Pregnancy is a time of profound transformation, centered around the development of a new life. At the heart of this process is the placenta, a remarkable organ that serves as the baby’s lifeline, providing oxygen and nutrients while removing waste. In most pregnancies, the placenta attaches high up in the uterus, away from the exit. However, in a small number of cases, a condition called placenta previa occurs, where the placenta is positioned unusually low, covering all or part of the cervix. While a diagnosis of placenta previa can be unsettling and automatically classifies a pregnancy as high risk, it is important to know that with modern ultrasound diagnosis and careful medical management, the vast majority of women with this condition can have a safe delivery and a healthy baby.

Placenta previa is a pregnancy complication where the placenta partially or completely covers the cervix, the opening of the uterus. The cervix is the outlet of the uterus, through which a baby must pass during a vaginal delivery.

To better understand this, it is helpful to use an analogy. Think of the uterus as a safe, pear-shaped room where the baby grows and develops. The cervix is the only doorway leading out of this room. The placenta is like a specialized, life-support cushion that attaches firmly to one of the walls to nourish the baby through the umbilical cord. In a typical pregnancy, this cushion attaches to the top or side wall, far away from the doorway. In placenta previa, however, the cushion has attached low on the wall, directly over or next to the doorway. This positioning can block the baby’s exit path and, more critically, can lead to severe bleeding as the uterus grows and the cervix begins to change in preparation for birth.

Healthcare providers classify placenta previa into different types based on the exact location of the placenta in relation to the cervical opening:

• Complete Previa: The placenta completely covers the entire cervical opening. This is the most serious type.
• Partial Previa: The placenta only covers part of the cervical opening.
• Marginal Previa: The edge of the placenta is right at the margin of the cervical opening but does not cover it.
• Low-Lying Placenta: The edge of the placenta is positioned within 2 centimeters of the cervical opening but is not directly touching it. Many cases of previa identified on an early ultrasound are initially classified as low-lying.

It is important to note the concept of “placental migration.” In a vast majority of cases where a low-lying placenta is detected on an ultrasound in the second trimester, the issue resolves itself by the third trimester. This does not mean the placenta detaches and moves up the wall. Rather, as the uterus grows significantly, the lower uterine segment expands and lengthens, which effectively “pulls” the placental attachment site further away from the cervix.

The exact cause of why a placenta implants in the lower part of the uterus is not always known. It is not caused by anything the mother does during pregnancy, you can’t prevent it through diet or activity. It is believed to be a random event, although certain conditions affecting the uterus may make it more likely.

The leading theory is that placenta previa can be related to the health of the uterine lining (the endometrium). If the upper portion of the uterus has scarring from a previous surgery or has a less-than-optimal blood supply, the fertilized egg may be more likely to travel lower down and implant in the healthier, more vascular tissue of the lower uterine segment. The placenta will naturally seek out the best location to establish the rich blood supply it needs to support the growing fetus, and sometimes that location is in the lower part of the uterus.

While the direct cause is unknown, researchers have identified several clear risk factors that increase a woman’s likelihood of developing placenta previa. These factors often relate to conditions that can cause scarring or alterations to the uterine lining.

The most significant risk factors for placenta previa include:

• Previous Uterine Surgery: Any surgery that creates a scar on the uterine wall can increase the risk. This is the strongest risk factor.
  • A prior cesarean section (C-section). The risk increases with each subsequent C-section.
  • A prior dilation and curettage (D&C) procedure.
  • Surgery to remove uterine fibroids (myomectomy).
• A Previous Pregnancy with Placenta Previa: A woman who has had placenta previa in one pregnancy has a higher chance of it occurring in a future pregnancy.
• Being Pregnant with Multiples: Carrying twins, triplets, or more means there is more placental tissue, which increases the chance that some of it will cover the cervix.
• Advanced Maternal Age: Women who are pregnant over age 35 have a higher risk.
• Multiple Prior Pregnancies: Women who have had many previous pregnancies are at a slightly higher risk.
• Smoking or Cocaine Use: Both these factors have been shown to increase the risk of developing placenta previa.

The classic and most common sign of placenta previa is painless, bright red vaginal bleeding in the second or third trimester.

This hallmark symptom has several key characteristics:

• It often starts without any warning and no obvious trigger.
• Bleeding can range from light spotting to a heavy, dangerous hemorrhage.
• It typically stops on its own, but it almost always recurs days or weeks later.
• While classically painless, some women may experience cramping, contractions, or abdominal pain along with the bleeding.

