The 36-week mark represents a significant milestone in the third trimester, signaling the final weeks before birth. An ultrasound performed around this time is a focused examination designed to confirm the continued well-being of the fetus and gather important data points needed for delivery preparation. This late-term scan provides a final, detailed assessment of the fetus’s growth trajectory and its surrounding environment. The information collected helps healthcare providers make informed decisions about the timing and method of delivery.
The Purpose of the Late-Term Scan
The primary goal of the 36-week ultrasound is to ensure the fetus is thriving as it approaches full term. Unlike the earlier anatomy scan, which focused on structural development, this scan is a surveillance tool for fetal health and growth trends. It is particularly useful for pregnancies where the size of the uterus, measured by fundal height, suggests the fetus may be growing too slowly or too quickly.
While not universally standard for every low-risk pregnancy, the utility of a third-trimester scan for detecting potential complications is increasingly recognized. This examination is especially common in situations where the pregnancy is considered higher risk or if there were concerns raised during previous appointments. The scan offers a final opportunity to identify issues that could complicate labor, allowing the medical team to plan for a safer birth.
The most immediate and practical goals are to determine the estimated fetal weight and confirm the fetus’s orientation in the uterus. This data helps the care team decide whether to proceed with a standard labor plan or if interventions, such as a scheduled cesarean section or a procedure to turn the fetus, might be necessary.
Detailed Fetal Measurements
The technician records specific biometric measurements to calculate the Estimated Fetal Weight (EFW), a key component of the assessment. These measurements are used to track growth and ensure the fetus is developing appropriately. The four standard measurements taken for EFW calculation are:
- Biparietal Diameter (BPD)
- Head Circumference (HC)
- Abdominal Circumference (AC)
- Femur Length (FL)
The BPD measures the width of the head, while the HC measures the distance around the outside of the head. The AC measures the circumference around the fetus’s abdomen at a specific level. The FL measures the length of the thigh bone. All these measurements are combined in complex formulas to produce the EFW, which is an estimate and not an exact birth weight, carrying an inherent margin of error.
Beyond size, the scan also confirms the Fetal Presentation, which is the part of the fetus lowest in the birth canal. The ideal presentation is cephalic, or head-down. If the buttocks or feet are positioned to deliver first, the fetus is described as being in a breech presentation. A transverse lie means the fetus is lying sideways across the abdomen.
Interpreting Growth and Position
The Estimated Fetal Weight (EFW) is plotted on a growth chart to determine its percentile rank. This percentile indicates how the fetus’s size compares to the population average. For example, an EFW in the 50th percentile means the fetus is average size for 36 weeks.
The EFW is used to identify fetuses that are potentially Small for Gestational Age (SGA) or Large for Gestational Age (LGA). A fetus with an EFW below the 10th percentile is typically considered SGA, which may signal a need for closer monitoring or intervention due to potential growth issues. Conversely, a fetus with an EFW above the 90th percentile is considered LGA, which can raise concerns about potential birth complications like shoulder dystocia.
The determination of Fetal Presentation is also a significant finding that directly influences the delivery plan. If the fetus is confirmed to be breech at 36 weeks, the likelihood of it turning spontaneously to the head-down position before labor is low. Options often include a planned cesarean section or attempting an External Cephalic Version (ECV), a procedure where a doctor manually turns the fetus from the outside of the abdomen.
Assessment of the Fetal Environment
A significant portion of the late-term scan focuses on the environment surrounding the fetus, specifically the amniotic fluid and the placenta. The Amniotic Fluid Index (AFI) or the Maximum Vertical Pocket (MVP) are the primary measurements used to assess the volume of amniotic fluid. The AFI is calculated by summing the deepest pocket of fluid found in each of the four quadrants of the uterus, while the MVP measures only the single deepest pocket.
A low fluid volume, known as oligohydramnios (AFI less than 5 cm or MVP less than 2 cm), can signal placental insufficiency or other fetal concerns and may prompt a discussion about delivery timing. Conversely, an excessive amount of fluid, called polyhydramnios (AFI greater than 24 cm or MVP greater than 8 cm), can be associated with complications like preterm labor or fetal swallowing issues.
The placenta is assessed for its location and appearance, ensuring it is positioned safely for delivery. The most critical check is to confirm that the placenta is not covering the cervix. If placenta previa is found at 36 weeks, a scheduled cesarean section is typically necessary to prevent severe bleeding during labor. The sonographer may also observe the placenta for signs of maturity or calcification.