Detailed Fetal Anatomy Ultrasound (Mid-trimester Anomaly Scan)
What is a detailed fetal anatomy scan and when is it performed?
The mid-trimester anomaly scan, also known as a detailed fetal anatomy scan, is a comprehensive pregnancy ultrasound performed between 18 and 22 weeks of gestation.
This scan is a standard part of antenatal care and is performed to carefully assess your baby’s growth and anatomy, as well as important structures that support a healthy pregnancy.
What Is Assessed During the Anomaly Scan?
During the mid-trimester anomaly scan, your doctor will systematically evaluate the following:
1. Number of Babies and Fetal Heartbeat
– Confirmation of how many babies are present
– Assessment of fetal cardiac activity and heart rate
2. Fetal Growth and Estimated Weight
Your baby’s growth is assessed by measuring the size of the baby’s head and abdomen, as well as the length of their femur (thigh bone). These measurements are used to calculate an estimated fetal weight and confirm that growth is appropriate for the age of the pregnancy.
3. Gestational Age (If Not Previously Confirmed)
If early pregnancy dating was not performed, this scan may help estimate gestational age, and check whether it corresponds to the age of the pregnancy from your menstrual history. However, early pregnancy ultrasound in the first trimester remains the most accurate for dating.
4. Detailed Fetal Anatomy
A structured review of anatomy of the baby is performed, including:
– Head and brain (skull shape and major brain structures)
– Face (orbits, nose and lips)
– Spine (alignment and continuity)
– Limbs (arms, legs, hands, and feet)
– Chest and heart (heart structures and position)
– Abdominal organs, which include the bowels, stomach, kidneys and urinary bladder
– Umbilical cord insertion into the fetal abdomen
– Fetal sex (fetal gender), if the parents wish to know
5. Placenta Assessment
The scan evaluates the placenta for location and appearance. This helps identify low lying placenta or placenta previa.
6. Amniotic Fluid Volume
Amniotic fluid levels are assessed to ensure they are within the normal range.
7. Cervical Length
The cervix is evaluated for shortening or opening, and the cervical length is measured to identify those who may be at risk of preterm birth.
How Long Does the Anomaly Scan Take?
The mid-trimester anomaly scan usually takes 30–40 minutes.
The duration may vary depending on gestational age, maternal body habitus (thickness of the mother’s abdomen) and baby’s position.
To improve visualization, your doctor may ask you to change position or take a short walk, empty your bladder, or have a light snack.
Occasionally, a repeat visit may be needed if certain structures cannot be clearly visualized. This is common and does not necessarily indicate a problem.
Are there any limitations to the anomaly scan?
The mid-trimester anomaly scan provides valuable information about fetal anatomy and can identify many structural abnormalities. However, not all conditions can be detected on ultrasound, and some abnormalities may develop later in pregnancy. A normal scan does not guarantee the absence of all medical conditions. Once the baby is born, he or she will undergo another physical examination by the pediatrician.
Is the anatomy scan the same as Down Syndrome screening?
The purpose of the fetal anatomy scan is to look at the baby’s structure and growth, as well as assess the placenta and amniotic fluid level. Although some babies with Down Syndrome may demonstrate certain abnormalities on ultrasound, many babies may have completely normal ultrasound findings. Therefore, the fetal anatomy scan does not replace genetic screening or diagnostic testing, first trimester screening or any ongoing antenatal care.
References
1. Salomon LJ, Alfirevic Z, Berghella V, et al. ISUOG Practice Guidelines: performance of the routine mid-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2011;37(1):116–26.
2. Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, et al. Williams Obstetrics. 26th ed. New York: McGraw-Hill Education; 2022.
Maternal–Fetal Medicine Specialist in Bangkok
Dr. Narisra Srikureja Firer (Dr. Niti)
Dr. Niti is a Bangkok-based OBGYN with subspecialty training in Maternal–Fetal Medicine (MFM). She provides comprehensive care for women across all stages of life, including pregnancy care, deliveries, and general gynecological conditions.
She dedicates her time to private practice at Ruamjairak Hospital and Mission Hospital, and serves as a Maternal–Fetal Medicine Consultant at a large government tertiary care center.
To read Dr. Niti’s full bio, click here.
