Pregnancy Ultrasound: Types, What to Expect & Safety

What Is a Pregnancy Ultrasound Scan?

A pregnancy ultrasound scan (also called a prenatal sonogram) is a non-invasive imaging test that uses high-frequency sound waves to create real-time images of the developing baby, placenta, uterus, and surrounding structures. It is the primary imaging tool used in prenatal care worldwide.

Ultrasound technology works by sending sound waves through the body using a handheld device called a transducer. When these sound waves encounter different tissues — such as bone, fluid, or soft tissue — they bounce back at different speeds. A computer processes these echoes and converts them into a visual image displayed on a monitor. The entire process is painless and does not involve any form of ionizing radiation, making it fundamentally different from X-rays or CT scans.

Obstetric ultrasound has been used in clinical practice since the 1960s, making it one of the most well-established diagnostic tools in modern medicine. Over the past five decades, extensive research involving millions of pregnancies has consistently demonstrated its safety for both the mother and developing baby. The World Health Organization (WHO), the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG), and the American College of Obstetricians and Gynecologists (ACOG) all endorse ultrasound as a standard component of prenatal care.

During a typical prenatal checkup, your healthcare provider may recommend ultrasound scans at specific intervals to monitor the pregnancy's progress. These scans serve multiple purposes: confirming the pregnancy is viable, estimating the gestational age and due date, checking for multiple pregnancies (twins or more), assessing fetal growth and development, identifying structural abnormalities, and evaluating the placenta and amniotic fluid levels.

Types of Ultrasound Technology

Several types of ultrasound technology are used during pregnancy, each serving specific clinical purposes. The most common is two-dimensional (2D) ultrasound, which produces flat, cross-sectional images and is used for most routine examinations. This type provides the detailed anatomical information needed for clinical assessment and has the strongest evidence base supporting its use.

Three-dimensional (3D) ultrasound creates still, three-dimensional images that can show the baby's surface features such as the face, hands, and feet. While 3D images can sometimes be helpful in evaluating specific structural abnormalities such as cleft lip, they are not routinely necessary for a healthy pregnancy. Four-dimensional (4D) ultrasound adds the element of real-time movement, producing video-like images of the baby. Both 3D and 4D ultrasound are primarily used for specific medical indications or, in some cases, as elective "keepsake" scans.

Doppler ultrasound is a specialized technique that measures blood flow through the baby's blood vessels, the umbilical cord, and the placenta. It is particularly valuable in high-risk pregnancies where monitoring blood flow patterns can help detect conditions such as fetal growth restriction, preeclampsia, or placental insufficiency. Doppler studies are often integrated into routine scans in later pregnancy.

How Many Ultrasound Scans Do You Get During Pregnancy?

Most healthy pregnancies include at least two routine ultrasound scans: a dating scan between weeks 8 and 14, and an anatomy scan (anomaly scan) between weeks 18 and 22. Additional scans may be recommended for high-risk pregnancies, multiple pregnancies, or when complications arise during pregnancy.

The number and timing of ultrasound scans can vary depending on your country, healthcare system, and individual pregnancy circumstances. However, international guidelines from the WHO and ISUOG recommend at least one ultrasound scan before 24 weeks of gestation, with many healthcare systems providing two standard scans as part of routine prenatal care.

In a low-risk, uncomplicated pregnancy, two scans are typically sufficient to gather the clinical information needed. The first scan (dating scan) establishes the gestational age and confirms the pregnancy is developing normally. The second scan (anatomy scan) provides a comprehensive evaluation of the baby's structures and development. These two scans form the backbone of prenatal imaging in most healthcare systems worldwide.

However, your healthcare provider may recommend additional scans in certain situations. Pregnancies complicated by gestational diabetes, preeclampsia, reduced fetal movement, abnormal growth patterns, or bleeding may require more frequent monitoring. Women carrying twins or higher-order multiples typically have scans every two to four weeks from mid-pregnancy onward. Similarly, if the first two scans reveal any concerns — such as a low-lying placenta, unusual amniotic fluid levels, or suspected growth issues — follow-up scans will be scheduled.

