Pregnancy ultrasounds use sound waves to create non-invasive images, allowing medical providers to monitor the developing fetus. This imaging tool assesses growth, checks the baby’s position, and confirms expected milestones throughout gestation. While many people seek a fixed date for the “last” ultrasound, the timing of the final scan is highly variable. It depends almost entirely on the specific medical needs and risk profile of the expectant parent and baby. For many low-risk pregnancies, there is no final scheduled appointment beyond the main structural check.
The Standard Ultrasound Timeline
The typical timeline for ultrasound imaging begins early in the first trimester with the dating or viability scan. This initial scan is usually performed between six and twelve weeks of gestation. Its primary function is to confirm the location of the pregnancy, establish an accurate estimated due date, and determine the number of fetuses present.
The next routine scan is the detailed anatomical survey, which is the most comprehensive structural check. This second routine scan is performed midway through the pregnancy. For many patients, these two appointments make up the entire set of imaging procedures.
This standard approach reserves imaging for specific diagnostic or screening purposes. If the pregnancy progresses without complications, subsequent physical examinations and external measurements provide sufficient information for monitoring fetal health.
The Last Routine Scan: The Anatomy Check
The anatomy scan is often conducted between 18 and 22 weeks of gestation and represents the most detailed structural evaluation. During this appointment, the sonographer systematically examines the baby’s major organs, including the heart, brain, kidneys, and spine, checking for proper formation. Measurements of the head circumference, abdominal circumference, and femur length are taken to track growth parameters against established norms.
This scan also confirms the location of the placenta, ensuring it is positioned safely away from the cervix. For low-risk pregnancies where the results of this structural survey are normal, this 20-week appointment is typically the final ultrasound. The detail collected during this mid-pregnancy scan is sufficient to clear the patient from needing further routine imaging.
Ongoing monitoring relies on external measurements, physical examinations, and listening to the fetal heart rate during regular prenatal visits. Unless a specific concern arises later, a follow-up imaging session is not scheduled, making the anatomy scan the effective “last routine ultrasound” for most healthy pregnancies.
Diagnostic Reasons for Third Trimester Ultrasounds
While the anatomy scan is the last routine appointment, many pregnancies require additional third-trimester ultrasounds due to specific diagnostic needs.
Growth Concerns
One common reason is to investigate a discrepancy between the measured size of the uterus and the estimated gestational age. This leads to a scan to check for suspected Fetal Growth Restriction, or to confirm if the baby is measuring larger than average.
Placental Re-evaluation
Another frequent indication is the re-evaluation of the placenta’s position, especially if the anatomy scan flagged a low-lying placenta or placenta previa. Providers need to confirm whether the placenta has moved away from the opening of the cervix before the due date. This confirmation check is commonly performed closer to 32 to 36 weeks.
Fetal Presentation
Late-term imaging is also used to confirm the baby’s presentation in preparation for delivery. If the baby is suspected of being in a breech (feet or buttocks first) or transverse (sideways) position, an ultrasound provides a clear image of the fetal lie. Knowing the presentation is important for determining the safety of a vaginal delivery.
Maternal Health Conditions
Maternal conditions like preeclampsia, gestational diabetes, or other chronic health issues can necessitate regular ultrasound monitoring of fetal well-being. These specialized scans help assess how the maternal condition is affecting the baby’s environment and growth rate. Diagnostic imaging sessions are scheduled based on the severity of the condition and may be performed weekly or more frequently until delivery.
What the Final Assessment Measures
Any ultrasound performed in the late third trimester focuses on several specific metrics, whether for diagnostic reasons or high-risk surveillance.
The sonographer first calculates the Estimated Fetal Weight (EFW), using measurements of the head, abdomen, and femur to project the baby’s weight. This projection helps inform delivery planning and confirms appropriate growth.
Another important measurement is the Amniotic Fluid Index (AFI). This index measures the overall volume of the fluid surrounding the baby, which is an indirect indicator of placental function and fetal hydration. The scan also confirms the baby’s positioning, ensuring the head is down (cephalic) for a vaginal delivery.
In situations concerning fetal well-being, the provider may order a Biophysical Profile (BPP). This comprehensive test combines the ultrasound assessment with a non-stress test, evaluating five specific parameters:
- Fetal breathing movements
- Body movements
- Muscle tone
- Amniotic fluid volume
- Heart rate reactivity
The results of these final assessments determine the immediate steps for management and the timing of delivery.