Most pregnant people have at least one ultrasound, typically scheduled between 18 and 22 weeks. But depending on your circumstances, you could have your first scan as early as 6 weeks or additional scans well into the third trimester. Here’s what happens at each stage and why.
Early Pregnancy: 6 to 10 Weeks
A first-trimester ultrasound isn’t standard for every pregnancy, but many providers order one. The main goals at this stage are confirming the pregnancy is in the uterus (not ectopic), checking for a heartbeat, and establishing how far along you are. Dating accuracy is highest in these early weeks: measuring the embryo from head to bottom (called the crown-rump length) pins down your due date to within about five days. That’s more precise than a second-trimester measurement, which has a margin of roughly eight days.
You’re most likely to get this early scan if your periods are irregular, you conceived through fertility treatment, you’ve had a previous miscarriage, or you’re experiencing bleeding or cramping. At 6 weeks, you’ll probably see a tiny flicker of a heartbeat. By 8 or 9 weeks, limb buds and a more defined shape become visible, though it’s still too early to examine organs in any detail.
For very early scans, your provider may use a transvaginal probe rather than the one that glides over your belly. This gives a clearer picture when the embryo is still very small. It’s painless, though it can feel a bit uncomfortable.
Screening for Chromosomal Conditions: 11 to 13 Weeks
Between 11 and 13 weeks, you can opt for a nuchal translucency (NT) scan. This measures the small pocket of fluid at the back of the baby’s neck. A thicker-than-expected measurement can signal a higher chance of chromosomal conditions like Down syndrome or certain heart defects. The timing window is narrow because the fluid gets reabsorbed after 14 weeks, making the measurement unreliable after that point.
At this stage, the fetus measures roughly 45 to 84 millimeters from head to bottom. The NT scan is typically combined with a blood test to give you a more complete risk estimate. It’s a screening, not a diagnosis, so an abnormal result doesn’t mean something is definitely wrong. It means further testing, such as amniocentesis or cell-free DNA screening, may be offered.
The Anatomy Scan: 18 to 22 Weeks
This is the one ultrasound recommended for all pregnancies. Often called the “20-week scan” or level II ultrasound, it’s a systematic head-to-toe examination of the baby’s developing body. The sonographer works through a detailed checklist: brain structures are measured across multiple views, the face is checked for intact lips and proper eye spacing, and each of the four heart chambers is evaluated. The spine, stomach, kidneys, bladder, and both sides of the diaphragm are all visualized. All twelve long bones (upper arms, forearms, thighs, and lower legs) are measured individually.
The scan also evaluates the placenta’s position and your amniotic fluid levels, both of which affect how the rest of your pregnancy is managed. If the placenta is lying low over the cervix, for example, you’ll likely get a follow-up scan later to see if it’s moved.
This is usually the appointment where you can learn the baby’s sex, if you want to know. It’s also the scan most people look forward to because the baby is big enough to look like a baby on screen, but still small enough to fit in a single frame.
What the Anatomy Scan Can and Can’t Find
The anatomy scan picks up about 75% of major structural birth defects overall. That’s a strong detection rate, but it means some conditions are missed, particularly those that develop later in pregnancy or are subtle on imaging. Heart defects, for instance, can be difficult to catch because the structures are tiny and the baby’s position matters. Some findings are also easier to see at certain gestational ages. Only about 28% of anomalies that are eventually detected in utero are identified by 22 weeks, with the rest discovered on later scans. If your provider sees something concerning, they may refer you for a more detailed scan at a specialized center.
Third Trimester: 28 Weeks and Beyond
Routine third-trimester ultrasounds aren’t part of the standard schedule for low-risk pregnancies. Instead, your provider monitors the baby’s growth by measuring your belly (fundal height) at each visit. If that measurement doesn’t line up with your gestational age, a growth scan is ordered to check the baby’s estimated weight, head size, and abdominal circumference.
Other reasons you might get a late-pregnancy ultrasound include checking amniotic fluid levels, confirming the baby is head-down before delivery, or monitoring a placenta that was low-lying earlier. If you develop gestational diabetes, preeclampsia, or other complications, you can expect more frequent scans from this point on.
Biophysical Profiles
If your provider needs a deeper look at how the baby is doing in the final weeks, they may order a biophysical profile. This combines an ultrasound with a heart rate monitor (nonstress test) to assess five things: the baby’s heart rate patterns, breathing movements, body movements, muscle tone, and amniotic fluid volume. Each area is scored on a two-point scale for a total of 10 points. A score of 8 to 10 is considered normal. Six is uncertain and usually means the test gets repeated soon. Four or below is abnormal and may lead to earlier delivery depending on how far along you are.
Twin and High-Risk Pregnancies
If you’re carrying twins, your scan schedule ramps up considerably. How often depends on whether the twins share a placenta. Twins with separate placentas (dichorionic) are generally scanned every four weeks. Twins sharing a placenta (monochorionic) are monitored every two weeks because they face additional risks, including a condition where blood flow between the babies becomes unbalanced. These frequent scans track growth differences and fluid levels to catch problems early enough to intervene.
Other conditions that increase scan frequency include chronic high blood pressure, a history of preterm birth, blood clotting disorders, and pregnancies where the baby has a known anomaly being monitored over time. In these situations, you might have ultrasounds at nearly every prenatal visit during the second and third trimesters.
How to Prepare for Your Scan
For early pregnancy ultrasounds, you’ll often be asked to come with a full bladder. A full bladder pushes the uterus into a better position for imaging and creates a clearer acoustic window. Drinking a few glasses of water about an hour before your appointment and avoiding the bathroom is usually enough. For later scans, when the baby is larger and easier to visualize, a full bladder is less important, but check with your provider’s office if you’re unsure.
Most ultrasounds take 20 to 45 minutes, though the anatomy scan can run longer if the baby is in a tricky position or the sonographer needs additional views. You won’t feel anything beyond the slight pressure of the probe and the cool ultrasound gel on your skin.
Safety of Prenatal Ultrasound
Ultrasound has been used in pregnancy for over 20 years and relies on sound waves, not radiation. It doesn’t carry the risks associated with X-rays or CT scans. That said, ultrasound energy can slightly warm tissues and, in rare cases, create tiny gas pockets in fluid. The long-term effects of these phenomena aren’t fully understood, which is why medical organizations recommend keeping exposure to the minimum needed for a clear diagnostic result, a principle known as ALARA (as low as reasonably achievable).
This is also why the FDA discourages “keepsake” ultrasound sessions at commercial studios. A scan ordered by your provider serves a medical purpose and is performed by trained staff who know how to limit exposure. A boutique session for a 3D video offers no medical benefit while adding unnecessary exposure time.