Pregnancy Ultrasound: Types, Timing & What to Expect

What Is a Pregnancy Ultrasound?

A pregnancy ultrasound (also called a sonogram or prenatal ultrasound) is a diagnostic imaging technique that uses high-frequency sound waves to create real-time images of the fetus, placenta, and amniotic fluid inside the uterus. It is the primary method for monitoring fetal development throughout pregnancy and is considered extremely safe for both parent and baby.

During a pregnancy ultrasound, a trained sonographer or physician moves a small device called a transducer across your abdomen (or in some cases, inserts it into the vagina for early pregnancy scans). The transducer emits sound waves that bounce off tissues and organs, creating echoes that are converted into moving images displayed on a monitor. Unlike X-rays or CT scans, ultrasound does not use ionizing radiation, making it safe for repeated use during pregnancy.

Prenatal ultrasound technology has been used since the 1950s and has undergone continuous refinement. Modern ultrasound machines can produce remarkably detailed two-dimensional (2D), three-dimensional (3D), and even four-dimensional (4D) images that show the baby's movements in real-time. The technology allows healthcare providers to visualize fetal anatomy, assess growth patterns, detect many structural abnormalities, and monitor overall pregnancy health.

The sound waves used in medical ultrasound are too high-pitched for humans to hear, hence the term "ultrasound." The energy levels used in diagnostic imaging are carefully controlled and have been extensively studied. Major medical organizations worldwide, including the World Health Organization (WHO), the American College of Obstetricians and Gynecologists (ACOG), and the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG), confirm that diagnostic ultrasound has an excellent safety record when performed by trained professionals using appropriate equipment.

How Ultrasound Works

The science behind ultrasound imaging is based on the principle of sound wave reflection. When the transducer is placed on the skin (with conducting gel to eliminate air gaps), it sends pulses of high-frequency sound waves into the body. These waves travel through soft tissue until they encounter a boundary between different tissue types, such as the interface between amniotic fluid and fetal skin. At these boundaries, some of the sound energy bounces back as an echo.

The transducer receives these returning echoes and measures both their intensity and the time taken to return. A computer processes this information to calculate the distance and density of various structures, assembling thousands of these measurements into a real-time image. Different tissues appear in varying shades of gray based on their density: fluids appear dark (black), bones appear bright (white), and soft tissues display various shades in between.

Types of Ultrasound Technology

Several types of ultrasound technology are used during pregnancy, each serving specific purposes. 2D ultrasound is the most common, producing flat, cross-sectional images that trained professionals can interpret to assess fetal anatomy and development. 3D ultrasound combines multiple 2D images to create a three-dimensional picture that can show the baby's surface features more clearly. 4D ultrasound adds the element of time to 3D imaging, showing movement in real-time, which can be particularly helpful for evaluating fetal behavior and detecting certain abnormalities.

Doppler ultrasound is a specialized technique that measures blood flow through the umbilical cord, placenta, and fetal heart. This information helps assess the baby's cardiovascular health and can detect problems with blood circulation that might affect fetal growth. Doppler is often used in conjunction with standard ultrasound during routine examinations.

When Are Pregnancy Ultrasounds Performed?

Most pregnant women receive at least two routine ultrasound examinations: a dating scan between 8-14 weeks to confirm the pregnancy and estimate the due date, and an anatomy scan between 18-22 weeks to check the baby's development in detail. Additional scans may be recommended based on individual circumstances, including high-risk pregnancies, suspected complications, or monitoring of identified concerns.

The timing and number of ultrasound examinations during pregnancy can vary depending on healthcare guidelines in different countries, individual risk factors, and the preferences of both healthcare providers and pregnant women. While some women may have only the two standard scans, others may receive additional examinations throughout pregnancy to monitor specific concerns or conditions.

First Trimester Ultrasound (Weeks 6-14)

Early pregnancy ultrasounds serve several important purposes. In very early pregnancy (weeks 6-8), ultrasound may be performed transvaginally to confirm a viable pregnancy, determine the location of the pregnancy (ruling out ectopic pregnancy), and detect a fetal heartbeat. This is particularly common when there are concerning symptoms such as vaginal bleeding or abdominal pain, or when the pregnancy was achieved through assisted reproductive technology.

