What causes placenta previa

Content on WhatAnswers is provided "as is" for informational purposes. While we strive for accuracy, we make no guarantees. Content is AI-assisted and should not be used as professional advice.

Last updated: April 4, 2026

Quick Answer: The exact cause of placenta previa is not fully understood, but it's generally believed to occur when a portion of the placenta implants too low in the uterus, potentially due to factors like scarring from previous uterine surgeries or a larger placental surface area. This abnormal positioning can lead to the placenta covering all or part of the cervix.

Key Facts

Placenta previa occurs when the placenta covers the internal opening of the cervix.
Risk factors include a history of C-sections (up to 1 in 100 for one previous C-section, rising to 1 in 20 for four or more).
Previous uterine surgeries, such as myomectomy or D&C, increase the risk.
Maternal age over 35 years is associated with a higher incidence.
Multiple pregnancies (twins, triplets, etc.) are linked to an increased likelihood.

What is Placenta Previa?

Placenta previa is a pregnancy complication where the placenta entirely or partially covers the cervix, the mother's lower uterus that opens into the vagina. This condition can cause severe bleeding during pregnancy and labor, and it often necessitates a Cesarean delivery. The placenta is a vital organ that provides oxygen and nutrients to the growing baby and removes waste products. Normally, it attaches to the top or side of the uterus. In placenta previa, however, it implants in the lower part of the uterus.

Causes and Risk Factors of Placenta Previa

The precise reason why the placenta implants in the lower part of the uterus remains unclear in many cases. However, research suggests that certain factors can increase a woman's risk of developing placenta previa. These factors often relate to changes or scarring in the uterine lining, or conditions that might lead to a larger placental surface area than usual.

Uterine Scarring and Previous Surgeries:

One of the most significant risk factors is a history of uterine surgeries. Procedures that involve the uterine lining can create scar tissue, which may affect where the placenta implants. This includes:

Previous Cesarean Sections (C-sections): The more C-sections a woman has had, the higher her risk. Studies indicate that the risk can increase significantly with each subsequent C-section. For instance, after one C-section, the risk might be around 1%; after four or more, it can rise to approximately 10%.
Other Uterine Surgeries: Procedures like myomectomy (removal of fibroids), dilation and curettage (D&C) for miscarriage or abortion, and endometrial ablation can also leave scars that influence placental implantation.

Previous Pregnancies and Births:

Women who have had multiple pregnancies or births (multiparity) are at a higher risk. This is often linked to the cumulative effect of changes in the uterine lining over time and potentially a larger placental size needed to support multiple pregnancies.

Maternal Age:

Advanced maternal age is another recognized risk factor. Women aged 35 and older are more likely to develop placenta previa compared to younger women. This is often associated with age-related changes in the uterus.

Multiple Gestations:

Carrying more than one baby (twins, triplets, or more) increases the risk. A larger placental surface area is required to support multiple fetuses, making it more likely for a portion of the placenta to implant low in the uterus.

Other Potential Factors:

While less definitively proven, other factors that have been suggested to increase risk include:

Smoking during pregnancy.
Assisted reproductive technology (ART) such as IVF.
A history of placenta previa in a previous pregnancy.
Maternal smoking.

How Placenta Previa Develops

The exact mechanism is not fully understood, but it's hypothesized that the scar tissue or altered uterine lining in the lower segment might be less hospitable for the initial implantation of the fertilized egg. Alternatively, if the placenta develops a larger surface area than usual, it may extend into the lower uterine segment simply due to space constraints or because the vascularization is more favorable there. As the pregnancy progresses and the uterus grows, the lower segment stretches and thins, which can cause the placenta to detach from the uterine wall, leading to bleeding. The location of the placenta is usually determined early in pregnancy via ultrasound, but it can sometimes move as the uterus grows. However, if the placenta is firmly implanted over the cervix, it is unlikely to move sufficiently to resolve the previa.

Types of Placenta Previa

Placenta previa is classified based on how much of the cervix is covered:

Complete Previa: The placenta completely covers the internal cervical os.
Partial Previa: The placenta partially covers the internal cervical os.
Marginal Previa (Low-Lying Placenta): The placenta is located at the edge of the cervix but does not cover it. Sometimes, a low-lying placenta can resolve as the pregnancy progresses.

It's important for pregnant individuals to be aware of these risk factors and to discuss any concerns with their healthcare provider. Regular prenatal check-ups, including ultrasounds, are crucial for monitoring the placenta's position and managing any potential complications.