Ultrasound is an important examination for monitoring the baby’s health. Prenatal ultrasound can detect and diagnose various congenital structural malformations in the fetus and is a necessary examination during pregnancy.
Generally, about 4-5 ultrasounds are needed during pregnancy, but the exact number varies from person to person. Ultrasound can monitor fetal development and screen for abnormalities. The doctor will adjust the frequency of examinations based on the pregnant woman’s health and the fetus’s condition.
Generally, the examination can be divided into two stages: the first trimester (5-8 weeks), which is for confirming pregnancy and preliminary screening, and usually involves two ultrasound examinations.
The second trimester (20-26 weeks) involves systematic screening for fetal structural abnormalities with 1-2 ultrasound examinations, and the third trimester (after 37 weeks) includes a final ultrasound examination as an assessment before delivery.
First Ultrasound (5-8 weeks)
It is generally recommended to have the first ultrasound examination between 5 and 8 weeks of pregnancy (approximately 50 days after the last menstrual period).
This ultrasound examination will check the condition of the gestational sac, measure the length of the embryo, and help estimate a more accurate gestational age. If a heartbeat is seen, it indicates that the embryo has a primitive heartbeat, which is an important sign of early embryonic survival.
Another important purpose of early pregnancy ultrasound is to confirm intrauterine pregnancy and rule out the possibility of ectopic pregnancy, as well as ectopic pregnancy and molar pregnancy.
It also helps determine whether it is a singleton or multiple pregnancy, providing a basis for subsequent prenatal checkups and pregnancy management.
Second Ultrasound ( 11-14 weeks)
The second ultrasound examination, usually performed between the 11th and 14th weeks of pregnancy, is often called the “NT scan.”
It measures the width of the nuchal translucency (NT) and determines whether the fetal nasal bone (NB) is visible. Combined with the pregnant woman’s basic information, a risk assessment is conducted. Low-risk cases proceed with routine prenatal checkups, while high-risk cases require prenatal diagnosis.
This examination mainly includes the following:
Measuring NT Thickness: The thickness of the nuchal translucency (NT) is measured via ultrasound. The NT is a fluid-filled area under the skin at the back of the fetal neck, and its thickness is associated with the risk of certain chromosomal abnormalities (such as Down syndrome) and structural malformations.
Observing the Nasal Bone (NB): The nasal bone of the fetus is examined for visibility. Absence or underdevelopment of the nasal bone is also associated with an increased risk of chromosomal abnormalities.
Comprehensive Risk Assessment: The NT value, nasal bone condition, and basic information such as the mother’s age and gestational age are combined to conduct an early risk assessment for Down syndrome and other chromosomal abnormalities. The assessment results are generally categorized as low risk or high risk.
Low-risk pregnant women can continue with routine prenatal checkups; high-risk pregnant women need further prenatal diagnosis (such as non-invasive prenatal testing or amniocentesis).
Studies have shown that performing NT screening at this stage, combined with early Down syndrome screening, can achieve a detection rate of over 80% for fetal Down syndrome, making it one of the important methods for early screening of fetal chromosomal abnormalities.
Third Ultrasound ( 20-24 weeks)
The third ultrasound examination is usually performed between 20 and 24 weeks of gestation and is often referred to as the “major anomaly scan.”
At this stage, the fetal organ structures are largely formed, and the amount of amniotic fluid in the uterus is relatively sufficient, facilitating systematic observation. This is a crucial period for screening for fetal structural abnormalities.
During this period, the general structure of the fetal head, limbs, and organs can be observed, six serious and fatal malformations can be screened, and common structural malformations of the fetal face, nervous system, digestive system, urinary system, and other systems can be screened and judged.
Comprehensive Structural Screening: A systematic scan of the fetal head, face, chest, abdomen, spine, and limbs is performed to assess the normality of the gross structures.
Focus on Excluding Severe Malformations: Screening is focused on six major lethal malformations, including anencephaly, severe encephalocele, severe open spina bifida, severe thoracoabdominal wall defects with evisceration, single ventricle, and lethal skeletal dysplasia.
Detailed Observation of Each System: The fetal nervous system (e.g., cerebrum and cerebellum), digestive system (e.g., stomach bubble and intestines), urinary system (e.g., kidneys and bladder), and other commonly suspected abnormal structures are evaluated one by one.
This examination is the most important and systematic screening among the five key ultrasounds during pregnancy, aiming to detect any potential structural abnormalities promptly.
Because the examination is detailed and involves a systematic process, it typically takes a considerable amount of time.
Fourth Ultrasound ( 28-32 weeks)
The fourth ultrasound examination, usually performed between 28 and 32 weeks of gestation, is commonly known as the “minor anomaly scan” and is a systematic screening in late pregnancy.
This examination is not only an important supplement to the major anomaly scan in mid-pregnancy, but also a crucial step in assessing fetal growth and development and the intrauterine environment.
Some fetal structural abnormalities (such as certain digestive or urinary system problems, or minor cardiac abnormalities) may not be fully visible in mid-pregnancy; this examination can further identify them as the fetus continues to grow.
Building upon the mid-trimester screening, this examination will further examine the structure of the fetal organs, focusing on screening for any morphological abnormalities not detected in the mid-trimester.