First Trimester Scans

During the first trimester, your healthcare provider may offer an early pregnancy scan between 6 and 10 weeks, particularly if you have experienced bleeding, pain, previous miscarriage, or if the dates of your pregnancy are uncertain. This early scan is typically performed transvaginally (through the vagina) because the embryo is still very small and the transvaginal approach provides much clearer images at this stage.

The first trimester screening scan, usually performed between weeks 11 and 14, serves several important purposes. It measures the crown-rump length (CRL) of the baby to establish an accurate due date, confirms whether you are carrying one baby or multiples, and may include a nuchal translucency (NT) measurement. The NT measurement assesses the fluid-filled space at the back of the baby's neck, which, combined with blood tests, can indicate the likelihood of certain chromosomal conditions such as Down syndrome (trisomy 21).

Second and Third Trimester Scans

The anatomy scan at 18-22 weeks is the most comprehensive routine scan of the pregnancy. It typically takes 30-45 minutes and involves a systematic examination of the baby's head, brain, face, spine, heart, stomach, kidneys, bladder, limbs, and other structures. This detailed assessment can detect a wide range of structural abnormalities and provides valuable information about the placenta's location and the volume of amniotic fluid.

Third trimester scans are not routine in most low-risk pregnancies but may be recommended if there are concerns about fetal growth, decreased fetal movement, or other complications. Growth scans measure the baby's head circumference, abdominal circumference, and femur length to estimate weight and assess whether growth is proceeding normally.

What Is a Dating Scan and Why Is It Important?

A dating scan is an ultrasound performed between weeks 8 and 14 of pregnancy to determine the gestational age and estimate the due date. It is the most accurate method for establishing when the baby is expected, with accuracy within 3-5 days when performed in the first trimester.

Accurate dating of pregnancy is fundamentally important for prenatal care. The estimated due date influences the timing of all subsequent screening tests, the interpretation of blood work results, and decisions about when interventions might be needed. While many women know the date of their last menstrual period (LMP), menstrual cycle lengths vary considerably, and ovulation does not always occur on day 14. Research shows that the LMP-based estimate can differ from the ultrasound-based estimate by a week or more in up to 30% of pregnancies.

During a dating scan, the sonographer measures the crown-rump length (CRL) — the distance from the top of the baby's head to its bottom. Before 14 weeks of gestation, CRL measurements are extremely accurate for determining gestational age because all embryos grow at nearly the same rate during this period, regardless of genetic or environmental factors. After the first trimester, individual variations in growth rates become more significant, making later measurements less reliable for dating purposes.

The dating scan also confirms that the pregnancy is developing in the correct location (inside the uterus, ruling out ectopic pregnancy), that a heartbeat is present, and whether you are carrying a single baby or multiples. For women who did not have earlier scans, this may be the first opportunity to hear and see the baby's heartbeat, which is typically visible from around 6 weeks and audible from around 8 weeks of gestation.

What Does the Anatomy Scan at 20 Weeks Check For?

The anatomy scan (also called the anomaly scan or morphology scan) is a detailed ultrasound performed between weeks 18 and 22. It systematically examines the baby's brain, heart, spine, kidneys, limbs, face, and abdominal organs for structural abnormalities. It also assesses the placenta position, amniotic fluid volume, and umbilical cord.

The 20-week anatomy scan is often considered the most important ultrasound of the entire pregnancy. It is a thorough, systematic examination that follows standardized protocols established by ISUOG and other international bodies. The scan typically takes 30 to 45 minutes and is performed transabdominally (over the belly). The sonographer works through a detailed checklist, examining each organ system in a specific order to ensure nothing is missed.