The dating scan, typically performed between weeks 11-14, is often the first routine ultrasound offered to all pregnant women. This examination establishes the gestational age and expected due date by measuring the crown-rump length (CRL) of the fetus - the distance from the top of the head to the bottom of the buttocks. Dating scans are most accurate during this period because early fetal growth follows a predictable pattern, and the measurements are less affected by individual variations in growth.

In many countries, the first trimester scan may also include an assessment of the nuchal translucency (NT), a fluid-filled space at the back of the baby's neck. Combined with blood tests measuring specific pregnancy hormones, this forms part of the combined first trimester screening (sometimes called combined screening or the nuchal translucency test) that estimates the probability of certain chromosomal abnormalities, including Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13).

Second Trimester Anatomy Scan (Weeks 18-22)

The anatomy scan, also known as the mid-pregnancy scan, anomaly scan, or 20-week scan, is the most comprehensive routine ultrasound examination during pregnancy. This detailed evaluation typically takes 30-45 minutes and systematically examines all visible fetal structures to assess development and detect potential abnormalities.

During the anatomy scan, the sonographer examines the baby's brain, face, spine, heart, stomach, kidneys, bladder, arms, legs, hands, feet, and other organs. They also assess the placenta's location and function, measure the amount of amniotic fluid, and evaluate the umbilical cord. The fetal heartbeat is checked, and various body measurements are taken to confirm appropriate growth.

This examination can detect a range of structural abnormalities including neural tube defects (such as spina bifida), major heart defects, kidney problems, limb abnormalities, and certain genetic conditions that have visible physical features. However, it's important to understand that not all abnormalities are detectable by ultrasound - some conditions have no visible signs, and detection rates vary depending on the specific condition, the baby's position, maternal body habitus, and the quality of the equipment and expertise available.

Third Trimester Ultrasounds

Routine third trimester ultrasounds are not universally recommended for all pregnancies, though practices vary between healthcare systems. When performed, these scans typically focus on fetal growth assessment, amniotic fluid levels, placental function, and fetal position in preparation for delivery.

Growth scans may be recommended for various reasons, including suspected large-for-gestational-age or small-for-gestational-age babies, maternal conditions like gestational diabetes or preeclampsia, multiple pregnancies, or when the baby hasn't been moving as expected. These examinations help healthcare providers make decisions about the timing and method of delivery.

What Can a Pregnancy Ultrasound Show?

Pregnancy ultrasound can reveal the number of fetuses, gestational age, fetal heartbeat, placenta location, amniotic fluid levels, and many structural features of the developing baby including the brain, heart, spine, limbs, and major organs. It can detect various abnormalities such as neural tube defects, certain heart conditions, and skeletal problems, though not all conditions are visible on ultrasound.

The information provided by pregnancy ultrasound serves both immediate clinical needs and longer-term pregnancy planning. Understanding what ultrasound can and cannot show helps pregnant women make informed decisions about their prenatal care and prepare for various outcomes.

Confirming Pregnancy Details

Early ultrasound provides essential baseline information for pregnancy management. It confirms that the pregnancy is located within the uterus (ruling out ectopic pregnancy, where the embryo implants outside the uterus, most commonly in a fallopian tube). It also determines whether there is one baby or multiple babies, which significantly affects prenatal care planning and delivery preparation.

The dating function of ultrasound is particularly valuable because many women don't have regular menstrual cycles or aren't certain about their last menstrual period. Accurate dating is important for timing various prenatal tests and interventions, and for determining when a baby is truly overdue. First trimester dating is accurate to within about 5-7 days; this accuracy decreases as pregnancy progresses because individual growth patterns become more variable.

Monitoring Fetal Development

Throughout pregnancy, ultrasound tracks fetal growth and development. Measurements of the head circumference, abdominal circumference, and femur (thigh bone) length are compared to established growth charts to ensure the baby is developing appropriately. Significant deviations from expected growth patterns may indicate underlying problems requiring further investigation.

