The anatomy scan is typically performed between 18 and 22 weeks of pregnancy to evaluate the baby’s growth, development, and organ function.
Reaching the halfway mark of pregnancy brings one of the most anticipated prenatal appointments. Parents often call this the “20-week scan,” but the timing can vary slightly depending on your provider and schedule. This detailed ultrasound offers a thorough look at your baby from head to toe. It serves as a medical checkup for the fetus, checking organs, limbs, and overall well-being. Knowing what to expect helps you prepare for this detailed look at your growing baby.
When Is The Anatomy Scan Done?
Medical providers schedule this ultrasound strictly within the second trimester. If you are asking when is the anatomy scan done, the standard window is between 18 and 22 weeks of gestation. This specific timeframe gives the sonographer the best opportunity to view fetal structures clearly.
Before 18 weeks, the baby’s organs may be too small to evaluate properly. The heart structures, in particular, need to be large enough to visualize the chambers and valves. On the other hand, waiting too long after 22 weeks can make the scan difficult. As the baby grows, the bones become denser and cast shadows that block views of internal organs. The baby also becomes more crowded in the uterus, making it harder to get the necessary angles for a complete assessment.
Most clinics prefer to book this appointment right at 20 weeks. This midpoint strikes a balance between size and visibility. Your doctor will likely give you a referral or order for this scan during your 16-week checkup. Booking early is smart, as these appointments are longer than routine checks and slots fill up fast.
Detailed Timing For The Mid-Pregnancy Scan
Understanding the “why” behind the timing helps reduce anxiety. The 18 to 22-week period aligns with significant developmental milestones. By this stage, the major organ systems have formed and are growing in complexity. The sonographer needs this specific developmental window to confirm that the anatomy is forming as expected.
Some factors might push your scan to the earlier or later side of this window. If you have a high BMI, the sonographer might suggest waiting until 22 weeks to improve image clarity. Conversely, if you have a history of preterm labor or cervical issues, they might look earlier. In cases of twins or multiples, the timeline might shift slightly to accommodate the monitoring of multiple babies.
You might wonder if you can combine this with other tests. The anatomy scan is distinct from the NIPT or first-trimester screenings. Those look for genetic probabilities, while this scan looks for physical structural details. They complement each other but serve different purposes in your prenatal care.
Comprehensive Anatomy Checklist
The sonographer follows a rigorous protocol during this hour-long appointment. They do not just snap a few cute pictures; they methodically systematically check every system. This list is extensive because this scan screens for potential birth defects and health conditions.
| Body System | Specific Structures Checked | Why This Matters |
|---|---|---|
| Brain & Head | Ventricles, cerebellum, cavum septum pellucidum, cisterna magna, skull shape. | Checks for fluid buildup, neural tube defects, and proper brain development. |
| Face | Profile, nasal bone, upper lip, eyes/lenses, chin. | Screens for cleft lip, cleft palate, and markers associated with genetic conditions. |
| Spine | Cervical, thoracic, lumbar, sacral vertebrae, skin covering. | Confirms vertebrae alignment and rules out open neural tube defects like spina bifida. |
| Heart | Four chambers, outflow tracts (LVOT, RVOT), heart rate, rhythm, aortic arch. | The most complex part; checks for congenital heart defects and proper blood flow. |
| Abdomen | Stomach bubble, liver, gallbladder, bowel appearance, abdominal wall. | Verifies organs are inside the body and the stomach is filling (swallowing works). |
| Kidneys & Bladder | Presence of two kidneys, renal pelvis size, bladder filling/emptying. | Assesses urinary tract function and amniotic fluid production. |
| Limbs | Femur, humerus, radius/ulna, tibia/fibula, hands, feet, fingers, toes. | Measures bone growth and checks for limb abnormalities or skeletal dysplasia. |
| Placenta & Cord | Placenta location, texture, cord insertion, three-vessel cord (2 arteries, 1 vein). | Rules out placenta previa and confirms the baby receives proper nutrients. |
Preparing For Your Appointment
Preparation for this ultrasound is simple but helps the process run smoothly. You do not need to fast. In fact, eating a light meal or snack 30 minutes before your appointment can help. A boost in blood sugar often wakes the baby up, making them move around. Movement helps the sonographer see different angles of the body.
Hydration is another factor. Many clinics ask you to come with a comfortably full bladder. A full bladder pushes the uterus up and out of the pelvis, providing a clearer window for the sound waves. You do not need to be in pain, but drinking 20-30 ounces of water an hour before can improve image quality significantly. Once the tech checks the cervix and placenta, they often let you use the restroom to empty your bladder for the rest of the scan.
Wear loose, comfortable clothing. A two-piece outfit works best. You will need to lower your waistband to expose your belly, so a dress can be inconvenient. The gel used during the scan is water-based and wipes off, but wearing clothes you don’t mind getting slightly messy is a good idea.