The baby's brain is carefully examined for normal development of the cerebral hemispheres, ventricles, cerebellum, and other structures. The four chambers of the heart are visualized along with the major blood vessels leaving the heart, as congenital heart defects are among the most common structural abnormalities detected at this stage. The spine is checked from the neck to the tailbone for proper alignment and closure (to detect conditions like spina bifida). The kidneys, bladder, and stomach are assessed, along with the diaphragm and abdominal wall.

Beyond organ assessment, the anatomy scan evaluates several other important aspects of the pregnancy. The position of the placenta is documented — if it is lying low or covering the cervix (placenta previa), follow-up scans will be planned as the placenta often moves upward as the uterus grows. The volume of amniotic fluid is assessed, as both too much (polyhydramnios) and too little (oligohydramnios) can indicate problems. The umbilical cord is checked to confirm it contains the normal three vessels (two arteries and one vein).

It is important to understand that while the anatomy scan can detect many structural abnormalities, it cannot detect all birth defects. The detection rate varies depending on the condition, the baby's position during the scan, the mother's body habitus, and the experience of the sonographer. On average, routine anatomy scans detect approximately 50-70% of major structural abnormalities. Some conditions, particularly mild heart defects or subtle limb differences, may not be visible at this stage.

What the Anatomy Scan Examines

• Head and brain: Skull shape, brain ventricles, cerebellum, and midline structures
• Face: Eye sockets, nose, lips (checking for cleft lip), and profile
• Heart: Four-chamber view, outflow tracts, and major blood vessels
• Spine: Vertebral alignment from neck to sacrum, checking for spina bifida
• Abdominal organs: Stomach, kidneys, bladder, and intestines
• Limbs: Arms, legs, hands, and feet including bone length
• Placenta: Position, appearance, and relationship to the cervix
• Amniotic fluid: Volume assessment (amniotic fluid index)
• Umbilical cord: Number of vessels (normally three) and insertion point

Learning the Baby's Sex

The anatomy scan at 18-22 weeks is typically when parents can learn the baby's sex, if they wish to know. By this stage, the external genitalia are usually developed enough to be identified on ultrasound. However, the accuracy depends on the baby's position during the scan — if the baby's legs are crossed or the angle is unfavorable, it may not be possible to determine the sex. If knowing the sex is important to you, mention this to your sonographer at the beginning of the scan.

It is worth noting that in some healthcare systems, sonographers may not routinely disclose the baby's sex unless specifically asked. In certain countries or cultures, there may be policies regarding sex disclosure. If you have a preference about knowing or not knowing, communicate this clearly with your healthcare team.

What Happens During a Pregnancy Ultrasound?

During a transabdominal ultrasound, you lie on your back while the sonographer applies gel to your abdomen and moves a handheld transducer over it. Images appear on a screen in real time. For transvaginal scans, a slim probe is gently inserted into the vagina. Both methods are safe and painless.

Understanding what to expect during an ultrasound scan can help reduce anxiety, especially if this is your first pregnancy. The procedure itself is straightforward and non-invasive. You will be asked to lie on an examination table, usually on your back with your abdomen exposed. The room may be dimly lit to allow better viewing of the monitor screen.

For a transabdominal ultrasound, the sonographer applies a clear, water-based gel to your abdomen. This gel helps transmit the sound waves between the transducer and your skin, eliminating air pockets that would interfere with the image quality. The transducer is then moved across different areas of your abdomen, applying gentle pressure to obtain views from various angles. You may feel slight pressure but should not experience pain.

For a transvaginal ultrasound, typically used in early pregnancy or when a transabdominal scan does not provide adequate images, a narrow, specially designed probe is covered with a protective sheath and lubricated before being gently inserted into the vagina. While some women find this mildly uncomfortable, it should not be painful. The transvaginal approach allows the transducer to be positioned much closer to the uterus and developing embryo, providing significantly more detailed images in early pregnancy.

During the scan, the sonographer takes various measurements and captures images for the medical record. They may point out features on the screen and explain what they are seeing, though this can vary depending on the setting and whether the sonographer is also a doctor. In some systems, a separate appointment with your doctor may be scheduled to discuss the results in detail.

