The detailed anatomy ultrasound, sometimes called the second trimester scan or the anatomical scan, is a routine ultrasound that is available to all pregnant women in Ontario.

Timing

Ultrasounds can be done throughout the course of pregnancy. This particular ultrasound is typically done in the second trimester, ideally between 18 and 22 weeks gestation.

Who can have this ultrasound?

The detailed anatomy ultrasound is a routine part of prenatal care and is important even if you have had previous ultrasounds. This ultrasound allows your health care provider to get a more detailed look at your pregnancy.

How to prepare for your ultrasound

Every ultrasound clinic has different requirements so it is important that you ask the clinic if there are any special instructions before your appointment. Commonly you will be asked to drink 1 litre of water before the ultrasound in order to fill your bladder. Filling your bladder makes it easier for the sonographer or physician to take the ultrasound images.

What this ultrasound will look at

Your baby has now grown enough in size that the sonographer should be able to have a detailed look at your baby, much more so than ultrasounds done in the first trimester of pregnancy. The ultrasound at this stage will focus on the fetal anatomy to make sure that everything is developing as expected. The sonographer or physician will take measurements and assess all major fetal organs and structures including:

• Brain
• Heart
• Kidneys
• Bladder
• Stomach
• Spine
• Limbs
• Sex organs

This ultrasound can also provide information about:

• The fetal position, movement, breathing, and heart rate
• An estimate of the fetal size and weight
• The amount of amniotic fluid in the uterus
• The location of the placenta
• The number of fetuses
• Fetal sex – dependent on fetal position

Soft markers

The detailed anatomy ultrasound will also look for the presence of soft markers which are features that may be associated with an increased risk for fetal chromosome differences. It is important to know that soft markers are often found in pregnancies that are healthy and that not all fetuses with soft markers will have a chromosome difference. If a soft marker is found on your ultrasound, your health care provider will review the ultrasound report with you in detail. You should have the option of being referred for genetic counselling to obtain more information and to discuss your options for further testing.

Examples of soft markers:

• Echogenic intracardiac focus (EIF)
• Choroid plexus cyst (CPC)
• Echogenic bowel
• Increased nuchal fold
• Ventriculomegaly

If you had screening for trisomy 21 (Down syndrome) or trisomy 18 early in your pregnancy (e.g., eFTS or MSS), the presence of a soft marker may increase the risk that was reported by these tests. Refer to the most recent Society of Obstetricians and Gynecologists of Canada (SOGC) guidelines for more information on the impact of ultrasound soft markers on fetal chromosome differences.

Limitations

Currently, there is no evidence that ultrasound is harmful to a developing fetus. No links have been found between ultrasound and birth defects, childhood cancer, or developmental problems later in life.