Ultrasound examinations are a crucial tool throughout pregnancy management. As a non-invasive, radiation-free, and highly accurate imaging modality, ultrasound technology allows for dynamic observation and assessment of fetal and maternal health at different stages of pregnancy, providing doctors with crucial clinical evidence.
This article systematically explains the timing, purpose, visual indicators, and key screening tests for ultrasound examinations, based on the three stages of pregnancy—the first, second, and third trimesters. This will help expectant mothers understand ultrasound examinations more clearly and plan accordingly.
1. First Trimester (1-3 Months): Confirming Pregnancy and Preliminary Fetal Assessment
First trimester ultrasounds are typically performed between 6 and 13 weeks of gestation. Most hospitals schedule the first routine ultrasound between 10 and 13 weeks. However, if symptoms such as abdominal pain, lower abdominal distension, or vaginal bleeding occur between 6 and 8 weeks of gestation, doctors may schedule an ultrasound examination earlier to determine if the pregnancy is normal.
Main examination objectives:
Confirm the normality of the intrauterine pregnancy and exclude ectopic pregnancy
Determine embryo viability and the presence of a fetal heartbeat
Determine whether the pregnancy is single or multiple
Estimate gestational age and calibrate the due date
Key points of the examination:
Position and morphology of the gestational sac: In a normal intrauterine pregnancy, the gestational sac should be located in the center or upper part of the uterine cavity, with a clear, regular shape. If the gestational sac is located too low or has an irregular shape, and is accompanied by symptoms such as vaginal bleeding and abdominal pain, the risk of miscarriage should be considered.
Embryo and fetal heart rate: Ultrasound can visualize the embryo and the presence of a fetal heartbeat. A normal fetal heart rate is 120-160 beats per minute. Ultrasound reports often use "present/regular/strong" to indicate a strong and robust fetal heartbeat. If "absent" or "weak" is displayed, further evaluation is required to determine if there is embryonic arrest.
Observation of the yolk sac: In early pregnancy, the yolk sac is a crucial structure supporting embryonic development and should be clearly visible. An enlarged or abnormally shaped yolk sac may also indicate early pregnancy problems.
Second Trimester (4-6 Months): A Critical Period for Systematic Fault Screening and Developmental Monitoring
The second trimester is a crucial period for rapid development and formation of fetal organ systems. A systematic ultrasound examination, commonly known as a "major fetal screening," is recommended between 20 and 24 weeks.
Main Purposes of the Examination:
Assess fetal development consistent with gestational age
Check the normality of the fetal vital organs and systems
Determine the position of the placenta and rule out placenta previa
Monitor amniotic fluid volume and umbilical cord condition
Key Examination Points:
Fetal Growth Parameters: Ultrasound measurements will measure parameters such as biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femoral length (FL) to assess overall fetal development. The doctor will also use these data to estimate fetal weight and determine the risk of intrauterine growth restriction (IUGR) or macrosomia.
Placental Position and Maturity: A normal placenta should be attached to the anterior or posterior wall of the uterus, or to the fundus. If it is attached to the lower uterine segment or covering the internal cervical os, it may be placenta previa, a high-risk pregnancy. Pregnant women in this situation require close follow-up and be aware of the risk of antepartum hemorrhage.
Amniotic fluid index (AFI): Amniotic fluid plays an important role in fetal movement and lung development. The normal AFI is 5-18 cm. Too little or too much amniotic fluid should raise concerns about fetal developmental abnormalities or pregnancy complications.
Fetal malformation screening: Systematic ultrasound can observe the fetal nervous system (such as ventricles and brain fissures), facial structures (cleft lip, cleft palate), four-chamber heart structure, kidneys, gastric alveoli, spine, and limb length and symmetry. If structural abnormalities are detected, further MRI or prenatal genetic counseling is necessary.
Spinal and limb development: The spine should be a continuous, hyperechoic line. If the ultrasound report shows "continuous," it indicates normal; if it shows "defective" or "interrupted," it raises concerns about spina bifida or other neural tube defects.
Late Trimester (7-10 Months): Prenatal Assessment to Ensure Safe Delivery
Late trimester ultrasounds are typically performed between 32 and 36 weeks of gestation. They are primarily used to assess fetal health, determine fetal position, estimate weight, and develop an appropriate delivery plan.
Main Purposes:
Assess fetal weight, length, and limb development
Determine whether the fetal position is normal (cephalic, breech, etc.)
Check for placental maturity and function
Assess amniotic fluid status to exclude oligohydramnios/polyhydramnios
Determine whether the umbilical cord is wrapped around the neck
Key Examination Points:
Fetal Weight Estimation: By measuring parameters such as head circumference, abdominal circumference, and femur length, the ultrasound system can roughly estimate fetal weight, providing a reference for the physician to determine whether a vaginal delivery or a cesarean section is appropriate.
Placental Maturity Grading: The placenta is generally graded on an ultrasound report from 0 to 3. A grade III placenta after 36 weeks of gestation indicates near-periodic placental function, potentially leading to insufficient oxygen supply to the fetus. If accompanied by decreased amniotic fluid, a comprehensive evaluation by the physician is warranted to determine whether early intervention is warranted.
Monitoring Amniotic Fluid Volume: An AFI below 8cm in late pregnancy is cause for concern. A figure below 5cm indicates severe oligohydramnios, potentially endangering the fetus's life and typically requires hospitalization for further monitoring or termination of pregnancy.
Evaluating Nuchal Cord: Nuchal cord wrapping can be clearly visualized on ultrasound, typically marked with a "V" to indicate one wrap and a "W" to indicate two wraps. While most fetuses with nuchal cord wrapping can be delivered safely, doctors will frequently monitor fetal heart rate during delivery to prevent complications due to insufficient oxygen supply.
Fetal Position: The normal fetal position is occiput anterior (head down), which is most conducive to vaginal delivery. If ultrasound reveals abnormal fetal positions such as breech, transverse, occiput posterior, or face-up, doctors may recommend external version or a direct cesarean section.
Summary: Ultrasound is a crucial "life monitor" during pregnancy and should not be neglected at any stage.
Ultrasound examinations are used throughout pregnancy, from early pregnancy confirmation and gestational age estimation, to mid-term systematic fetal anomaly screening, and finally, late-term fetal position assessment and delivery assessment. Ultrasound provides pregnant women and doctors with detailed and intuitive health data, helping to identify abnormalities promptly and enable informed intervention.
While many worry about whether frequent ultrasounds may harm the fetus, modern ultrasounds emit non-ionizing radiation, which is harmless to both mother and baby. Especially when performed by qualified physicians in reputable hospitals, ultrasounds are completely reliable.
