An OB ultrasound (short for obstetric ultrasound) is an imaging exam that uses sound waves to create a picture of a baby, placenta, and uterus during pregnancy. It involves no radiation and poses no known risk to the mother or fetus. Most pregnant people will have at least one, typically between 18 and 22 weeks, though many will have two or more depending on how the pregnancy progresses.
What It’s Used For
At its core, an OB ultrasound answers a few big questions: Is the pregnancy developing normally? How far along are you? Is there one baby or more than one? Beyond that, the specific purpose depends on when during pregnancy the scan takes place.
In the first trimester, ultrasound is used to confirm a pregnancy is in the uterus (rather than ectopic), establish a due date, detect a heartbeat, and identify twins or triplets. Later in pregnancy, the focus shifts to checking the baby’s physical development, estimating size and weight, measuring amniotic fluid levels, and locating the placenta. If a problem is suspected at any point, such as poor growth or abnormal bleeding, a targeted ultrasound can be ordered to investigate.
Two Types of Scans
There are two main approaches: transabdominal and transvaginal. For a transabdominal scan, a technician moves a handheld probe across your belly using a layer of gel. This is the version most people picture when they think of a pregnancy ultrasound. It works well in the second and third trimesters when the baby is large enough to image clearly through the abdomen. You may be asked to drink water beforehand so your bladder is full, which helps create a clearer image by pushing the uterus into a better position.
A transvaginal scan uses a smaller, wand-shaped probe inserted into the vagina. Because the probe sits closer to the uterus and ovaries, it produces sharper images in early pregnancy when the embryo is still tiny. It also avoids interference from abdominal fat and intestinal gas. You don’t need a full bladder for this type. Transvaginal ultrasounds are standard in the first trimester and for evaluating complications like ectopic pregnancy or threatened miscarriage.
First Trimester Ultrasound
Not every pregnancy includes a first trimester scan, but it’s common. The primary goals are confirming a heartbeat, dating the pregnancy, and ruling out ectopic pregnancy. During the earliest weeks, the embryonic heart rate is a key sign of viability. Before about 6 weeks, a heart rate of at least 100 beats per minute is considered normal. By 6 to 7 weeks, that threshold rises to 120 beats per minute.
Between 11 and 13 weeks, you may be offered a specialized scan called a nuchal translucency screening. This measures a small pocket of fluid at the back of the baby’s neck. The measurement, combined with a blood test, helps estimate the risk of certain chromosomal conditions like Down syndrome. For the measurement to be accurate, the baby needs to be in a specific position, and the crown-to-rump length should fall between 45 and 84 millimeters. The timing window is narrow, closing at 13 weeks and 6 days.
The Anatomy Scan
The anatomy scan, performed between 18 and 22 weeks, is the most comprehensive OB ultrasound most people will have. This is the one where the technician methodically examines the baby from head to toe. If the baby cooperates, it’s also when you can find out the sex.
The scan evaluates a long list of structures. On the baby’s head, the technician measures the diameter and circumference of the skull and checks brain structures. The face is examined for proper eye spacing, a normal nose, and an intact upper lip. The heart gets special attention through multiple views that assess its chambers, valves, and the major blood vessels leaving it. Heart rate and rhythm are recorded.
In the abdomen, the technician checks the umbilical cord insertion, the bladder, and both kidneys. The spine is imaged in three planes to look for defects. All twelve long bones (arms and legs) are visualized, and the hands and feet are examined separately. Four standard measurements, the head diameter, head circumference, abdominal circumference, and thigh bone length, are taken to estimate the baby’s size and growth.
The anatomy scan also looks beyond the baby. The placenta’s location matters because a placenta covering the cervix (placenta previa) affects delivery planning. The volume of amniotic fluid is measured since too much or too little can signal a problem.
Later Pregnancy Scans
In the third trimester, ultrasounds are typically ordered only when there’s a specific concern. If your provider suspects the baby isn’t growing well, serial growth scans can track weight gain over time. If you’re past your due date or have a high-risk condition, you may receive a biophysical profile, which scores five indicators of fetal well-being: body movements, muscle tone, breathing motions, amniotic fluid volume, and heart rate patterns. Each component gets a score of 0 or 2, for a maximum of 10. The observation period lasts up to 30 minutes to give the baby enough time to demonstrate normal activity.
A limited ultrasound may also be done during labor to quickly check the baby’s position (head down or breech) or to investigate sudden vaginal bleeding.
3D and 4D Ultrasound
Standard OB ultrasounds produce flat, two-dimensional images. 3D ultrasound compiles multiple 2D images into a three-dimensional picture, offering more surface detail. 4D adds the element of real-time movement, essentially a 3D video. These are not routine. Their main medical value is in diagnosing specific structural abnormalities, particularly cleft lip, which is easier to identify when you can see the surface of the face in three dimensions. 4D imaging can also help evaluate musculoskeletal and heart abnormalities by showing how structures move.
Many commercial studios offer 3D and 4D scans as a keepsake experience. While the technology itself is the same used in medical settings, medical guidelines emphasize that ultrasound should be used when it serves a clinical purpose, not purely for entertainment or souvenir photos.
What the Experience Is Like
A standard OB ultrasound takes roughly 20 to 45 minutes, depending on the baby’s position and how many structures need to be evaluated. The anatomy scan tends to run longer. You’ll lie on an exam table, and the sonographer will apply warm gel to your abdomen before moving the probe. There’s no pain involved, though the pressure on a full bladder can be uncomfortable during early transabdominal scans.
The sonographer typically takes measurements and captures images quietly, then a radiologist or your OB provider reviews them. Don’t be alarmed if the sonographer doesn’t narrate everything or seems focused. They’re working through a systematic checklist. Your provider will go over the results with you, sometimes the same day, sometimes at a follow-up appointment. If something looks unusual on the scan, you may be referred for a more detailed ultrasound at a maternal-fetal medicine specialist’s office.