Notching refers to a distinct dip or indentation in a waveform or structure that doctors use as a diagnostic marker. The term appears most often in three medical contexts: uterine artery Doppler ultrasound during pregnancy, the normal arterial pulse waveform of the heart, and rib changes visible on a chest X-ray. Each type of notching tells a different story about blood flow and vascular health.
Uterine Artery Notching in Pregnancy
This is the most commonly searched type of notching, and it comes up during routine ultrasound screening in the second trimester. When a sonographer uses Doppler ultrasound to measure blood flow through the uterine arteries (the vessels feeding the placenta), the flow pattern appears as a wave on screen. In a healthy pregnancy, that wave has a smooth, rounded shape. A “notch” is a small downward dip that appears on the descending part of the wave, right after the peak of each heartbeat.
Early in pregnancy, some degree of notching is normal. The placenta is still developing, and the small spiral arteries in the uterus haven’t fully remodeled yet to accommodate the high blood flow a growing baby needs. By about 24 weeks, those arteries should have opened up enough that the notch disappears. When notching persists beyond 24 weeks, it signals that the placenta’s blood supply may not have developed properly, a condition linked to inadequate remodeling of the spiral arteries that supply the placental bed.
Why Persistent Notching Matters
The exact mechanism behind the notch isn’t fully settled, but the leading theory is straightforward: when resistance in the placental blood vessels stays too high, blood flow hitting that resistance bounces back like a wave reflecting off a wall, creating the visible dip in the waveform. Animal studies support this explanation. When researchers progressively blocked the spiral arteries in pregnant animals, a diastolic notch only appeared once blood flow had dropped to roughly one-third of normal and vascular resistance had tripled or quadrupled.
That degree of reduced flow has real consequences. When notching is detected in both uterine arteries (bilateral notching) between weeks 19 and 22, it’s associated with a significantly higher risk of serious complications: fetal growth restriction, severe preeclampsia, preterm birth before 32 weeks, placental abruption, and dangerously low amniotic fluid levels. Bilateral notching is a stronger warning sign than a notch on just one side, though any notch detected during screening is flagged for follow-up.
As a screening tool, bilateral notching picks up about 77 to 83 percent of cases that go on to develop pregnancy-induced hypertension with protein in the urine, depending on the population studied. For the most severe cases requiring delivery before 34 weeks, the sensitivity rises to about 81 percent. The positive predictive value, however, is lower (around 27 percent for hypertensive disease), meaning most women with notching will still have normal outcomes. It’s a risk flag, not a diagnosis.
What Happens After Notching Is Found
If your ultrasound shows uterine artery notching or abnormally high resistance in the uterine arteries, the typical next step is closer monitoring rather than immediate intervention. Current guidelines call for serial growth scans starting at 24 weeks, repeated every three weeks until birth. These scans track the baby’s estimated weight, amniotic fluid levels, and blood flow through the umbilical artery to catch any signs of the baby not growing well.
Your care will usually shift to a consultant-led team, and low-dose aspirin may be recommended if it hasn’t already been started. Aspirin, taken early enough in pregnancy, can help improve placental blood flow and reduce the risk of preeclampsia in higher-risk pregnancies. The goal of all this monitoring is early detection: catching complications like growth restriction or preeclampsia before they become dangerous, so delivery timing can be planned carefully.
The Dicrotic Notch in Arterial Pulse Waves
Outside of pregnancy, “notching” also describes a completely normal feature of every heartbeat. The dicrotic notch is a small, brief dip that appears on the downslope of an arterial blood pressure waveform, the kind displayed on a hospital monitor or measured with a finger pulse sensor. It marks the exact moment the aortic valve snaps shut after the heart finishes pumping blood out. The pressure wave traveling through the arteries bounces briefly off the closed valve, creating that characteristic dip.
The dicrotic notch is the dividing line between the pumping phase (systole) and the resting phase (diastole) of each heartbeat. It’s always present in a healthy arterial waveform, and its shape and position can give clinicians useful information about heart valve function and arterial stiffness. This type of notching is not a concern on its own. It’s a normal part of cardiovascular physiology.
Rib Notching on Chest X-Ray
A third type of notching shows up on chest X-rays as small scalloped indentations along the undersides of the ribs. This is most classically associated with coarctation of the aorta, a condition where a section of the body’s largest artery is abnormally narrow. The narrowing forces blood to find alternative routes to reach the lower body, and the intercostal arteries (small vessels running along the ribs) become those detour routes.
Over time, these rerouted vessels enlarge significantly under the increased pressure. The expanded arteries press against the undersides of the ribs, gradually eroding the bone and leaving visible notches. On an X-ray, the notching typically appears on the middle ribs (roughly ribs 3 through 8) while sparing the first two or three ribs and the last two. That’s because the blood supply to those end ribs comes from arteries that aren’t affected by the aortic narrowing.
Rib notching can sometimes be the first clue that coarctation exists, particularly in adults whose condition went undetected in childhood. It’s an incidental finding, often spotted on a chest X-ray ordered for an unrelated reason, that prompts further imaging to confirm the diagnosis.