The amount of amniotic fluid surrounding the baby is assessed during ultrasound examinations. Both too much fluid (polyhydramnios) and too little fluid (oligohydramnios) can indicate problems with the baby or the pregnancy and may require closer monitoring or intervention. The placenta is also evaluated for its location, appearance, and function. A low-lying placenta (placenta previa) or signs of placental problems can affect delivery planning and overall pregnancy management.

Detecting Abnormalities

The anatomy scan in particular focuses on detecting structural abnormalities. Some conditions that can potentially be identified include:

• Brain and spine: Anencephaly, spina bifida, hydrocephalus
• Heart: Major cardiac defects affecting the heart's structure and function
• Abdominal wall: Gastroschisis, omphalocele
• Kidneys: Missing kidney, enlarged kidneys, urinary tract blockages
• Limbs: Missing or shortened limbs, clubfoot
• Face: Cleft lip (though cleft palate is harder to detect)

However, it's crucial to understand that ultrasound has limitations. Some abnormalities, including many genetic conditions, have no visible features on ultrasound. The detection rate for major abnormalities is typically estimated at 50-70%, meaning that a normal ultrasound cannot guarantee a baby without health problems. Factors affecting detection include the specific abnormality, the timing of the scan, the baby's position, maternal body characteristics, and the quality of the equipment and examiner's expertise.

Determining the Baby's Sex

Many parents want to know their baby's sex before birth, and this can typically be determined during the anatomy scan at 18-22 weeks. The sonographer looks for visible genital anatomy, though accuracy depends on the baby's position and other factors. It's important to note that sex determination is not a medical necessity and some parents choose not to learn this information before birth.

Accurate sex determination becomes possible around 16-20 weeks for most pregnancies. Earlier determinations are less reliable, though some practitioners may offer an educated guess at earlier scans. If knowing the sex is important to you, communicate this to your healthcare team, as policies vary regarding when and whether this information is shared.

How Should You Prepare for a Pregnancy Ultrasound?

Preparation for a pregnancy ultrasound depends on the type of examination. For abdominal scans in early pregnancy and the anatomy scan, you're typically asked to drink 2-3 glasses of water about an hour before the appointment so that your bladder is moderately full. For transvaginal scans used in very early pregnancy, an empty bladder is preferred. Wear comfortable, loose-fitting clothing for easy access to your abdomen.

Proper preparation can help ensure the best possible images and a smoother examination experience. The requirements differ depending on when in pregnancy the scan is performed and whether it's an abdominal or transvaginal examination.

Bladder Preparation

For most abdominal ultrasounds in the first and second trimesters, a moderately full bladder is beneficial. The fluid-filled bladder acts as an acoustic window, helping sound waves travel more effectively and pushing the uterus into a better position for imaging. You're typically advised to drink about 2-3 glasses of water approximately one hour before your appointment and avoid emptying your bladder until after the scan.

However, there's no need to be extremely uncomfortable. If you arrive with an overly full bladder, the sonographer may ask you to partially empty it before proceeding. If you accidentally urinated before arriving, don't worry - the examination can usually still be performed, though images might be slightly less clear in early pregnancy.

For transvaginal ultrasounds, which are sometimes used in very early pregnancy for clearer views, an empty bladder is preferred. A full bladder can actually push the uterus away and make imaging more difficult with this approach. The transvaginal transducer is inserted into the vagina (with a protective cover) and provides close-up views of the uterus and early pregnancy.

What to Wear

Comfortable, loose-fitting clothing is recommended for ultrasound appointments. Two-piece outfits are ideal as you'll need to expose your abdomen for the examination. You'll typically be asked to lower your pants or skirt to below the hip bones and lift your top to just below your bra. The sonographer will tuck paper towels or drapes around your clothing to protect it from the ultrasound gel.

What to Bring

Most facilities allow a partner or support person to accompany you to the ultrasound, which many people find reassuring. Check with your specific facility about their policy, especially as some may have restrictions on the number of people allowed in the room.

