An OB scan, short for obstetric scan, is an ultrasound examination performed during pregnancy to check on the developing baby and monitor the health of the pregnancy. Most people will have at least two OB scans: one in the first trimester (around 11 to 14 weeks) to date the pregnancy and one in the second trimester (around 18 to 20 weeks) to check the baby’s anatomy. Some pregnancies require additional scans depending on risk factors or complications.
How an OB Scan Works
An OB scan uses sound waves to create images of the baby, placenta, and surrounding structures inside the uterus. There are two main approaches. A transabdominal scan places a probe on your belly and sends sound waves through the abdominal wall. A transvaginal scan uses a smaller probe inserted into the vagina, which sits closer to the uterus and produces sharper images, especially in early pregnancy.
Transabdominal scans require a full bladder to help transmit the sound waves. You’ll typically be asked to drink at least 24 ounces of clear fluid about an hour before your appointment and avoid using the bathroom until after the exam. For a transvaginal scan, the opposite applies: you empty your bladder right before the procedure. The transvaginal approach doesn’t require a full bladder because the probe is already close to the structures being examined, and it avoids interference from abdominal fat or bowel.
The Dating Scan (First Trimester)
Your first OB scan usually happens between 11 and 14 weeks. Its primary job is to establish how far along you are. The sonographer measures the baby from head to rump (called the crown-rump length), which pins down your due date within about five days. That’s more precise than dating by a second trimester scan, which has a margin of roughly eight days, and considerably more reliable than counting from your last period.
This scan also confirms how many babies you’re carrying, checks for a heartbeat, and screens for certain chromosomal conditions by measuring a small pocket of fluid at the back of the baby’s neck. If the baby is positioned well, the sonographer may attempt an early look at biological sex, but predictions made before 12 weeks are correct only about 54% of the time. Accuracy jumps to around 77% between 12 and 13 weeks, and reaches near 100% after 14 weeks. Most providers will advise against relying on any sex prediction made before 12 weeks.
The Anatomy Scan (Second Trimester)
The most detailed OB scan takes place between 18 and 22 weeks, often called the anatomy scan or anomaly scan. This is a thorough, head-to-toe evaluation of the baby’s developing body. The checklist is extensive:
- Head and brain: The sonographer checks skull shape, brain ventricles, the cerebellum, and the midline structures that divide the two brain hemispheres.
- Face: Eye sockets, lenses, nose, lips, jawline, and the roof of the mouth are examined for cleft lip and other facial differences.
- Heart: Position, size, the four chambers, and the major outflow vessels are assessed. Heart rate and rhythm are also recorded.
- Abdomen: The stomach, kidneys (in two views), bladder, diaphragm, and the spot where the umbilical cord attaches to the belly are all checked.
- Spine and limbs: The length of the thigh bone is measured, along with overall limb movement and positioning.
Beyond the baby itself, this scan evaluates the placenta’s location, how far it sits from the cervix, and where the umbilical cord inserts into it. If the placenta is found to be low-lying (less than 20 mm from the cervix opening), a follow-up scan in the third trimester is typically recommended, since the placenta often migrates upward as the uterus grows. Amniotic fluid levels get a visual check as well.
What the Measurements on Your Report Mean
After an OB scan, you may receive a report filled with abbreviations. The four standard growth measurements are BPD (the width of the baby’s head, measured between the two parietal bones), HC (head circumference), AC (abdominal circumference), and FL (femur length, the thigh bone). Together, these give your provider a picture of whether the baby is growing on track compared to established reference ranges for each gestational week. If any measurement falls outside the expected range, your provider may schedule additional scans to track the trend over time rather than relying on a single reading.
Third Trimester Scans
Not every pregnancy requires a third trimester OB scan, but they’re common when there are concerns about the baby’s growth, fluid levels, or positioning as delivery approaches. One specific type of late-pregnancy assessment is the biophysical profile, typically performed after 32 weeks. It combines an ultrasound with a heart rate monitor and scores the baby in four areas: body movement, muscle tone (opening and closing hands, bending limbs), rhythmic breathing movements, and amniotic fluid volume. Each category receives a score, and the total helps your provider decide whether the baby is doing well or needs closer monitoring.
Low amniotic fluid on a biophysical profile always triggers closer follow-up, even when all other scores look normal. Third trimester scans also recheck placental position if it was low-lying at the anatomy scan and confirm whether the baby is head-down as the due date nears.
Who Needs Extra Scans
Some pregnancies call for more frequent OB scans than the standard two. Twin pregnancies are a common example. With fraternal twins (who each have their own placenta), guidelines recommend a first trimester scan, a detailed second trimester scan, and then a scan every four weeks after that. Pregnancies complicated by certain infections, like parvovirus, chickenpox, or Zika virus, also typically require serial monitoring to watch for effects on the baby. Pregnant adolescents and people at higher risk for fetal anomalies may be offered additional imaging as well.
Safety of OB Scans
Diagnostic ultrasound is widely regarded as safe. Standard B-mode imaging, the type used for routine OB scans, sends short pulses of sound with gaps in between, producing negligible heat in tissue. Under current guidelines, there is no reason on thermal grounds to limit B-mode scanning for any clinical indication, including scans of a normal pregnancy.
Doppler modes, which measure blood flow and use higher acoustic output, do require more caution. Providers aim to keep exposure time short and power levels low when using Doppler, following the ALARA principle: as low as reasonably achievable. Some newer techniques like elastography (which measures tissue stiffness) are not used on a fetus in any trimester because safety data for developing tissues doesn’t exist yet.
3D and 4D Scans
Standard OB scans produce flat, two-dimensional images. 3D ultrasound builds a still, three-dimensional image of the baby, while 4D adds real-time movement so you can see the baby yawning or stretching. In a medical setting, 3D imaging can help clarify facial differences like cleft lip or complex heart defects that are harder to visualize in 2D.
Outside the clinic, many businesses offer elective 3D and 4D scans as a bonding experience or keepsake opportunity. These sessions can be emotionally meaningful, but they aren’t diagnostic. They can’t replace the structured anatomy scan your provider orders, and no medical decisions should be based on their images.