Determining The Biological Sex
For many parents, this appointment reveals the baby’s sex. The sonographer looks at the external genitalia to determine if you are having a boy or a girl. This is usually accurate at 20 weeks, provided the baby is not modest and keeps their legs open. If you want to keep it a surprise, tell the technician immediately when you walk in the room so they can hide that part of the screen.
In some cases, parents of multiples have more questions. For example, wondering if identical twins share the same gender is common. The anatomy scan confirms this by looking at the genitalia of each baby and the placental type. The scan clarifies whether twins share a placenta (monochorionic) or have separate ones, which also aids in determining zygosity.
Common Reasons For A Rescan
It is common to leave the appointment without every single picture needed. Babies are stubborn. If your little one stays curled in a ball or faces your spine, the sonographer cannot see the face or heart clearly. This does not mean something is wrong. It simply means the baby did not cooperate.
In these situations, the clinic schedules a follow-up scan in one or two weeks. This brief appointment focuses only on the missing views. You might also be asked to do jumping jacks, drink cold water, or roll side-to-side during the initial exam to get the baby to shift positions.
Soft markers are another reason for follow-ups. These are small findings that might indicate a higher risk for genetic conditions but are often normal variants. Examples include a bright spot on the heart (echogenic intracardiac focus) or a small cyst in the brain (choroid plexus cyst). Seeing a marker usually triggers a referral for a more detailed Level II ultrasound to take a closer look.
Understanding The Measurements
The sonographer takes several standard measurements to estimate the baby’s weight and gestational age. These numbers are compared to population averages. A baby measuring a week ahead or behind is usually normal. Growth spurts happen, and due dates are estimates. Significant discrepancies might lead to further monitoring.
If you are still unclear on when is the anatomy scan done for growth checks specifically, note that providers might order extra growth scans in the third trimester if the measurements at 20 weeks are very small or very large. The 20-week measurements set the baseline for this growth curve.
Standard Biometric Data Points
Your report will contain acronyms that describe fetal size. Understanding these helps you read the results without confusion.
| Measurement Acronym | Full Name | What It Indicates |
|---|---|---|
| BPD | Biparietal Diameter | The distance across the head from side to side. Helps date the pregnancy. |
| HC | Head Circumference | The distance around the head. confirms brain growth and skull shape. |
| AC | Abdominal Circumference | The distance around the belly. This is the most variable number and reflects the baby’s weight and fat stores. |
| FL | Femur Length | The length of the thigh bone. Used to track longitudinal growth and skeletal health. |
| EFW | Estimated Fetal Weight | A calculation combining BPD, HC, AC, and FL to guess the baby’s current mass. |
| AFI | Amniotic Fluid Index | Measures the pockets of fluid around the baby to ensure kidneys and placenta are working well. |
The Role Of The Sonographer Vs. The Doctor
The person performing the scan is a diagnostic medical sonographer. They are highly trained to capture images and spot abnormalities. However, they generally cannot give you a diagnosis during the scan. They may point out limbs or the heartbeat, but they must remain quiet about medical findings until a radiologist or obstetrician reviews the images.
This silence can feel unnerving. If the tech gets quiet or focuses on one area for a long time, try to stay calm. They often need to take multiple measurements of complex organs like the heart. Capturing a clear image of a moving target takes concentration. After the scan, a doctor reviews the full report and discusses the results with you, usually within 24 hours or immediately if a doctor is on-site.
Safety And Technology
Ultrasound technology uses high-frequency sound waves to create images. It does not use radiation like X-rays. Decades of use in obstetrics show it is a safe tool for monitoring fetal health. The ACOG guidelines for ultrasound support its use for medical purposes. However, medical bodies advise against “keepsake” ultrasounds performed purely for entertainment at non-medical facilities, as these expose the fetus to prolonged energy without medical benefit.
The anatomy scan usually uses 2D imaging. This is the black-and-white, grainy look you are used to. Some medical offices offer 3D or 4D previews if the baby is in a good position, showing the surface of the face. While 3D images are fun for parents, the 2D cross-sections provide the actual medical data the doctor needs to assess internal organs.
Common Timing Concerns
Parents often worry about scheduling conflicts. If you cannot get an appointment exactly at 20 weeks, 19 or 21 weeks is perfectly fine. The window is a guideline, not a wall. The clarity of the images matters most. If you inadvertently schedule too early, the clinic will simply bring you back.
Knowing when is the anatomy scan done helps you plan work leave or childcare. The appointment is lengthy, and policies regarding children in the exam room vary. Many clinics restrict siblings from attending because the sonographer needs extreme focus. Check your clinic’s policy beforehand to avoid having to reschedule.
This mid-pregnancy milestone makes the pregnancy feel real for many parents. Seeing the spine, counting the fingers, and watching the heart beat transforms the abstract idea of a baby into a visual reality. Whether you learn the sex or keep it a surprise, the reassurance of a healthy scan provides peace of mind for the second half of the pregnancy.