Key Landmarks in Second Trimester Ultrasound: What Are They and Why Do They Matter?

Understanding the significance of each key landmark in a second trimester ultrasound scan is crucial for both healthcare professionals and expectant parents. This scan, typically conducted between 18–22 weeks gestation, is a comprehensive examination designed to assess foetal development, health, and growth. Conducted with a high-frequency transducer and primarily utilising a transabdominal approach, the second trimester ultrasound is an essential step in prenatal care, as it allows sonographers to check critical structures and measurements of the developing foetus.

In this article, we will go through each important landmark and explain why it matters. The guide includes detailed attributes and a systematic protocol checklist, ensuring nothing vital is missed in the scan.

What Are the Key Landmarks in a Second Trimester Ultrasound?

Each foetal landmark has unique characteristics and purposes, and knowing them is key to performing an effective scan. Let’s explore the most crucial landmarks:

1. Biparietal Diameter (BPD)

The Biparietal Diameter (BPD) is one of the primary measurements taken during a second trimester ultrasound. It is obtained by measuring the distance between the two parietal bones of the foetal skull. BPD is particularly helpful in estimating gestational age, especially when conducted alongside other measurements. According to standard guidelines set by the International Society of Ultrasound in Obstetrics and Gynaecology (ISUOG), a precise alignment of the skull is required to avoid any miscalculation.

1. Head Circumference (HC)

Head circumference is another critical measurement that assesses the growth of the foetus. Unlike BPD, which focuses on a single dimension, HC offers an overall view of the skull’s structure. To ensure accuracy, the skull should have a smooth, oval shape when scanned, without any abnormalities or asymmetries. Accurate measurement of the HC is integral for estimating foetal growth and identifying potential issues in head development.

1. Abdominal Circumference (AC)

The Abdominal Circumference (AC) is used to calculate foetal weight and assess growth patterns. It is measured at the level of the stomach and spine, ensuring the sonographer has a clear, unobstructed view. AC measurements are vital for detecting any signs of restricted growth or, conversely, signs of macrosomia (overly large growth). Consistent growth patterns and accurate measurements play a vital role in evaluating the baby’s health in utero.

1. Femur Length (FL)

Femur Length (FL) measures the longest bone in the foetus’s body and provides insights into skeletal development. The measurement is taken from the head to the distal end of the femur and serves as an additional parameter for estimating gestational age. Femur length also helps identify skeletal disorders and growth issues, making it a crucial part of the second trimester ultrasound checklist.

1. Spine

The foetal spine is assessed in both longitudinal and transverse views to detect possible abnormalities such as spina bifida or other spinal conditions. The spine’s alignment and closure are carefully examined, as they are significant markers of healthy foetal development. Additionally, assessing the spine’s integrity ensures that there is no open spinal defect, which can pose risks post-birth.

1. Heart (Four-Chamber View)

The four-chamber view of the foetal heart is essential for detecting structural abnormalities. It provides a clear view of the heart’s ventricles and atria, ensuring that blood flow is well-balanced and that there are no issues with the cardiac structure. Proper visualisation of all chambers is necessary to confirm normal heart development, and any deviation can indicate a need for further investigation by a specialist.

1. Kidneys

The foetal kidneys are examined to ensure proper renal development and detect any signs of obstruction. The kidneys are visible in transverse and longitudinal planes, appearing on either side of the spine. Renal function is vital, as it contributes to amniotic fluid levels, a key factor in foetal wellbeing. Poorly developed kidneys may indicate complications that can be addressed postnatally.

1. Stomach and Bladder

Both the stomach and bladder are checked for normal positioning, size, and function. The stomach appears as a dark, circular structure in the scan, indicating that the foetus is swallowing amniotic fluid, a sign of good gastrointestinal function. The bladder, similarly, should appear filled during the scan, signalling normal urinary function.

1. Placenta

The position of the placenta is monitored in relation to the cervix to rule out any cases of placenta previa (when the placenta covers the cervix). Placenta previa can lead to complications during delivery and, if detected early, can prompt additional care measures. Placental location and health are both key markers in second trimester ultrasound protocols.

1. Amniotic Fluid Levels

Amniotic fluid serves as a protective buffer for the foetus and helps with lung and muscle development. The ultrasound measures the amount of amniotic fluid surrounding the foetus to ensure it is within the normal range. Both low (oligohydramnios) and high (polyhydramnios) levels of amniotic fluid are flagged for further observation as they can indicate issues with foetal development or maternal health.

