Obstetric ultrasound is a non-invasive imaging diagnostic technique primarily used to assess fetal development during pregnancy, confirm pregnancy location, and monitor maternal health status. This examination utilizes high-frequency sound waves to produce real-time images, safely observing the intrauterine environment and providing clinicians with critical information such as fetal growth stages, organ development, and placental positioning.
The main purpose is to detect abnormalities early, such as ectopic pregnancy, fetal structural anomalies, or pregnancy complications, and to track pregnancy progress. The procedure is painless and non-invasive, regarded as a standard prenatal care process, applicable from early pregnancy diagnosis to pre-delivery stages.
Ultrasound examinations are mainly divided into transabdominal and transvaginal types. Transabdominal ultrasound requires applying gel on the lower abdomen before scanning and is suitable for mid to late pregnancy; transvaginal ultrasound involves inserting the probe into the vagina, providing clearer images during early pregnancy (6-12 weeks). 3D/4D ultrasound can create three-dimensional models to assist in evaluating fetal facial or cardiac structures.
The sound waves penetrate tissues and reflect back signals, which are converted into real-time images by a computer. Different tissue densities produce varying grayscale contrasts, allowing clinicians to identify fetal bones, internal organs, and amniotic fluid volume. Doppler technology measures blood flow dynamics to evaluate placental function or fetal oxygenation status.
Primary indications include confirming gestational age, determining fetal number, screening for fetal anomalies, and diagnosing uterine abnormalities. Ultrasound is the first-choice examination when fetal growth restriction, polyhydramnios/oligohydramnios, or placental previa are suspected.
The examination is usually performed by obstetricians or professional ultrasound technicians. The duration depends on the purpose of the scan, generally about 15-30 minutes per session. Early pregnancy may require a full bladder for clearer imaging, while mid to late pregnancy requires positioning adjustments to scan the fetus comprehensively.
The frequency of examinations depends on individual needs. Standard prenatal checks typically include early screening at 11-14 weeks, structural scans at 20-24 weeks, and fetal growth assessments after 32 weeks. There is no fixed "dose" limit, but the ALARA (As Low As Reasonably Achievable) principle should be followed.
The primary advantage is providing immediate diagnostic information through real-time imaging, allowing for the prompt detection of issues such as uterine fibroids, fetal skeletal abnormalities, or amniotic fluid problems. Its lack of ionizing radiation makes it the preferred safe imaging modality during pregnancy.
Currently, there is no conclusive evidence that diagnostic ultrasound causes harm to the fetus, but unnecessary "keepsake" scans should be avoided. Rarely, prolonged scanning may cause localized tissue heating, but routine clinical scans do not exceed safety limits.
Important Note: The procedure should be performed by trained professionals. Frequent scans without medical necessity may increase unnecessary psychological stress.
Contraindications include allergies to gel or cases where the examination cannot be tolerated. When ectopic pregnancy is suspected, immediate ultrasound is necessary. However, prolonged scans for multiple pregnancies without medical indication should be avoided. Pregnant women with diabetes or high BMI may require increased ultrasound frequency to penetrate more tissues.
Ultrasound itself does not interact with other treatments, but if combined with medications such as anticoagulants, ultrasound may be used to assess placental location. Before and after invasive procedures like amniocentesis, ultrasound provides real-time guidance and safety assessment.
Large-scale studies worldwide have confirmed that systematic ultrasound screening can reduce the missed detection rate of major structural anomalies by 20-30%. Detailed scans at 18-22 weeks have an accuracy rate of over 90% for detecting brain, cardiac, and spinal abnormalities, becoming a standard component of prenatal screening.
The 2018 WHO guidelines recommend nasal bone measurement and nuchal translucency (NT) scans at 11-14 weeks to effectively assess Down syndrome risk. Four-dimensional ultrasound has been supported by multicenter studies for its accuracy in diagnosing congenital heart defects.
Alternatives include MRI scans (used when ultrasound results are inconclusive), but MRI is more costly and not suitable for immediate use. X-ray examinations are contraindicated during pregnancy due to radiation exposure. Traditional palpation provides limited information and cannot replace imaging. Genetic testing (such as non-invasive DNA testing) can supplement genetic risk assessment but cannot replace structural evaluation. In certain cases, Doppler blood flow studies may be needed to assess placental function.
Do I need special preparations before a prenatal ultrasound?
It is generally recommended to drink 400-500 ml of water and avoid urinating beforehand to fill the bladder, which enhances abdominal ultrasound imaging clarity. However, in mid to late pregnancy or for transvaginal scans, this step may not be necessary. Specific preparations should follow the instructions of your healthcare provider.
Will the procedure cause pain or discomfort?
Most prenatal ultrasounds are painless and non-invasive. The probe may feel warm or exert gentle pressure on the skin, but it does not cause tissue damage. If the scan takes longer, communicate with medical staff to adjust positions and ease discomfort.
If an ultrasound shows fetal abnormalities, what should I do next?
When abnormalities are suspected, the doctor may recommend additional detailed anatomical scans or special angle scans for confirmation. Further tests such as genetic counseling or amniocentesis may be arranged, along with professional medical advice to guide decision-making.
How does ultrasound frequency vary across different pregnancy stages?
The first trimester ultrasound confirms fetal heartbeat and estimates due date; the second trimester involves comprehensive organ structure assessments; the third trimester monitors fetal position, amniotic fluid volume, and growth curves. High-risk pregnancies may require more frequent scans.
Is long-term exposure to multiple ultrasounds harmful to the fetus?
Current medical research indicates that diagnostic ultrasound used within standard parameters does not cause thermal effects or biological harm, making it a safe imaging technique. However, excessive use of non-medical 3D/4D keepsake videos should be avoided to reduce unnecessary exposure.