Placenta praevia is a pregnancy complication where the placenta covers part or all of the cervix, blocking the baby’s path during delivery. Most cases are discovered through routine ultrasound examinations during pregnancy, and while many resolve naturally as the pregnancy progresses, understanding when and how this condition is diagnosed is essential for ensuring the safety of both mother and baby.

Introduction: Who Should Undergo Diagnostics

Every pregnant woman should receive regular prenatal care, which includes ultrasound examinations that can detect placenta praevia. These routine screening examinations are typically performed in the second trimester of pregnancy, and they serve as the primary method for identifying whether the placenta has attached in an unusual position within the uterus.^[1]

Women who experience specific symptoms during pregnancy should seek immediate medical evaluation, regardless of whether they have had recent prenatal appointments. The most important warning sign is bright red vaginal bleeding that occurs after 20 weeks of pregnancy. This bleeding is typically painless, though it can sometimes be accompanied by mild cramping or contractions in the abdomen, belly, or back.^[2]

The bleeding associated with placenta praevia can be unpredictable. It may start suddenly without any clear trigger, or it may be brought on by sexual intercourse or a medical examination. In some cases, the bleeding appears as light spotting at first, then stops for a period of time before starting again days or weeks later. Some women may not experience any bleeding until labor begins, which is why regular prenatal screening is so important even in the absence of symptoms.^[3]

Certain groups of women have higher risk factors that make diagnostic vigilance even more important. Women who are 35 years or older, those who have had multiple previous pregnancies, and those carrying twins, triplets, or more babies should be particularly attentive to prenatal screening schedules. Women who have had previous cesarean sections or other uterine surgeries, including procedures such as dilation and curettage (D&C), also face elevated risks.^[4]

Women who smoke cigarettes or use cocaine during pregnancy, as well as those who have a history of placenta praevia in a previous pregnancy, should inform their healthcare providers of these risk factors. Those who have used in vitro fertilization (IVF) or other assisted reproductive technologies, or who have a history of uterine fibroids, should also be monitored closely throughout their pregnancy.^[5]

Classic Diagnostic Methods

The primary and most reliable method for diagnosing placenta praevia is ultrasound examination. Most cases are initially identified during routine prenatal ultrasound scans performed in the second trimester of pregnancy. These ultrasounds are standard components of prenatal care and do not require any special preparation beyond what is normally expected for a pregnancy examination.^[9]

The initial ultrasound examination is typically performed using an abdominal ultrasound device. During this procedure, a healthcare provider places a handheld device called a transducer on the outside of the pregnant woman’s abdomen. The transducer emits sound waves that create images of the uterus and the placenta’s position within it. This non-invasive examination is painless and poses no risk to the developing baby.^[1]

When abdominal ultrasound images suggest that the placenta may be positioned low in the uterus, healthcare providers often recommend a more detailed examination using transvaginal ultrasound. This technique involves inserting a specially designed wand-like ultrasound device into the vagina. While this may sound uncomfortable, transvaginal ultrasound provides much clearer and more accurate images of the placenta’s exact position relative to the cervix. Healthcare providers take great care during this procedure to position the device safely and avoid disturbing the placenta or causing bleeding.^[9]

The distinction between different types of placenta praevia becomes clear through these ultrasound examinations. Marginal placenta praevia means the placenta is positioned at the edge of the cervix, touching it but not covering the opening. Partial placenta praevia indicates that the placenta covers a portion of the cervical opening. Complete or total placenta praevia means the placenta entirely blocks the cervix, completely obstructing the birth canal.^[1]

It is important to understand that a diagnosis of placenta praevia early in pregnancy does not necessarily mean the condition will persist throughout the entire pregnancy. The placenta grows and the uterus expands significantly as pregnancy progresses. In many cases, particularly when diagnosed in the second trimester, the placenta appears to move away from the cervix as the uterus grows larger. This is why healthcare providers typically schedule follow-up ultrasound examinations to monitor whether the placenta’s position changes over time.^[5]

When a woman presents to medical care with vaginal bleeding after 20 weeks of pregnancy, ultrasound becomes an urgent diagnostic tool. Healthcare providers use ultrasound not only to confirm the presence of placenta praevia but also to check for other serious conditions that can cause bleeding, such as placental abruption, where the placenta detaches early from the uterine wall. Distinguishing between these conditions is critical because they require different management approaches.^[9]

In some specialized medical centers, magnetic resonance imaging (MRI) may be used if ultrasound images are unclear or if healthcare providers need more detailed information about the placenta’s attachment. However, MRI is not routinely necessary for diagnosing placenta praevia, as ultrasound is typically sufficient and more readily available.^[12]

One critical diagnostic principle that all healthcare providers follow is the strict avoidance of digital vaginal examinations once placenta praevia is suspected or diagnosed. Unlike normal prenatal examinations where a provider may check the cervix by inserting fingers into the vagina, this type of examination must never be performed when placenta praevia is present. The physical contact could disturb the placenta and trigger severe, life-threatening bleeding for both mother and baby.^[7]

Healthcare providers also use ultrasound to monitor the baby’s growth and position throughout the pregnancy. When the placenta covers the cervix, babies sometimes cannot move into the normal head-down position needed for vaginal delivery. Ultrasound examinations help providers track whether the baby is growing appropriately and whether any adjustments to the birth plan will be necessary.^[13]

Diagnostics for Clinical Trial Qualification

While placenta praevia itself is not typically the subject of clinical trials for new treatments, women with this condition may be included in studies examining optimal management strategies, timing of delivery, or prevention of complications. The diagnostic criteria used to qualify patients for such studies generally follow the same standard ultrasound-based diagnostic methods used in routine clinical care.^[4]

Clinical trials involving pregnant women with placenta praevia would require confirmed diagnosis through ultrasound examination showing the placenta’s position relative to the cervix. Researchers would typically classify participants according to the type of placenta praevia (marginal, partial, or complete) as this affects both the risk level and the management approach. The gestational age, which refers to how far along the pregnancy is, also serves as an important factor in determining eligibility for studies, as management strategies differ depending on whether the pregnancy is in the second or third trimester.^[11]

Studies examining outcomes for women with placenta praevia often include diagnostic monitoring of maternal vital signs and laboratory tests. Blood tests to check the mother’s blood count and blood type become particularly important because of the risk of significant bleeding. Women with Rh-negative blood type require special medication (Rhogam) to prevent complications, and this would be documented as part of trial protocols.^[3]

Continuous monitoring of the baby’s wellbeing through fetal heart rate monitoring and ultrasound assessments of fetal growth would be standard diagnostic criteria in clinical research settings. Some studies might also examine the fluid around the baby, called amniotic fluid, to determine whether the baby’s lungs have developed sufficiently if early delivery becomes necessary. This assessment helps researchers understand the optimal timing for delivery to balance maternal safety with fetal maturity.^[8]

Research examining the connection between placenta praevia and placenta accreta spectrum disorders, where the placenta attaches too deeply into the uterine wall, may use additional imaging techniques. Advanced ultrasound examinations looking at blood flow patterns in the placenta and uterus, or specialized MRI imaging, might be employed to diagnose these more serious complications. These diagnostic methods would be carefully documented as part of trial inclusion criteria.^[4]