The third trimester of pregnancy begins at week 28, marking the final stage before delivery. Ultrasound examinations during this period serve a distinct purpose from earlier scans, shifting the focus from anatomical development to monitoring growth and well-being. While first and second-trimester ultrasounds are generally standard, the need for third-trimester scanning is highly individualized and depends on the specific circumstances of the mother and fetus. The primary goal of any late-stage scan is to gather real-time data that helps healthcare providers ensure the pregnancy is progressing safely toward birth.
Standard Ultrasound Frequency in Low-Risk Pregnancy
For a low-risk pregnancy, where both the mother and fetus are healthy, routine third-trimester ultrasounds are often not required. The comprehensive anatomy scan performed between 18 and 22 weeks generally provides sufficient information for ongoing care. No additional ultrasounds are scheduled unless a specific clinical concern arises later on. Some providers or healthcare systems may opt for a single late scan, typically between 32 and 37 weeks, primarily to confirm the baby’s position and reassess placental location. The absence of a scheduled scan in the final trimester is usually a positive sign, indicating that the pregnancy is following a healthy and predictable course.
Medical Conditions Requiring Increased Monitoring
When a pregnancy is classified as high-risk, the frequency of ultrasound monitoring increases to ensure continuous fetal surveillance. Conditions like gestational diabetes or preeclampsia, a form of high blood pressure, can affect placental function and necessitate frequent assessments. Carrying multiples, such as twins or triplets, requires serial scans to monitor the growth of each fetus and manage potential complications like twin-to-twin transfusion syndrome.
Increased scanning is common for suspected fetal growth issues, either too slow (growth restriction) or too fast (macrosomia), often suspected during routine fundal height measurements. If a patient reports decreased fetal movement, an ultrasound is typically ordered immediately to assess the baby’s activity and overall status. A low-lying placenta (placenta previa) found during the mid-trimester scan requires a follow-up ultrasound to confirm its position has moved away from the cervix before delivery.
Key Assessments Performed During Late-Term Scans
Third-trimester ultrasounds focus on Fetal Growth Assessment, using measurements like the head circumference, abdominal circumference, and femur length to calculate an Estimated Fetal Weight (EFW). While EFW has a margin of error (typically around 10 to 15%), tracking the trend of these measurements over time is more informative than any single number. The scan also evaluates the volume of the amniotic fluid, calculated using the Amniotic Fluid Index (AFI) or by measuring the Maximum Vertical Pocket (MVP), to identify oligohydramnios (low fluid) or polyhydramnios (excess fluid).
The scan checks the baby’s Fetal Position, confirming if the baby is head-down (cephalic), buttocks-first (breech), or lying sideways (transverse) as the due date approaches. A specialized assessment is the Biophysical Profile (BPP), a scoring system that observes four specific fetal behaviors:
- Body movement.
- Muscle tone.
- Breathing movements.
- The amount of amniotic fluid.
In some higher-risk cases, Doppler flow studies are performed to measure blood flow velocity in vessels like the umbilical artery and the middle cerebral artery, providing insight into the efficiency of oxygen and nutrient transfer from the placenta to the fetus.
Interpreting Results and Follow-Up Care
The results of a third-trimester ultrasound guide clinical management. If the measurements suggest growth restriction or compromised blood flow, the immediate follow-up involves increasing the frequency of surveillance, often to weekly or twice-weekly scans and non-stress tests. Conversely, if the results are reassuring with appropriate growth and fluid levels, the monitoring may simply continue as planned, offering confidence in the pregnancy’s trajectory.
Abnormal findings, such as low amniotic fluid or evidence of placental insufficiency, may prompt a discussion about the optimal timing and method of delivery. These results help the care team weigh the risks of continuing the pregnancy against the risks associated with an early delivery. For example, if a severe growth issue is detected, the medical team may decide that the safest option is to plan for an induction or Cesarean section before the original due date. The interpretation of these late-term scans ensures necessary adjustments to the care plan are made promptly.