Consider bringing a list of any questions you have about your pregnancy or the scan. This is an opportunity to ask about what the sonographer is seeing and any concerns you might have. However, be aware that sonographers may have limited ability to discuss findings in detail, particularly if something unexpected is discovered, as this typically requires physician interpretation.

How Is a Pregnancy Ultrasound Performed?

During an abdominal pregnancy ultrasound, you lie on an examination table while the sonographer applies gel to your abdomen and moves a handheld transducer across the skin. The transducer sends sound waves into your body and receives the returning echoes, which are displayed as real-time images on a monitor. The examination is painless and typically takes 15-45 minutes depending on the type of scan.

Understanding what happens during the examination can help reduce anxiety and make the experience more comfortable. The procedure is straightforward and non-invasive, though it may feel unfamiliar if it's your first ultrasound.

The Examination Room

Ultrasound examinations are performed in rooms specially equipped with ultrasound machines and monitors. The room is often dimmed to allow better visualization of the screen images. You'll lie on a padded examination table, typically on your back with your head supported by a pillow. Later in pregnancy, if lying flat is uncomfortable, the sonographer can adjust your position slightly.

The Scanning Process

The sonographer will apply a water-based gel to your abdomen. This gel serves an important purpose: it eliminates the air gap between the transducer and your skin, allowing sound waves to travel efficiently into your body. The gel may feel slightly cool when first applied but quickly warms to body temperature.

The transducer, a smooth handheld device about the size of a small bar of soap, is then moved across your abdomen in various directions. The sonographer applies gentle pressure to obtain the clearest images. You may feel some pressure, particularly when the transducer passes over your bladder, but the examination should not be painful. If you experience significant discomfort, tell the sonographer.

As the transducer moves, you'll see images appearing on the monitor. Many sonographers position the screen so that you can watch during the examination, and most will explain what they're seeing and point out features like the baby's head, heart, and limbs. The moving images can be exciting for expectant parents, though it may be initially difficult to understand what you're seeing without guidance.

For transvaginal ultrasounds in early pregnancy, you'll be asked to undress from the waist down and may be given a drape for privacy. The transducer, which is narrow and designed for internal use, is covered with a protective sheath and lubricating gel. It's inserted a few inches into the vagina. This examination may feel unusual or slightly uncomfortable but shouldn't be painful. It provides clearer images of early pregnancy than abdominal scanning because the transducer is closer to the uterus.

During the Anatomy Scan

The anatomy scan is more comprehensive and takes longer than earlier scans. The sonographer systematically examines multiple fetal structures and takes numerous measurements. They may ask you to change position, take a short walk, or drink something if the baby is in a position that makes certain views difficult to obtain.

Occasionally, despite best efforts, not all views can be obtained during a single appointment. If important structures aren't adequately visualized - perhaps because of the baby's position, low amniotic fluid, or maternal body characteristics - you may be asked to return for a follow-up scan. This is relatively common and doesn't necessarily indicate a problem.

After the Examination

Once the examination is complete, the sonographer will offer you tissues or wipes to remove the gel from your abdomen. The gel is water-soluble and washes out of clothing, though it's worth bringing a small towel if you'd prefer not to have any residue on your clothes.

Many facilities offer printed images or digital copies of your ultrasound as keepsakes. There may be a small fee for additional prints. These images capture moments of your baby's development and can be meaningful mementos of your pregnancy.

Is Pregnancy Ultrasound Safe?

Yes, diagnostic ultrasound is considered very safe for both the pregnant person and the developing baby. Unlike X-rays or CT scans, ultrasound uses sound waves rather than ionizing radiation. Decades of research and clinical experience involving millions of pregnancies have found no confirmed harmful effects when ultrasound is performed appropriately by trained professionals using properly maintained equipment.

The safety of prenatal ultrasound is one of the most thoroughly studied aspects of this technology. Understanding the evidence can help expectant parents feel confident about this routine aspect of prenatal care.

