On ultrasound, placenta previa appears as placental tissue sitting over or very close to the opening of the cervix (the internal os), the narrow passage at the bottom of the uterus. Normally, the placenta attaches high on the uterine wall, well away from this opening. When previa is present, the sonographer can see the characteristic bright, grainy texture of placental tissue extending down to cover or overlap the cervical opening, rather than staying safely above it.
What the Sonographer Is Looking For
The placenta shows up on ultrasound as a thick, slightly brighter mass attached to the uterine wall. It has a distinctive speckled, granular texture that’s easy to distinguish from the smoother appearance of the uterine muscle and the darker amniotic fluid surrounding the baby. During a routine anatomy scan around 18 to 20 weeks, the sonographer identifies where the lower edge of this placental tissue sits in relation to the internal cervical os, which appears as a small funnel or closed line at the bottom of the uterus.
The key measurement is the distance, in millimeters, between the leading edge of the placenta and the center of the cervical opening. When the placenta overlaps the os, that distance is recorded as a positive number (for example, +15 mm means 15 mm of placental tissue covers the os). When the edge falls short of the os, it’s recorded as a negative number. This precise measurement determines the diagnosis and guides what happens next.
How Different Types Look on the Screen
Placenta previa falls into a few categories based on how much of the cervical opening is covered:
- Complete previa: The placental tissue extends fully across the internal os. On the screen, you’d see the bright placental mass clearly draped over the entire cervical opening, with no gap visible between the placenta’s edge and the os on either side.
- Partial previa: Only part of the cervical opening is covered. The placental edge reaches partway across the os, so the sonographer can see where the placental tissue ends and the uncovered portion of the os begins.
- Marginal previa: The placental edge comes within 2 cm of the internal os without actually covering it. On ultrasound, the placenta appears to stop just at or very near the edge of the cervical opening.
- Low-lying placenta: The edge sits within 2 to 3.5 cm of the os. This isn’t technically previa, but it’s close enough that it gets flagged for monitoring. On the screen, the placenta looks normal but its lower border is closer to the cervix than expected.
Transvaginal vs. Transabdominal Scans
Most people first hear about a low placenta after a standard transabdominal ultrasound, the one performed through the belly. But this approach gets the diagnosis wrong about 25% of the time. A full bladder, which is often required for transabdominal scanning, can compress the lower part of the uterus and push the uterine walls together, making the placenta appear closer to the cervix than it actually is. Emptying the bladder and rescanning can reduce these false positives.
Transvaginal ultrasound, where a slim probe is placed in the vaginal canal, is the gold standard for confirming placenta previa. Because the probe sits closer to the cervix, it provides a much sharper, more detailed view of the relationship between the placental edge and the cervical opening. Despite concerns some people have about inserting a probe when there’s a low placenta, studies have consistently shown transvaginal scanning is safe and does not increase vaginal bleeding. The probe doesn’t actually touch the cervix.
Why a Second-Trimester Diagnosis Often Changes
If your 20-week scan shows placenta previa, it doesn’t necessarily mean the placenta will still be covering the cervix at delivery. As the uterus grows during the second and third trimesters, the lower segment stretches and expands. This stretching effectively pulls the placental attachment site upward and away from the cervix, a process often called “placental migration.” The placenta isn’t actually sliding along the uterine wall. Rather, the wall itself is expanding beneath it.
The closer the placental edge is to the cervical opening at the time of the initial diagnosis, the less likely it is to clear by delivery. A marginal previa, where the edge is near but not over the os, has a much better chance of resolving than a complete previa where the tissue extends 20 or 30 mm across the opening. Anterior placentas (attached to the front wall of the uterus) and posterior placentas (attached to the back wall) can both migrate, though the dynamics differ slightly based on how the lower uterine segment stretches.
Most providers will schedule a follow-up transvaginal ultrasound around 28 to 32 weeks to remeasure the distance between the placental edge and the os. If the placenta has moved well clear of the cervix by then, the diagnosis is typically dropped.
What Can Mimic Previa on Ultrasound
A couple of things can create the appearance of previa when it’s not actually present. A full bladder is the most common culprit: the pressure it places on the uterus can artificially flatten the lower segment and make the placenta look like it’s reaching the cervix. This is why sonographers sometimes ask you to empty your bladder partway through the exam and rescan.
Focal uterine contractions, which are temporary tightenings of the uterine muscle, can also temporarily change the shape of the lower uterus and distort the apparent position of the placenta. These contractions are common during scanning and usually resolve on their own within minutes. If one is suspected, the sonographer may wait and recheck.
Vasa previa, a different and rarer condition, can also involve structures near the cervical opening. Instead of placental tissue, vasa previa involves fetal blood vessels running across or near the os without the cushion of placental tissue or umbilical cord around them. On a standard grayscale ultrasound, these vessels may appear as thin parallel or circular echogenic lines near the cervix. Color Doppler imaging, which highlights blood flow in color, makes the distinction clear: it shows active blood flow within small vessels crossing over the os, rather than the broad, solid mass of a placenta.
What the Millimeter Measurements Mean for You
The exact distance between your placenta’s edge and the cervical opening has direct implications for delivery planning. When the placental edge overlaps the os by any amount at term, a cesarean delivery is necessary because the placenta would be in the baby’s path during labor. Even when the edge is very close but not overlapping, with a distance of 1 to 10 mm from the os, a planned cesarean is typically recommended.
When the distance reaches 11 to 20 mm, vaginal delivery becomes a possibility depending on other factors. And once the edge clears 20 mm from the os, the placenta is generally considered far enough away that it no longer affects delivery planning. This is why your provider pays such close attention to those millimeter measurements on follow-up scans. A few millimeters of change between 28 weeks and 36 weeks can shift the entire delivery approach.