How Should You Prepare for a Pregnancy Ultrasound?

Preparation depends on the type of scan. For early transabdominal scans (before 12 weeks), you may need a full bladder to help lift the uterus for better images. For later scans and transvaginal scans, no special preparation is typically needed. Wear comfortable, two-piece clothing so your abdomen is easily accessible.

Preparation for a pregnancy ultrasound is generally straightforward, but the specific instructions may vary depending on the gestational age and type of scan being performed. Your healthcare provider or the ultrasound department will typically give you instructions when scheduling your appointment.

For early transabdominal scans (typically before 12-14 weeks), you may be asked to drink a certain amount of water beforehand and avoid emptying your bladder. A full bladder pushes the uterus upward out of the pelvis, providing a better "window" for the ultrasound waves. This is particularly important in early pregnancy when the uterus is still relatively small and positioned low in the pelvis. The amount of water recommended varies, but it is usually around 500-750 ml (about 2-3 cups) consumed about an hour before the appointment.

For scans performed later in pregnancy (after about 14 weeks), a full bladder is usually not necessary because the uterus has grown large enough to be easily visualized without this assistance. For transvaginal scans, an empty bladder is actually preferred as it allows the probe to get closer to the structures being examined.

Regardless of the type of scan, wearing comfortable, loose-fitting clothing is recommended. A two-piece outfit (separate top and bottom) is ideal so that you can easily expose your abdomen without having to change into a gown. Avoid applying lotion or cream to your abdomen on the day of the scan, as this can interfere with the gel and transducer contact. You can eat and drink normally before your scan unless specifically instructed otherwise.

Tips for a Successful Scan

• Arrive early: Give yourself 10-15 minutes to check in and settle in
• Bring your records: Have your prenatal notes or referral letter available
• Prepare questions: Write down anything you want to ask the sonographer or doctor
• Be patient: Sometimes the baby's position makes certain views difficult to obtain, and you may need to walk around or return for a repeat scan
• Discuss preferences: Let the sonographer know if you do or do not want to learn the baby's sex

Is Ultrasound Safe During Pregnancy?

Yes, diagnostic ultrasound is considered safe for both mother and baby when performed by trained professionals for medical purposes. Unlike X-rays, ultrasound does not use ionizing radiation. Over 50 years of clinical use and extensive research have not identified any harmful effects on human pregnancies.

The safety of diagnostic ultrasound in pregnancy is supported by decades of research and clinical experience. Major medical organizations worldwide — including the WHO, ISUOG, ACOG, and the Royal College of Obstetricians and Gynaecologists (RCOG) — all confirm that ultrasound is safe when used appropriately in a clinical setting. Importantly, diagnostic ultrasound works by sending and receiving sound waves; it does not involve any form of radiation, unlike X-rays, CT scans, or nuclear medicine imaging.

The primary safety consideration with ultrasound involves two physical effects: thermal effects (slight tissue heating) and mechanical effects (vibration and cavitation). Modern ultrasound machines are designed to operate well within safe limits, and the exposure levels used in diagnostic obstetric imaging are far below those that could cause biological harm. The Thermal Index (TI) and Mechanical Index (MI) displayed on the ultrasound screen help the operator keep exposure within recommended limits.

While ultrasound is safe, medical guidelines emphasize the ALARA principle (As Low As Reasonably Achievable), meaning that scans should be performed for valid medical reasons and the exposure time should be kept to the minimum necessary to obtain the required diagnostic information. This principle is why most guidelines do not recommend unlimited or purely recreational ("keepsake") ultrasound scans without medical indication.

Multiple large-scale studies and systematic reviews, including Cochrane reviews, have examined the long-term outcomes of children who were exposed to ultrasound in utero. No adverse effects have been identified in terms of growth, development, neurological function, hearing, or vision. Nevertheless, research continues, and the medical community maintains its commitment to the judicious use of this technology.