How Safety Has Been Established

Ultrasound has been used in obstetrics since the 1950s, providing decades of observational data on exposed populations. Large-scale studies have followed children who were exposed to prenatal ultrasound to look for any differences in development, health, or school performance compared to unexposed children. These studies have consistently failed to find any harmful effects.

The acoustic energy levels used in diagnostic ultrasound are carefully regulated and standardized. Modern ultrasound machines include output displays that show thermal and mechanical indices, allowing sonographers to monitor exposure levels during examinations. The ALARA principle (As Low As Reasonably Achievable) guides clinical practice, meaning that sonographers use the minimum output necessary to obtain good images.

Why Ultrasound Is Considered Safe

Ultrasound works by sending pulses of sound waves into the body and measuring the returning echoes. Unlike X-rays, gamma rays, or the radiation used in CT scans, sound waves do not ionize atoms or damage DNA. The two potential biological effects of ultrasound - heating and mechanical effects (cavitation) - have been extensively studied and are not considered significant at the energy levels used in diagnostic imaging.

Heating effects would require significantly higher exposure levels than those used in routine prenatal imaging. The pulsed nature of diagnostic ultrasound (sound waves are only transmitted for very brief periods during scanning) further minimizes any thermal effects. Mechanical effects (the formation and collapse of small gas bubbles) are also not considered significant because the amniotic fluid and fetal tissues do not contain gas pockets that would be susceptible to this phenomenon.

Recommendations and Guidelines

All major medical organizations, including the World Health Organization (WHO), the American College of Obstetricians and Gynecologists (ACOG), the Royal College of Obstetricians and Gynaecologists (RCOG), and the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG), support the safety of diagnostic ultrasound when used appropriately.

These organizations recommend that ultrasound be performed only for valid medical indications and by properly trained personnel. They advise against non-medical use of ultrasound purely for entertainment purposes (sometimes called "keepsake" ultrasounds), primarily because these examinations are often performed by less qualified operators using equipment that may not be optimally maintained, and because extended scanning times may be used to obtain interesting images rather than focusing on diagnostic needs.

What Are the Limitations of Pregnancy Ultrasound?

While pregnancy ultrasound is a valuable diagnostic tool, it has important limitations. It cannot detect all fetal abnormalities - estimated detection rates for major anomalies range from 50-70%. Some conditions have no visible features on ultrasound, and factors like the baby's position, maternal body habitus, and the timing of the scan affect what can be seen. A normal ultrasound does not guarantee a healthy baby.

Understanding what ultrasound cannot do is as important as knowing what it can accomplish. Realistic expectations help pregnant women interpret their results appropriately and make informed decisions about additional testing.

Detection Limitations

Not all fetal abnormalities are detectable by ultrasound. Some conditions, particularly genetic disorders, may have no visible physical features during pregnancy. Others may not become apparent until later in pregnancy or after birth. The detection rate varies significantly depending on the specific condition:

• Anencephaly (absence of major portions of the brain): Nearly 100% detection rate
• Major limb defects: 70-90% detection rate
• Spina bifida: 80-90% detection rate in specialized centers
• Major heart defects: 40-60% detection rate
• Cleft palate (without cleft lip): Less than 5% detection rate
• Down syndrome: Ultrasound cannot diagnose chromosomal conditions, though some features may increase suspicion

These figures represent averages and can vary significantly depending on the expertise of the examiner, the quality of equipment, the gestational age at the time of the scan, and individual patient factors. Even in the best centers with the most experienced practitioners, some abnormalities will be missed.