It is crucial to note that with the widespread use of routine prenatal ultrasounds, many women with placenta previa today have no symptoms at all. The condition is often diagnosed during a routine second trimester anatomy scan before any bleeding has occurred. This early detection is a major medical advancement that allows for careful planning and monitoring to prevent complications.

Any vaginal bleeding at any point during pregnancy should be reported to your healthcare provider immediately.

Diagnosis

The diagnosis of placenta previa is made definitively with an ultrasound.

• Transabdominal Ultrasound: This is the standard ultrasound where a probe is moved across the mother’s abdomen. It can usually identify a low-lying placenta.
• Transvaginal Ultrasound: If a low-lying placenta is suspected, a transvaginal ultrasound is often used to obtain a much clearer and more accurate picture. This involves placing a thin, wand-like probe into the vagina. It allows the doctor to see the exact relationship between the edge of the placenta and the cervical opening. This procedure is considered safe even with placenta previa.

Once placenta previa is diagnosed, the pregnancy will be managed as high-risk, and close monitoring will be a key part of the care plan. This will involve regular follow-up ultrasounds to track the position of the placenta (to see if it “migrates” away from the cervix), monitor the baby’s growth, and assess the well-being of both mother and baby.

If placenta previa is diagnosed or suspected, doctors will strictly avoid routine digital pelvic exams, as they can disturb the placenta and provoke a catastrophic bleed.

Treatment

There is no medical or surgical treatment to reposition the placenta. Therefore, the management of placenta previa is focused on a single goal: getting the pregnancy as close to the due date as safely as possible, minimizing the risk of bleeding, and planning for a safe delivery. The treatment depends on the type of previa, gestational age, bleeding severity, and maternal and fetal health.

Management with Little or No Bleeding: If placenta previa is diagnosed but there is no active bleeding, the management plan will focus on risk reduction. This usually includes:

• Pelvic Rest: This is a crucial recommendation. It means avoiding anything that could cause trauma to the cervix or trigger uterine contractions. This includes:
  • No sexual intercourse.
  • No douching or use of tampons.
  • Avoiding pelvic exams.
• Activity Restriction: Your doctor will likely recommend avoiding strenuous activities, heavy lifting, and intense exercise like running or jumping.

Management of a Bleeding Episode: Any significant bleeding episode will require immediate hospitalization for close monitoring. The goals are to stabilize the mother and allow the pregnancy to continue if possible.

• Hospital Bed Rest: The mother will be monitored closely.
• Intravenous (IV) Fluids and Blood Tests: An IV line will be placed, and blood will be drawn to check blood count and prepare for a potential blood transfusion.
• Corticosteroids: If the bleeding occurs before 34-36 weeks, the mother will be given steroid injections (like betamethasone) to rapidly mature the baby’s lungs in case an early delivery becomes necessary.
• Tocolytic Medications: If contractions are present, medications may be given to stop them and prevent the cervix from dilating.
• Blood Transfusion: If bleeding is heavy, a blood transfusion will be given to replace the lost blood.

Delivery Considerations: The method of delivery is the most critical decision in managing placenta previa.

• Cesarean Section is Medically Necessary: If the placenta covers any part of the cervix at the time of delivery, a vaginal birth is not possible. As the cervix dilates during labor, it would cause the placenta to tear away from the uterine wall, resulting in a massive hemorrhage that would be life-threatening for both mother and baby.
• Planned C-Section: The delivery is typically planned in advance. A scheduled C-section is usually performed between 36 and 37 weeks of gestation, before natural labor has a chance to begin.
• Emergency C-Section: An emergency C-section may be required at any gestational age if there is uncontrollable, heavy bleeding or if either the mother or baby shows signs of distress.

A diagnosis of placenta previa can undoubtedly bring stress and uncertainty to what should be a joyful time. It changes the landscape of a pregnancy, requiring more frequent monitoring, lifestyle adjustments, and a different plan for delivery. However, it is a condition that modern obstetrics is extremely well-equipped to manage. Thanks to the precision of ultrasound diagnosis, most cases are identified long before they become an emergency. With a clear management plan, a vigilant healthcare team, and a planned cesarean delivery, the overwhelming majority of women with placenta previa will have a safe outcome and welcome a healthy baby into the world.

• American College of Obstetricians and Gynecologists (ACOG). (2021). Placenta Previa. Retrieved from https://www.acog.org/womens-health/faqs/placenta-previa
• Mayo Clinic. (2022). Placenta previa. Retrieved from https://www.mayoclinic.org/diseases-conditions/placenta-previa/symptoms-causes/syc-20352768
• Royal College of Obstetricians and Gynaecologists (RCOG). (2018). Placenta praevia and placenta accreta: Diagnosis and management. https://www.rcog.org.uk