Why wasn’t my baby’s heart condition detected before birth?

This information aims to give parents a better understanding of the reasons behind why their baby’s heart condition was not detected before birth.

There are many different types of congenital heart defects. Some are mild and don’t need treatment, while some are life-threatening and need open heart surgery or other medical interventions.

Sometimes the term congenital heart “disease” is used instead of “defects” – either way, the name is commonly shortened to CHD.

Approximately 50% of congenital heart disease (CHD) is detected in the antenatal period. This is the current national statistic, though this figure varies by region and hospital.

There are several reasons why a heart defect might not be detected before birth, and we will discuss some of them here:

Variation in how the defect presents

• The ease of detecting a heart defect before birth depends on the type of defect. For instance, it is generally easier to identify if one of the four chambers of the heart hasn’t developed properly by 20 weeks of pregnancy (HLHS) than to detect a ventricular septal defect (VSD), which is a hole between the heart’s pumping chambers.
• There are some heart defects that cannot be detected in the antenatal period, like a bicuspid aortic valve and there are some that are trickier to diagnose, like coarctation of the aorta (CoA) , which mainly manifests itself after birth once the duct has closed.
• Some heart defects progress during pregnancy. While the heart may appear structurally normal at the 20-week scan, the defect can evolve and become significant by the time of birth.
• Some heart defects can be very subtle and difficult to spot at the 20 week anatomy scan

Limitations of ultrasound

Ultrasound scans rely on sound waves to create images of the baby’s heart. While this technology has significantly improved, it is not infallible. The resolution of the images may not be sufficient to detect all types of heart defects.

There are several reasons why the resolution may not be sufficient:

• At 20 weeks of pregnancy, the fetal heart is roughly the size of an acorn. When the image is magnified, it could reduce the amount of detail seen.
• It is a tiny moving structure, beating within the chest of the fetus which is also moving inside the womb of the mother. It can be technically very difficult to obtain adequate images to fully assess the structure of the heart.
• The amount and type of maternal tissue can affect the resolution. Extra adipose tissue absorbs the ultrasound waves and reduces the amount of detail seen in the resulting image.
• Abdominal scarring can also affect the signals from ultrasound waves decreasing the resolution and reducing the amount of detail seen.
• If there is limited amniotic fluid, it may be challenging to visualize the heart’s structure accurately.

Fetal position and gestational age

• The baby’s position during an ultrasound scan can influence the quality of the heart images. If the baby is positioned in a way that blocks the heart, such as lying with its spine facing upwards or curled tightly, it can be difficult to capture clear images of the heart. Typically, the sonographer will make efforts to encourage the baby to move, such as asking you to empty your bladder, change positions, or even take a short walk. If the baby still doesn’t shift, you may be offered a follow-up appointment in the hope that the baby will be in a more favourable position for better heart views.
• It is commonly believed that as the pregnancy progresses and the baby grows larger, assessing the fetal heart becomes easier. However, as the pregnancy advances, evaluating the heart’s structure actually becomes more difficult. This is often due to the baby’s position and the calcification of the bones. Since ultrasound waves cannot pass through bone, the increasing calcification of bones in later pregnancy can obstruct views of the heart. Additionally, as space in the uterus becomes more limited, the baby has less room to completely stretch out, making it harder for the sonographer to get clear views of the heart, especially when trying to capture images scanning between the baby’s ribs.
• A common question from parents who haven’t had an antenatal diagnosis of CHD is why their baby’s cardiac defect wasn’t detected after having multiple scans during the pregnancy. The optimal time to evaluate the fetal heart’s structure is during the 20-week anatomy scan. This scan is when a thorough assessment of the heart is made. Any subsequent scans in the third trimester, which focus on the baby’s growth and overall well-being, do not include a detailed evaluation of the heart, as this has already been done during the anatomy scan. Additionally, as mentioned earlier, it becomes more challenging to examine the fetal heart in the later stages of pregnancy.