What Happens If the Ultrasound Shows Something Abnormal?

If an abnormality is found, your healthcare provider will explain the findings and discuss next steps. This may include a follow-up scan, referral to a specialist in fetal medicine, additional tests such as amniocentesis or NIPT, or simply monitoring with repeat scans. Finding an abnormality does not always mean a serious problem.

Receiving news that something appears unusual on an ultrasound scan can be alarming for expectant parents. It is important to understand that not all unexpected findings indicate a serious problem. Some findings are normal variants, others are minor and resolve on their own before birth, and some require further investigation to determine their significance.

Common scenarios include "soft markers" — subtle ultrasound findings that are seen in a small percentage of normal babies but are slightly more common in babies with chromosomal conditions. Examples include a slightly bright spot on the heart (echogenic intracardiac focus), mildly dilated kidney pelvis (mild pyelectasis), or a shortened femur length. When a single soft marker is found in an otherwise normal scan, the risk is usually only slightly increased and may not require any additional testing.

If a structural abnormality is identified — such as a heart defect, spina bifida, or cleft lip — you will typically be referred to a maternal-fetal medicine specialist (perinatologist) for a more detailed assessment. This specialist may perform a higher-resolution scan, order additional testing such as amniocentesis (to check the baby's chromosomes), or recommend a fetal echocardiogram (detailed heart scan) or MRI for further evaluation.

Throughout this process, your healthcare team will provide information and support to help you understand the findings and the available options. Genetic counseling may be offered to help interpret test results and discuss implications. It is entirely appropriate to ask questions, seek a second opinion, or request time to process information before making any decisions.

If the placenta is found to be lying low (placenta previa), a follow-up scan is usually scheduled around 32-36 weeks. In many cases, the placenta migrates upward as the uterus grows, resolving the issue. Similarly, if amniotic fluid levels appear slightly outside the normal range, repeat monitoring may be all that is needed.

What Are the Different Types of Pregnancy Ultrasound?

The main types include transabdominal (over the belly), transvaginal (internal probe), Doppler (measuring blood flow), and 3D/4D (three-dimensional images). Transabdominal is most common for routine scans after the first trimester. Transvaginal provides clearer early pregnancy images. Doppler monitors blood flow in high-risk situations.

Understanding the different types of ultrasound available can help you know what to expect at each stage of your pregnancy. Each type has specific clinical applications and advantages, and your healthcare provider will choose the most appropriate method based on the gestational age and the purpose of the examination.

Transabdominal Ultrasound

This is the most familiar type of pregnancy ultrasound. The transducer is placed on the surface of the abdomen and moved around to obtain images. It is used for the majority of scans from the late first trimester onward. Transabdominal ultrasound is completely non-invasive, painless, and allows visualization of the entire uterus and its contents. The only slight discomfort may come from pressure on a full bladder during early pregnancy scans.

Transvaginal Ultrasound

A transvaginal scan uses a slim, specially designed transducer that is inserted into the vagina. Because the probe is positioned much closer to the uterus and ovaries, it provides significantly higher-resolution images in early pregnancy (typically before 12 weeks) when the embryo is still very small. It is the preferred method for confirming early pregnancy, dating very early pregnancies, evaluating bleeding or pain, and assessing cervical length in women at risk of preterm birth.

Doppler Ultrasound

Doppler ultrasound is a specialized technique that measures the speed and direction of blood flow through blood vessels. In obstetrics, it is used to assess blood flow in the umbilical artery, the baby's middle cerebral artery, the uterine arteries, and other vessels. This information is particularly valuable in monitoring high-risk pregnancies where there are concerns about preeclampsia, fetal growth restriction, or placental insufficiency. Doppler studies help guide clinical decisions about timing of delivery in complicated pregnancies.

