A late-term ultrasound at 38 weeks of pregnancy is common, though it is not a routine screening for every expectant mother. Most healthcare providers schedule standard ultrasounds in the first and second trimesters for low-risk pregnancies. When a scan is performed at 38 weeks, it is usually a targeted screening reserved for pregnancies with specific risk factors or clinical concerns. This late-stage imaging assesses fetal well-being and gathers final, time-sensitive information before labor begins. The goal is to identify factors that might affect the safety and planning of the delivery.
Indications for a Late-Term Ultrasound
A provider orders an ultrasound at 38 weeks when maternal or fetal indicators suggest a need for closer monitoring. Maternal medical conditions often necessitate this late-term check, including high blood pressure, gestational diabetes, or pre-eclampsia, which can affect placental function and fetal growth. Advanced maternal age or carrying multiple babies also increase the need for a final assessment.
Concerns about the baby’s growth, whether restricted (small-for-gestational-age) or excessive (macrosomia), prompt an ultrasound to estimate the final size. A reduction in the baby’s usual movement is another direct indication, as this change can signal fetal distress. The scan also confirms the location of the placenta, especially if there was an earlier concern about placenta previa.
Critical Assessments Performed at 38 Weeks
The 38-week scan focuses on time-sensitive metrics that directly impact delivery planning. A primary assessment determines the fetal presentation, confirming if the baby is positioned head-down (vertex) for a vaginal delivery or feet-first (breech). The baby’s position is usually fixed by this late stage.
The sonographer also measures the Amniotic Fluid Index (AFI), which is the total measurement of fluid pockets around the baby. Adequate fluid volume is important for fetal well-being; levels that are too low (oligohydramnios) or too high (polyhydramnios) require attention. Another calculation is the Estimated Fetal Weight (EFW), which uses measurements like head circumference, abdominal circumference, and femur length to estimate the baby’s current size. This estimate is relevant for predicting complications like shoulder dystocia.
In higher-risk scenarios, a comprehensive evaluation called a Biophysical Profile (BPP) may be performed. The BPP uses ultrasound to observe specific characteristics and, when performed as a complete test, includes a non-stress test to assess the fetal heart rate. The components assessed are:
- Fetal breathing movements
- Gross body movements
- Muscle tone
- Amniotic fluid volume
- Fetal heart rate (via non-stress test)
These components are assigned a score, providing a detailed picture of the baby’s overall well-being and oxygen status.
How Ultrasound Findings Guide Delivery Decisions
The data collected during the late-term ultrasound often dictates the final management plan and timing of labor. A finding of a breech presentation typically leads to a discussion of an External Cephalic Version (ECV) to manually turn the baby or scheduling a planned cesarean section. If the Estimated Fetal Weight suggests macrosomia (a significantly larger than average baby), providers may recommend inducing labor between 37 and 39 weeks. This intervention aims to reduce the risk of birth injuries like shoulder dystocia.
Abnormalities in the BPP score or AFI can prompt immediate interventions. A low AFI or an abnormal BPP score suggests the baby may not be receiving sufficient oxygen or nutrition, increasing the risk of adverse outcomes. In these situations, a provider may recommend immediate induction of labor or an emergency cesarean delivery. Conversely, if scan results are reassuring but a risk factor remains, the plan may involve increasing the frequency of monitoring or repeating the ultrasound in a few days.
When a 38-Week Scan Is Not Standard Practice
For a consistently low-risk and uncomplicated pregnancy, a 38-week ultrasound is often not included in the standard prenatal care schedule. Most guidelines do not recommend a routine third-trimester scan for all pregnant individuals. The primary emphasis in these low-risk cases shifts to non-imaging methods for monitoring.
Clinical assessments remain the focus, including measuring the size of the uterus externally and relying on the mother’s report of consistent fetal movement. If there are no clinical indications or maternal concerns, monitoring continues through regular prenatal visits without additional imaging. The absence of a late-term scan reflects a pregnancy progressing within expected parameters, where targeted screening is not warranted.