What is the normal distance between the lower edge of the placenta and the internal cervical os?

The placenta is a fetal accessory organ that supplies blood to the fetus. During prenatal examinations, particular attention is paid to the relationship between the lower edge of the placenta and the internal cervical os. So, what is the normal distance between the lower edge of the placenta and the internal cervical os? Below is an explanation.

What Is the Normal Distance Between the Lower Edge of the Placenta and the Internal Cervical Os?

A distance of more than 7 cm between the lower edge of the placenta and the internal cervical os is considered normal. Before 28 weeks’ gestation, if the distance is less than 7 cm, this is termed “placenta previa state.” After 28 weeks, if the lower edge of the placenta lies less than 7 cm from the internal cervical os—i.e., the placenta implants on the lower uterine segment, or its lower edge reaches or covers the internal cervical os, positioning it lower than the presenting fetal part—it is diagnosed as placenta previa. Placenta previa is one of the most common causes of abnormal vaginal bleeding during the second and third trimesters. Risk factors for placenta previa often include multiple pregnancies, induced abortions, or dilation-and-curettage (D&C) procedures. Moreover, if placenta previa is diagnosed, cesarean delivery is the safest mode of delivery; vaginal delivery is contraindicated. There is no specific medical treatment for placenta previa; management focuses on bed rest, avoidance of physical exertion, emotional stress, and sexual intercourse, along with increased intake of high-protein foods. As pregnancy progresses, the relative positions of the placenta and cervical os may change. In the absence of painless vaginal bleeding, continued observation—with adherence to the above precautions—is generally sufficient.

Knowledge Extension: Precautions for Placenta Previa

1. A caregiver must accompany the pregnant woman at all times; she should never be left alone. Bleeding associated with placenta previa is unpredictable and occurs without warning. Additionally, dietary considerations are especially important: constipation should be avoided, as increased intra-abdominal pressure may trigger placental separation and subsequent hemorrhage.

2. Sexual intercourse is strictly prohibited in women with placenta previa. Regular antenatal visits are essential to determine the specific type of placenta previa (e.g., marginal, partial, or complete). If complete (central) placenta previa is diagnosed, hospital admission should be undertaken promptly per physician recommendation. Furthermore, family members should consider donating blood in advance to ensure availability of compatible blood products in case of postpartum hemorrhage.

The above outlines the normal distance between the lower edge of the placenta and the internal cervical os. We hope this information is helpful to you.