What is the normal distance between the lower edge of the placenta and the internal cervical os at 29 weeks of gestation?

Under normal circumstances, at 29 weeks of gestation, the distance between the lower edge of the placenta and the internal cervical os should be ≥3 cm. A measurement below this value may indicate a risk of low-lying placenta, which should be confirmed through ultrasound examination. Detailed analysis is as follows:

At 29 weeks of pregnancy, which falls in the late second to early third trimester, the placental position gradually becomes fixed. A distance of ≥3 cm ensures sufficient space for fetal development and delivery, prevents the placenta from obstructing the cervical opening, and reduces risks such as antepartum hemorrhage and preterm birth. This is an important indicator of normal placental placement. In most pregnant women, as the uterus expands during pregnancy, the placenta naturally migrates upward over time.

If imaging shows the distance between the lower edge of the placenta and the internal cervical os is <3 cm, this indicates a low-lying placenta (placenta previa), with higher risks associated with shorter distances. This condition may lead to painless vaginal bleeding during pregnancy, particularly after physical exertion or sexual intercourse, and in severe cases, can compromise blood supply and oxygen delivery to the fetus. However, in some cases of mild placental low-lying, the placenta may still move upward into a normal position during the late stages of pregnancy as the lower uterine segment stretches and elongates.

Pregnant women should undergo regular ultrasound examinations as directed by their healthcare provider to monitor changes in placental position. Daily precautions include avoiding strenuous exercise, lifting heavy objects, prolonged squatting, or any activity that increases abdominal pressure. Maintaining a regular sleep schedule and a balanced diet rich in essential nutrients can help reduce the risk of bleeding.