Technical Limitations

Several factors can affect ultrasound image quality and what can be visualized:

• Fetal position: If the baby is facing toward your spine or has limbs positioned in front of key structures, visualization may be limited
• Maternal body habitus: Higher body weight can reduce image clarity because sound waves must travel through more tissue
• Amniotic fluid levels: Too little or too much fluid can affect visualization
• Gestational age: Some structures are easier to see at certain stages of pregnancy
• Equipment quality: Older or poorly maintained equipment may produce less clear images

What Ultrasound Cannot Do

Ultrasound examines visible physical structures but cannot assess:

• Genetic conditions: Chromosomal abnormalities like Down syndrome, Edwards syndrome, or cystic fibrosis cannot be diagnosed by ultrasound alone (though some may be suspected based on visible features)
• Intellectual development: Future cognitive abilities cannot be predicted
• Future health: Many health conditions that may develop later in life have no prenatal indicators
• Minor abnormalities: Small or subtle defects may not be visible

For genetic conditions, different types of testing are needed, such as non-invasive prenatal testing (NIPT), chorionic villus sampling (CVS), or amniocentesis. These tests analyze genetic material rather than physical structures.

What Happens If Ultrasound Shows Something Unusual?

If an ultrasound reveals unexpected findings, your healthcare team will explain what was observed and discuss next steps. This may include referral to a specialist for a more detailed examination, additional imaging, or diagnostic tests like amniocentesis. It's important to remember that many findings are benign or resolve on their own, and that having time and support to understand the situation and your options is essential.

Receiving unexpected news during an ultrasound can be distressing. Understanding the process that follows can help you feel more prepared and empowered to navigate this situation.

Types of Findings

Not all unusual findings on ultrasound indicate serious problems. Some observations are what medical professionals call "soft markers" - variations that may be slightly more common in babies with certain conditions but are also frequently seen in healthy babies. Examples include certain kidney findings, small cysts in the brain (choroid plexus cysts), or slightly thickened skin at the back of the neck.

When soft markers are found in isolation (without other concerning signs), the risk of an underlying problem is usually low, though additional testing may be offered. Multiple soft markers together may warrant more investigation.

More significant findings might include structural abnormalities (such as heart defects or neural tube defects), growth problems, or concerns about amniotic fluid levels or placental function. The significance and implications of these findings vary widely.

Next Steps

If something unusual is found, common next steps include:

• Repeat ultrasound: Sometimes the finding needs to be re-evaluated at a different stage of pregnancy or simply rechecked to confirm what was seen
• Referral to a specialist: A maternal-fetal medicine specialist or fetal cardiologist can provide more detailed evaluation and expertise
• Additional imaging: Fetal MRI or echocardiography may provide more information about certain conditions
• Genetic testing: Amniocentesis or NIPT may be recommended to assess for chromosomal abnormalities
• Counseling: Genetic counselors can help explain findings, risks, and options

Emotional Support

Receiving unexpected news about your baby can trigger a range of emotions including shock, fear, grief, and anxiety. These reactions are normal, and it's important to give yourself time and space to process the information.

Many hospitals and prenatal care centers offer counseling services for expectant parents dealing with concerning findings. Genetic counselors, in particular, are trained to help families understand complex medical information and make decisions that align with their values. Support groups for parents of children with specific conditions can also provide valuable connection and information.

Is Pregnancy Ultrasound Voluntary?

Yes, pregnancy ultrasound, like all prenatal testing, is voluntary. You have the right to accept or decline any medical examination or test. Your healthcare provider should explain the purpose, benefits, and limitations of ultrasound and obtain your informed consent before proceeding. Your decision to accept or decline ultrasound does not affect your access to other aspects of prenatal care.

The principle of informed consent is fundamental to prenatal care. Understanding your rights helps you make decisions that align with your values and preferences.

Making Informed Decisions

Before any prenatal test or examination, you should receive information about:

• The purpose of the test and what it can tell you
• How the test is performed
• The benefits and limitations of the test
• Any risks (for ultrasound, there are no known risks, but this should still be discussed)
• What might happen if findings are abnormal
• What decisions you might face based on the results

Some people choose not to have certain prenatal tests because they know they wouldn't act on the information or because they prefer not to know. Others want as much information as possible to prepare for any challenges. Neither approach is right or wrong - it's a personal decision.

Your Rights

You have the right to:

• Ask questions about any recommended test or procedure
• Receive clear, understandable information
• Take time to consider your options
• Decline any test without judgment or pressure
• Change your mind about previous decisions
• Receive respectful care regardless of your choices