The 20-week anatomy scan is the most comprehensive ultrasound performed during pregnancy, offering a detailed look at the developing fetus and placenta. This scan assesses the baby’s growth, checks the structure of major organs, and confirms the position of the placenta and umbilical cord. For most low-risk pregnancies, this represents the final routine ultrasound. Subsequent ultrasounds after 20 weeks are conditional, depending entirely on medical necessity or specific concerns that arise during routine prenatal care.
The Standard Prenatal Imaging Timeline
Prenatal care for uncomplicated pregnancies relies on two primary ultrasound milestones: the first-trimester dating scan and the 20-week anatomy scan. The initial scan confirms viability and establishes an accurate due date. The 20-week scan provides a detailed survey of the fetal anatomy and placental location. Once the 20-week scan confirms normal findings, the focus of care shifts away from routine imaging.
Standard monitoring during the second half of pregnancy involves non-imaging methods performed at regular appointments. These assessments include measuring the height of the top of the uterus, known as fundal height, to estimate fetal growth, and listening to the baby’s heart rate. This routine surveillance is sufficient for monitoring healthy progression. Further ultrasound imaging is only scheduled if a problem is suspected.
Medical Indications for Follow-Up Imaging
The need for a follow-up ultrasound after 20 weeks is driven by specific medical indications concerning the mother, the fetus, or the pregnancy environment. Maternal health conditions, such as pre-existing Type 1 or Type 2 diabetes or chronic hypertension, are common reasons for ordering additional growth checks. These conditions can affect placental function and impact the baby’s development, necessitating closer monitoring.
Fetal factors often prompt a repeat scan. This includes fundal height measurements suggesting the baby is growing slower or faster than expected, concerns about reduced fetal movement, or the need to re-evaluate a finding from the anatomy scan, such as a soft marker for a genetic condition. Pregnancies involving multiples, such as twins or triplets, inherently require more frequent ultrasounds to track the growth of each fetus.
The state of the placenta and amniotic fluid volume also indicate the need for late-term imaging. If the 20-week scan identified a low-lying placenta (placenta previa), a repeat scan is scheduled around 32 weeks to confirm if it has moved away from the cervix before delivery. Similarly, if a clinical assessment suggests the amniotic fluid volume is too low (oligohydramnios) or too high (polyhydramnios), an ultrasound is necessary to measure the fluid accurately.
Specific Timing and Function of Late-Term Ultrasounds
When a medical indication is present, follow-up imaging is scheduled within specific windows to assess fetal well-being. A common window for a Fetal Growth Scan is between 28 and 32 weeks of gestation. This scan measures the estimated fetal weight (EFW) using biometric measurements, including head circumference, abdominal circumference, and femur length. These measurements are plotted against growth curves to ensure the baby is tracking appropriately.
Later in the third trimester, typically after 36 weeks, a Biophysical Profile (BPP) may be ordered for high-risk pregnancies or when concerns exist about fetal movements. The BPP is a detailed assessment of fetal well-being that combines an ultrasound evaluation with a non-stress test (NST). The ultrasound portion scores four distinct physical parameters:
- Fetal breathing movements.
- Gross body movement.
- Muscle tone.
- The volume of amniotic fluid.
These four ultrasound parameters, along with the NST, are scored to provide a total assessment of the baby’s condition, with a maximum possible score of 10. Late-term imaging may also be used to confirm the baby’s presentation, ensuring they are head-down (vertex) for delivery, or identifying if they are breech. The timing of any subsequent scan is tailored to the specific concern and necessary clinical action.