**Core Concept**
Placenta previa is a condition in pregnancy where the placenta partially or completely covers the internal cervical os (the opening of the cervix). The risk factors for placenta previa are crucial to identify, as they can help in early detection and management of the condition.

**Why the Correct Answer is Right**
Placenta previa risk increases with multiple gestations, previous uterine surgery or cesarean section, and advanced maternal age. These factors lead to a higher incidence of placenta accreta spectrum disorders, which can cause the placenta to implant too deeply into the uterine wall, increasing the risk of placenta previa.

**Why Each Wrong Option is Incorrect**
**Option A:** Nulliparity (first pregnancy) is not a risk factor for placenta previa. In fact, placenta previa is more common in multiparous women (women who have had multiple pregnancies).
**Option B:** Previous uterine surgery or cesarean section is a known risk factor for placenta previa. These procedures can cause scarring in the uterine wall, leading to a higher risk of placenta accreta and placenta previa.
**Option C:** Advanced maternal age (older than 35 years) is a risk factor for placenta previa. As women age, the risk of placenta accreta and placenta previa increases due to the natural decline in uterine elasticity and the increased risk of placental implantation.
**Option D:** Multiple gestations (carrying twins or more) are a risk factor for placenta previa. The increased uterine distension and the higher risk of placental implantation in a multigestational pregnancy increase the risk of placenta previa.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that placenta previa can present with painless vaginal bleeding, which can be a sign of placental abruption or other complications. It is essential to investigate the cause of bleeding promptly and manage the patient accordingly.

**Correct Answer: A. Nulliparity**