Most pregnant women receive at least two ultrasounds: one between 10 and 13 weeks and another between 18 and 22 weeks. These two scans form the baseline for low-risk pregnancies, though your provider may schedule additional imaging depending on your health, your baby’s development, or specific risk factors like carrying multiples.
First Trimester: 7 to 13 Weeks
Your first ultrasound typically happens somewhere between 10 and 13 weeks, though some providers will schedule one as early as 7 weeks if there’s uncertainty about your due date, a history of miscarriage, or symptoms like bleeding. At 7 weeks, the embryo measures only about 5 to 9 millimeters, and the fluid-filled gestational sac surrounding it is roughly 18 to 24 millimeters. Even at this early stage, a heartbeat is often visible, sometimes beating at 110 beats per minute or faster.
Early scans use a transvaginal probe rather than the familiar wand-on-the-belly approach. The embryo is simply too small and too deep in the pelvis for an abdominal ultrasound to capture clearly. By the second and third trimesters, abdominal scanning takes over for most imaging.
The technician measures the distance from the top of the head to the bottom of the torso (crown-rump length) to estimate gestational age. This measurement is the most accurate way to set or confirm a due date, more reliable than counting from your last period. The scan also confirms the pregnancy is developing inside the uterus and checks whether you’re carrying one baby or more.
Nuchal Translucency Screening: 11 to 13 Weeks
Between 11 weeks and 13 weeks and 6 days, you may be offered a specialized scan that measures a small pocket of fluid at the back of your baby’s neck. This measurement, combined with a blood test, helps estimate the chance of certain chromosomal conditions. The baby needs to have a crown-rump length between 45 and 84 millimeters for the measurement to be accurate, which is why the timing window is narrow.
If this measurement comes back larger than expected or if another abnormality is spotted, your provider will typically offer genetic counseling and the option of diagnostic testing. The American College of Obstetricians and Gynecologists recommends that all pregnant patients be offered prenatal genetic screening regardless of age or risk level.
The Anatomy Scan: 18 to 22 Weeks
The second standard ultrasound is the most comprehensive one you’ll have. Often called the anatomy scan or level II ultrasound, it’s a systematic head-to-toe evaluation of your baby’s developing body. This is the appointment that tends to run longest, often 30 to 45 minutes, because the sonographer is working through an extensive checklist.
For the brain, three standard cross-sectional views are captured to assess the skull shape, brain structures, and the fluid-filled spaces within the brain. The face is examined from multiple angles to visualize the eye sockets, nose, and mouth separately. Moving down, the sonographer checks that the heart is positioned on the left side of the chest and evaluates its internal chambers and valves. Both sides of the diaphragm are visualized, and the lung fields are inspected for any cystic or unusually bright areas.
In the abdomen, the technician measures the circumference, images the umbilical cord insertion point to rule out abdominal wall defects, checks the kidneys in two planes, and confirms urine in the bladder. The spine is examined along its full length in three different orientations. All twelve long bones (upper arms, forearms, thighs, and lower legs) are individually visualized, and both hands and feet are imaged, with care taken to see left and right sides separately.
Beyond the baby, the sonographer evaluates the placenta for its position relative to the cervix, looks for any signs of bleeding or abnormal attachment, and checks for additional placental lobes. The cervical length may also be measured. Four key measurements of the baby (head width, head circumference, abdominal circumference, and thigh bone length) are recorded to confirm growth is on track.
Third Trimester Scans: Not Routine, but Common
A third trimester ultrasound isn’t standard for every pregnancy, but many women end up having one. Your provider may order imaging after 28 weeks to check whether the baby is growing appropriately, especially if your belly is measuring larger or smaller than expected. These scans assess fetal size, the volume of amniotic fluid, the baby’s position (head-down or breech), and placental location.
Certain situations make third trimester imaging particularly important. If you had a prior cesarean delivery or uterine surgery and your placenta is sitting low, providers need to evaluate for a condition where the placenta grows too deeply into the uterine wall. The risk of this increases with each prior cesarean. Low-lying placentas and abnormal blood vessel positions near the cervix also warrant follow-up imaging later in pregnancy.
In high-risk pregnancies, your provider may also use Doppler ultrasound to measure blood flow through the umbilical cord and other key vessels. This helps gauge whether the baby is getting adequate oxygen and nutrition.
Biophysical Profiles: Checking Fetal Well-Being
If your provider has concerns about your baby’s health in the third trimester, they may order a biophysical profile. This combines an ultrasound with fetal heart rate monitoring to generate a score out of 10. The ultrasound portion watches for four things over a 30-minute window: at least two distinct body or limb movements, at least one episode where the baby extends and then flexes a limb or opens and closes a hand, at least one episode of breathing-like chest movement lasting more than 20 seconds, and at least one pocket of amniotic fluid measuring more than 2 centimeters deep. The fifth component is heart rate reactivity, assessed through external monitoring. Each component earns 2 points if present, 0 if absent.
Twin and Multiple Pregnancies
If you’re carrying multiples, expect significantly more ultrasounds. The schedule depends on whether the babies share a placenta, which is determined at your first scan around 12 weeks.
- Twins with separate placentas: Scans at 12 and 20 weeks, then every 4 weeks until delivery.
- Twins sharing a placenta but in separate sacs: Scans at 12 and 16 weeks, then every 2 weeks until delivery. These pregnancies carry a risk of unequal blood sharing between the twins, so closer monitoring is essential.
- Twins sharing both a placenta and a sac: The same schedule as above (every 2 weeks from 16 weeks on), with even closer surveillance since there is no membrane separating the babies.
For any type of twin pregnancy, if the babies show a size difference greater than 15%, unequal amniotic fluid levels, or abnormal blood flow patterns, the monitoring interval tightens to weekly scans.
Elective 3D and 4D Imaging
Elective 3D and 4D ultrasounds are not part of routine medical care, but many parents schedule them to see detailed images of their baby’s face. The best window is between 26 and 30 weeks. Earlier than 24 weeks, the baby hasn’t developed enough facial fat for clear features. Later than 34 weeks, there’s less room in the uterus for the baby to move, which limits the angles available. The sweet spot around 27 to 30 weeks offers the best combination of developed features and enough space for the baby to shift into a good position for imaging.