Placenta previa is most often diagnosed with ultrasound, either during a routine mid-pregnancy anatomy scan around 20 weeks or after an episode of painless vaginal bleeding in the second half of pregnancy. Transvaginal ultrasound is the most accurate method, measuring the exact distance between the edge of the placenta and the opening of the cervix to confirm whether previa is present.
What Triggers the Diagnostic Workup
There are two common paths to a placenta previa diagnosis. The first is incidental: your provider spots a low-lying placenta during the routine anatomy scan performed around 18 to 20 weeks. The second is symptom-driven, typically prompted by bright red vaginal bleeding that appears in the second half of pregnancy. This bleeding often starts and stops, sometimes returning days or weeks later, and is usually painless or accompanied by only mild cramping. The amount can range from light spotting to heavier flow.
Bleeding after intercourse is another symptom that can lead to evaluation. Any vaginal bleeding after 20 weeks warrants investigation, because providers need to determine whether the placenta is covering or sitting near the cervix before performing any physical exam. A manual cervical check when previa is present can disturb the placenta and trigger sudden, heavy bleeding. For this reason, a digital vaginal exam is not performed until ultrasound has ruled out previa.
How Ultrasound Confirms the Diagnosis
Transvaginal ultrasound is the gold standard for diagnosing placenta previa. A narrow ultrasound probe is placed in the vagina, which gives a much clearer view of the relationship between the placenta’s lower edge and the internal opening of the cervix (called the internal os) than an abdominal scan alone. This is safe and does not increase bleeding risk.
Transabdominal ultrasound, the type where a probe glides over your belly, is often done first and can raise suspicion. But it’s less precise at measuring exact distances near the cervix, so transvaginal ultrasound is used to confirm or rule out the diagnosis. In rare cases where ultrasound images are unclear, MRI may be used instead.
What the Measurements Mean
The diagnosis hinges on how close the placenta sits to the cervical opening. If the placental edge is within 20 millimeters of the internal os, or overlaps it entirely, the diagnosis is placenta previa. When the edge sits more than 20 mm away but is still lower than usual, it’s classified as a low-lying placenta rather than true previa. This distinction matters: a placental edge less than 20 mm from the cervical opening is associated with a significantly higher likelihood of needing a cesarean delivery, according to guidelines from the Royal College of Obstetricians and Gynaecologists and the Society of Obstetricians and Gynecologists of Canada.
Complete previa, where the placenta fully covers the cervix, is the most straightforward to identify on imaging. Partial previa, where only part of the opening is covered, and marginal previa, where the edge just reaches the os, require more careful measurement.
The 20-Week Scan and Why Many Cases Resolve
A placenta previa found at the mid-pregnancy anatomy scan does not necessarily mean you’ll still have it at delivery. As the uterus grows, the lower segment stretches, and the placenta effectively “migrates” upward and away from the cervix. Research published in the American Journal of Obstetrics and Gynecology found that 91.6% of placenta previa cases diagnosed at the mid-trimester scan resolved before delivery.
Because of this high resolution rate, a diagnosis at 20 weeks typically leads to a follow-up ultrasound later in pregnancy, usually around 32 weeks, to reassess placental position. If the placenta has moved well clear of the cervix, the diagnosis is lifted. If it remains close to or over the os, your provider will plan delivery accordingly, which nearly always means a scheduled cesarean.
Previas diagnosed later in pregnancy, or those where the placenta completely covers the cervix, are less likely to resolve on their own. Your care team will monitor with additional ultrasounds in the third trimester to track any changes.
How Previa Is Distinguished From Abruption
Vaginal bleeding in the second half of pregnancy can also signal placental abruption, where the placenta separates from the uterine wall. The two conditions look quite different. Previa bleeding is typically painless, bright red, and intermittent. Abruption bleeding tends to come with sudden abdominal or back pain, a tender uterus, and symptoms that worsen over time. In some abruption cases, there’s no visible bleeding at all because blood is trapped behind the placenta.
Ultrasound is the key tool for telling them apart. In previa, the scan clearly shows the placenta sitting over or near the cervix. Abruption is harder to see on ultrasound and is often diagnosed through a combination of physical exam findings, fetal monitoring, and blood work alongside imaging.
Who Gets Screened More Closely
Certain factors increase the likelihood of placenta previa, and providers may pay closer attention to placental position in these pregnancies. A prior cesarean delivery is one of the most well-studied risk factors. Research from the American Journal of Obstetrics and Gynecology found that women whose previous cesarean was a scheduled (prelabor) procedure had roughly 2.6 times the risk of previa in their next pregnancy compared to women who previously delivered vaginally. The overall incidence rose from 0.24% after a vaginal delivery to 0.98% after a prelabor cesarean. Interestingly, a cesarean performed after labor had already begun carried a much smaller, statistically insignificant increase in risk.
Other factors that raise the probability include having had placenta previa in a previous pregnancy, carrying multiples, a history of other uterine surgeries, smoking, and being over age 35. None of these factors change how the diagnosis is made, but they do influence how carefully and how often the placental location is checked throughout pregnancy.