Sometimes called an anatomy scan, an 18-week ultrasound can help with evaluating fetal development and detecting complications.
The 18th week of pregnancy is usually the earliest that a healthcare provider can do the anatomy scan.
During the 18-week ultrasound, a doctor or ultrasound technician will use an ultrasound machine to look at many different parts of the developing fetus, including the brain, heart, stomach, kidneys, skull, and genitals. It is usually possible to determine the sex of the fetus at this scan.
Ultrasounds work by sending sound waves into the body. Parts of the body, including the developing fetus, send these sound waves back. Based on how long it takes the waves to return to the ultrasound machine, the machine can determine how far away various body parts are and render a picture displaying this information.
In most cases, people do not need to do anything special to prepare for an anatomy scan.
During an 18-week ultrasound, an ultrasound technician or a doctor uses a transducer, which looks like a remote control. They apply a gel to the lower portion of the stomach and rub the transducer over the area to create clear images.
Sometimes, the provider also inserts the transducer into the vagina to get a clearer picture. Learn more about transvaginal ultrasounds here.
An ultrasound at 18 weeks usually takes longer than earlier ultrasounds, which doctors use to date the pregnancy. The provider may ask the woman to move, drink water, or use the bathroom to encourage the fetus to change positions.
The doctor or technician will check a range of factors during this ultrasound, including:
- genital development and the sex of the fetus
- development of the skull and brain
- development of the heart, including whether the heart has four chambers
- development of organs, such as the kidneys, lungs, and intestines
- signs of cleft palate and other genetic anomalies
- development of the placenta, including whether it is in the right place
- amniotic fluid levels
The 18-week ultrasound provides many important pieces of information, including:
- Whether the fetus is developing normally: The healthcare provider can identify whether the baby is likely to need medical care right after birth.
- The overall health of the pregnancy: Problems with the placenta or amniotic fluid may mean that the pregnant woman needs to switch to a provider specializing in high risk pregnancies. In some cases — if the pregnant woman has placenta accreta, for example — the delivery may need to take place in an operating room or a specialized hospital.
- Delivery decisions: The anatomy scan may guide delivery decisions. For example, if a woman is considering a home birth, the anatomy scan may reveal whether this presents any particular risks.
The 18-week ultrasound is not a perfect diagnostic tool. It cannot detect all congenital disabilities. Moreover, the images that it produces are not photos and may not provide fully accurate information. Sometimes, it is not possible to see all parts of the fetus. Small shadows and positioning issues can make diagnosing the health of the pregnancy challenging.
As the fetus is still relatively small at 18 weeks, people who choose to have an anatomy scan at this early point may not be able to get a clear image. Their provider may recommend that they have a vaginal ultrasound or return later in the pregnancy if any images are unclear, or there are signs that there might be a problem with the fetus.
Traditional ultrasounds, sometimes called 2D ultrasounds, produce blurry and grainy images that may be hard for an untrained person to understand. While most people can make out larger shapes, such as the skull and torso, it is more difficult to see fine details on these ultrasounds.
People may see the fetus moving or kicking in the ultrasound. The sucking reflex develops around this age, so they may see the fetus sucking their thumb.
Many providers now routinely offer 3D ultrasounds. These scans use the same basic process as 2D ultrasounds, but they render a photo based on sounds from many different angles to create a composite image of the fetus. This technique makes it easier to see the fetus’s features, including smaller parts of the body, such as fingers, toes, and even genitals.
A 4D ultrasound offers even more precision. These scans take many images per second to create a very detailed depiction of the fetus. People can see the fetus move and sometimes even smile or suck their thumb.
As 4D ultrasounds offer so much more detail, doctors sometimes use them to assess whether a fetus’s behavior is normal. This assessment can provide useful information about development, especially when a doctor cannot get a clear image, or there are signs of an abnormality on a 2D or 3D ultrasound.
For many people, the 18-week ultrasound is the first chance to see the fetus up close and to learn the sex. For others, it can be a scary procedure, especially if they have worries about complications.
In most cases, the ultrasound offers reassurance that the fetus and pregnancy are healthy. Even when there is a problem with the ultrasound, subsequent ultrasounds may offer better images that ease any concerns. People should discuss the benefits, risks, and limitations of ultrasounds with a provider they trust.