What it is (overview)

An obstetric ultrasound (also called a pregnancy scan, baby scan, or obstetric scan) is a safe, noninvasive imaging test that uses sound waves to create pictures of the uterus, placenta, amniotic fluid, and the developing baby (fetus). Unlike X-rays, ultrasound does not use ionizing radiation. The scan may be done across the abdomen (transabdominal) and, especially early in pregnancy, sometimes with a small internal probe (transvaginal) to get clearer images.

Depending on the stage of pregnancy and the reason for the exam, an obstetric ultrasound can help measure and assess:

Gestational age and due date (dating): Measurements such as crown–rump length in early pregnancy can help estimate how far along the pregnancy is and refine the estimated due date.

Fetal growth and development: Later scans may measure the baby’s head, abdomen, and thigh bone to estimate growth and fetal weight trends over time.

Number of babies and location: Confirms a single pregnancy vs. twins/multiples and helps determine where the pregnancy is located in the uterus.

Heartbeat and general well-being: Confirms fetal cardiac activity and can support assessment of overall fetal health.

Placenta and amniotic fluid: Evaluates placenta location (including whether it may cover the cervix) and estimates amniotic fluid levels.

Basic anatomy and possible abnormalities: A mid-pregnancy “anatomy scan” looks at major structures (brain, spine, heart, kidneys, limbs, and more) to screen for structural concerns.

Cervix and uterus: In some situations, it can help assess cervical length or look for uterine issues that may affect pregnancy.

What the results mean: Most results describe whether findings are within the expected range for the stated gestational age (for example, growth measurements matching the due date, normal amniotic fluid, and a normally positioned placenta). If something is “out of range,” it doesn’t always mean there is a serious problem—dates may be off, the baby may simply be small/large, or images may be limited by fetal position. Your clinician may recommend a follow-up ultrasound, additional monitoring, or other tests depending on what is seen.

When & why it's usually done

An obstetric ultrasound is routinely used during pregnancy and may also be ordered urgently when symptoms suggest a possible complication. Common reasons include:

Confirming an early pregnancy: To verify an intrauterine pregnancy, confirm viability (heartbeat), and estimate gestational age—especially when the last menstrual period is uncertain or cycles are irregular.

Bleeding or cramping in pregnancy: To evaluate causes of first-trimester bleeding, pelvic pain, or cramping and to check for concerns such as miscarriage risk, ectopic pregnancy, or subchorionic hematoma.

Dating and routine screening: A dating scan in early pregnancy and a mid-pregnancy anatomy scan (often around 18–22 weeks) are common parts of prenatal care. Some people also have a third-trimester scan to check growth, position, placenta, or fluid levels.

Checking fetal growth: If fundal height measurements differ from expected, or if there are risk factors for growth restriction or a large baby, ultrasound helps track growth over time.

Assessing fetal position: To determine if the baby is head-down (cephalic) vs. breech or transverse—especially later in pregnancy.

Placenta concerns: If there is bleeding, a low-lying placenta is suspected, or a prior scan showed placenta previa or other placental findings.

Amniotic fluid concerns: To evaluate possible low or high amniotic fluid suggested by symptoms (leaking fluid) or clinical measurements.

High-risk pregnancy monitoring: More frequent pregnancy scans may be recommended with conditions such as diabetes, high blood pressure, preeclampsia risk, autoimmune disease, kidney disease, prior pregnancy complications, multiple gestation (twins), or reduced fetal movement.

Follow-up of earlier findings: If a prior scan suggests an anatomic concern or “soft marker,” a targeted ultrasound, fetal echocardiogram, or additional testing may be advised.

Common diseases related to it

• Ectopic pregnancy
• Threatened miscarriage and early pregnancy loss
• Subchorionic hematoma
• Multiple pregnancy (twins or higher-order multiples)
• Fetal growth restriction (intrauterine growth restriction, IUGR)
• Macrosomia (large-for-gestational-age baby)
• Placenta previa / low-lying placenta
• Placental abruption (assessment support in the right clinical setting)
• Oligohydramnios (low amniotic fluid) and polyhydramnios (high amniotic fluid)
• Congenital anomalies detectable by ultrasound (e.g., neural tube defects, certain heart/kidney/limb abnormalities)
• Breech or transverse presentation
• Cervical insufficiency / short cervix (when cervical length is measured)

Health goals where it may help

• Confirming pregnancy location and gestational age to plan appropriate prenatal care
• Tracking fetal growth and development across pregnancy
• Screening for structural concerns and guiding next-step testing when needed
• Monitoring placenta position and amniotic fluid to reduce complications
• Supporting safer delivery planning by assessing fetal position (head-down vs. breech) and growth
• Enhanced monitoring for high-risk pregnancies (e.g., diabetes, hypertension, twins)
• Providing reassurance and timely evaluation when symptoms occur (bleeding, pain, reduced fetal movement)