Why Are Measurements So Critical in a Second Trimester Ultrasound?

Accurate measurements are central to assessing the foetus’s health and growth. Each measurement allows the sonographer to compare the foetus’s size and development against standard growth charts for the given gestational age. By systematically measuring landmarks like BPD, HC, AC, and FL, any irregularities or risks are detected early, allowing healthcare providers to develop a care plan tailored to the mother and baby’s needs.

What Does a Second Trimester Ultrasound Protocol Checklist Look Like?

A systematic approach is vital to ensure all critical aspects are covered in the second trimester ultrasound. Here’s a sample checklist for sonographers to follow:

1. Patient Preparation
   • Brief patient on the steps involved in the scan.
   • Ensure patient is comfortable, especially if a full bladder is required.
2. Transducer Setup
   • Use a high-frequency transducer (3-5 MHz), adjusting settings based on the patient’s needs.
3. Systematic Landmark Identification
   • Head: BPD and HC measurements, assessing shape and symmetry.
   • Abdomen: AC measurement, noting stomach and spine alignment.
   • Limbs: FL and other long bones as needed.
   • Spine: Longitudinal and transverse views.
   • Heart: Four-chamber view with ventricles and atria visible.
   • Other Organs: Kidneys, bladder, and stomach position and function.
4. Placental and Amniotic Assessment
   • Assess placental location and ensure it’s clear of the cervix.
   • Measure amniotic fluid to identify any irregularities.
5. Documentation
   • Record all measurements and images.
   • Note any findings requiring follow-up.

Key Measurements and Landmarks Table

Landmark/Measurement	Gestational Range (Weeks)	Purpose	Key Features
Biparietal Diameter	18–22	Foetal age estimation	Measurement between parietal bones
Head Circumference	18–22	Growth assessment	Skull outline should be oval-shaped
Abdominal Circumference	18–22	Foetal weight estimation	Measured at the level of the stomach and spine
Femur Length	18–22	Skeletal assessment	Long bone measured from head to distal end
Heart (Four-chamber view)	18–22	Structural heart assessment	Clear view of ventricles and atria
Spine	18–22	Detection of anomalies	Longitudinal and transverse views
Kidneys	18–22	Renal development check	Located on either side of the spine
Stomach	18–22	GI tract assessment	Visualized as a dark, circular structure
Bladder	18–22	Urinary system check	Normally appears filled in ultrasound
Placenta	18–22	Placental health	Positioned relative to the cervix
Amniotic Fluid	18–22	Foetal wellbeing indicator	Assessed around the foetus; measures fluid pockets

How Is a Second Trimester Ultrasound Interpreted?

After the ultrasound is completed, the findings are reviewed by a radiologist or obstetrician specialising in prenatal imaging. This analysis includes interpreting the foetal growth, placental position, and organ development. If abnormalities are detected, further monitoring or diagnostic procedures may be advised, such as additional scans or referrals to specialists.

Border Radiology Specialists provides reliable and accurate second trimester ultrasound services. We prioritise quality imaging and follow best practices, ensuring a comprehensive view of your baby’s development at this crucial stage. For a trusted ultrasound provider, reach out to our expert team.

References

1. International Society of Ultrasound in Obstetrics and Gynaecology (ISUOG). Second Trimester Ultrasound Guidelines
2. Royal Australian and New Zealand College of Obstetricians and Gynaecologists. Routine Antenatal Care

Frequently Asked Questions (FAQs)

1. What’s the ideal timing for a second trimester ultrasound?
   The scan is typically performed between 18–22 weeks to obtain detailed foetal anatomy and measurements.

1. What conditions can a second trimester ultrasound detect?
   It can detect anatomical abnormalities, monitor growth, and assess organs such as the heart, kidneys, and brain.

1. Is the second trimester ultrasound mandatory?
   While not mandatory, it’s recommended as it provides crucial information about foetal development.

1. Can the gender of the baby be determined in a second trimester ultrasound?
   Yes, gender can often be determined around this time if the baby’s position is favourable.

1. How accurate are the measurements taken in this scan?
   Measurements are typically accurate but may vary based on foetal position and equipment quality.

1. What follow-up actions are taken if abnormalities are detected?
   Further diagnostic tests or specialist referrals may be recommended to provide a clear picture of any potential issues.