Simultaneously, fetal head circumference, abdominal circumference, and femur length will be measured to assess whether the fetal growth is consistent with gestational age and to identify any instances of excessively rapid or slow growth.
This examination will also check the placental location, thickness, and maturity, assess the amniotic fluid volume, and measure the umbilical artery blood flow S/D ratio to determine the adequacy of the umbilical cord blood supply to the fetus. This helps to understand placental function and whether the fetus may be at risk of hypoxia.
Fifth Ultrasound ( 37-40 weeks)
Week 37 of pregnancy is a significant milestone. At this stage, the fetus is fully developed and medically termed a “full-term baby,” meaning the baby is essentially capable of surviving outside the womb.
Ultrasound scans at this time primarily measure key data such as the biparietal diameter, head circumference, abdominal circumference, and femur length to estimate current weight, determine if the size is consistent with gestational age, and predict birth weight.
Furthermore, based on amniotic fluid index and fetal position analysis, crucial information is gathered to help clinicians choose the appropriate delivery method (e.g., vaginal delivery or cesarean section) and develop a delivery plan.
Situations Requiring Additional Ultrasound Scans
In certain specific situations, doctors may recommend increasing the frequency of ultrasound examinations to more closely monitor the mother’s and baby’s condition and ensure a safe pregnancy. This typically includes the following three categories:
1. Pregnant women with high-risk factors: If a pregnant woman has gestational hypertension, diabetes, placenta previa, or is carrying multiples, it is considered a high-risk pregnancy.
In order to promptly assess placental function, fetal development, and the intrauterine environment, doctors will arrange more frequent ultrasound examinations to dynamically track changes in the condition.
2. Abnormal symptoms or signs: If vaginal bleeding, persistent abdominal pain, significantly decreased or suddenly increased fetal movement occurs during pregnancy, it may indicate problems such as placental abruption, fetal hypoxia, or abnormal amniotic fluid volume.
In such cases, an ultrasound examination should be performed as soon as possible to help determine the cause and allow for timely intervention.
3. Fetal development does not match gestational age. When an ultrasound indicates that the fetus is growing slower than expected or larger than expected, it may indicate intrauterine growth restriction, nutritional problems, or gestational diabetes.
Therefore, doctors usually recommend regular ultrasound follow-ups to monitor the fetus’s growth curve and trends, assess its health status, and guide subsequent management.
In these cases, additional ultrasound examinations are intended to ensure safer pregnancy management. All examinations will be strictly controlled within safe limits to ensure no adverse effects on the mother and baby. Please be sure to follow your doctor’s instructions and cooperate in completing the necessary examinations.
Why Is It Necessary to Repeat Prenatal Ultrasound Multiple Times?
This is because fetal growth and development are dynamic processes. During fetal malformation screening, repeated examinations are sometimes required, and a diagnosis can only be made after dynamic observation of fetal growth and development.
Sometimes, due to fetal reasons, certain organs and malformations are not detected, such as a diaphragmatic hernia, which can only be discovered incidentally after the abdominal pressure of the fetus increases and the abdominal contents herniate into the thoracic cavity.
Can Ultrasound Detect Fetal Malformations 100% of the Time?
No. While ultrasound can detect most fetal malformations, it cannot detect all of them. Furthermore, ultrasound examinations are affected not only by the equipment itself but also by the operator’s skill, the fetal position, and the amount of amniotic fluid.
The main purpose of prenatal ultrasound examinations is to observe whether the fetus has obvious malformations, such as those of the limbs, brain, internal organs, and heart.
However, some malformations, such as ear deformities, finger/toe abnormalities, and external genital malformations, cannot be definitively diagnosed by ultrasound.
Is Ultrasound Safe?
Ultrasound is a safe medical examination technique; its safety has been proven through long-term, widespread clinical application and scientific validation.
Ultrasound imaging utilizes sound waves, unlike the radiation used in X-rays or CT scans. It is non-radioactive and will not cause radiation damage to human tissue or genetic material. Under the usual examination time and intensity, the physical effects of ultrasound are extremely weak, far below the safe range that could affect human tissue.
Even for pregnant women, ultrasound examinations during pregnancy are widely recognized as a safe and reliable monitoring method, helping to observe fetal development.
During the examination, the doctor will apply a layer of coupling gel for sound transmission to the skin. This gel has safe ingredients and is harmless to the skin; it can be wiped off after the examination. The entire process is non-invasive and painless; generally, patients only need to cooperate with the doctor by exposing the area to be examined and maintaining a stable posture.
Does Ultrasound Have Any Effect On the Fetus?
Ultrasound examinations during pregnancy are generally safe for the fetus. Ultrasound is a radiation-free imaging method that uses sound waves to observe fetal growth and development, and it is a routine and safe prenatal checkup.
The ultrasound frequencies used in ultrasound examinations are within safe limits and do not produce harmful radiation or significant thermal effects.
Routine prenatal ultrasound examinations are short, with the probe typically in contact with the abdomen for only a few minutes, and the sound wave output intensity is strictly limited by medical standards.
Currently, there is no evidence that standardized ultrasound examinations increase the risk of fetal malformations or affect normal development. In practice, doctors adhere to the principle of “reasonable control and minimization” to ensure that the examination duration and intensity are within safe limits.
