teeth erupt. If there is a problem, your dentist may elect to take an xray or two. Xray usage is individually based on need.