3D and 4D Ultrasound

Three-dimensional ultrasound constructs a still, volumetric image from multiple 2D "slices," allowing visualization of the baby's surface features. Four-dimensional ultrasound adds real-time movement to these 3D images. While these technologies can provide remarkable images of the baby's face and movements, their clinical utility beyond standard 2D ultrasound is limited to specific diagnostic situations, such as evaluating facial clefts, skeletal abnormalities, or complex congenital anomalies. They are not considered medically necessary for routine prenatal care.

What Are the Limitations of Pregnancy Ultrasound?

Ultrasound cannot detect all birth defects or abnormalities. Detection rates vary by condition, averaging 50-70% for major structural anomalies. Image quality can be affected by the baby's position, maternal body habitus, gestational age, and amniotic fluid volume. Ultrasound also cannot diagnose genetic or chromosomal conditions directly.

While ultrasound is an invaluable tool in prenatal care, it is important to have realistic expectations about what it can and cannot show. Understanding these limitations helps parents process the results in context and prevents false reassurance or unnecessary alarm.

The detection rate for structural abnormalities varies significantly depending on the specific condition. Heart defects, for example, are among the most common congenital anomalies but can be difficult to detect because the fetal heart is small and the four-chamber view alone may miss certain types of defects. Conditions like spina bifida and anencephaly have relatively high detection rates (over 90% in experienced centers), while others, such as certain limb defects or mild kidney abnormalities, may be harder to identify.

Several factors influence image quality and the ability to visualize structures clearly. A higher maternal body mass index (BMI) can make it more challenging to obtain clear images because the ultrasound waves must travel through more tissue. The baby's position during the scan is also crucial — if the baby is facing away from the transducer or is in an awkward position, certain structures may be obscured. Reduced amniotic fluid (oligohydramnios) or an early gestational age can also limit visualization.

It is also important to understand that ultrasound provides anatomical information — it shows the physical structure of the baby's organs and body. It cannot directly diagnose genetic or chromosomal conditions such as Down syndrome, cystic fibrosis, or sickle cell disease. While certain ultrasound findings may suggest an increased risk for these conditions, definitive diagnosis requires genetic testing through procedures like amniocentesis, chorionic villus sampling (CVS), or non-invasive prenatal testing (NIPT).

When Are Extra Ultrasound Scans Needed?

Extra scans may be recommended for twin or multiple pregnancies, gestational diabetes, preeclampsia, suspected growth restriction, decreased fetal movement, placenta previa, previous pregnancy complications, or advanced maternal age. The frequency depends on the specific condition and its severity.

While most low-risk pregnancies proceed smoothly with just two routine scans, certain situations require closer monitoring through additional ultrasound examinations. Your healthcare provider will determine the appropriate scan schedule based on your individual risk factors and pregnancy progression.

Multiple pregnancies (twins, triplets, or more) require significantly more frequent monitoring than singleton pregnancies. The recommended scan frequency depends on the type of twin pregnancy — monochorionic twins (sharing a placenta) are at higher risk for complications like twin-to-twin transfusion syndrome (TTTS) and typically require scans every two weeks from mid-pregnancy. Dichorionic twins (separate placentas) may be scanned every four weeks.

Gestational diabetes may necessitate growth scans in the third trimester to monitor for excessive fetal growth (macrosomia), which can complicate delivery. Similarly, preeclampsia or chronic hypertension may require Doppler studies and growth scans to monitor blood flow to the baby and assess growth patterns.

If there are concerns about fetal growth restriction (the baby measuring smaller than expected), regular scans with Doppler assessments help monitor the baby's well-being and guide decisions about timing of delivery. Decreased fetal movement or vaginal bleeding in later pregnancy may also prompt additional ultrasound evaluation to ensure the baby is doing well.

Women who have experienced complications in previous pregnancies — such as preterm birth, growth restriction, or stillbirth — may be offered additional monitoring in subsequent pregnancies. This may include cervical length measurements in the second trimester to assess the risk of preterm birth, or regular growth scans to monitor fetal development.