Having the placenta develop along the posterior wall and reaching up to the internal os is not necessarily a problem in itself. The location of the placenta can vary from person to person, and as long as it is not covering or partially covering the cervix (known as placenta previa), there should be no cause for concern. It's important to note that at 12 weeks 5 days, there is still plenty of time for the placenta to move away from the cervix as your pregnancy progresses. Your healthcare provider will continue monitoring your pregnancy through regular ultrasounds to ensure that there are no complications related to placental positioning. If any concerns arise, they will discuss appropriate measures or interventions with you.