What Causes a Complete AVSD During Pregnancy?

A complete atrioventricular septal defect (CAVSD) is a structural difference in the heart that develops very early in pregnancy, usually before a woman even knows she is pregnant. It is important to know that this diagnosis is not your fault. While certain maternal health conditions can occasionally influence heart development, the vast majority of cases are caused by complex biological and genetic factors that are entirely outside of a parent’s control ^[1]. Your daily activities, a missed prenatal vitamin, or a stressful day at work did not cause your baby’s heart to develop this way ^[1].

How Does CAVSD Happen?

To understand how a CAVSD forms, it helps to look at how a baby’s heart develops. The heart begins as a simple tube and quickly grows into four chambers with valves between them. Between the 4th and 8th week of pregnancy, specialized tissues called endocardial cushions grow towards the middle of the heart to form the dividing walls (the septum) and the valves ^[2].

In a baby with CAVSD, these cushions fail to fuse together properly ^[2][3]. Because this fusion doesn’t happen, a large hole remains in the center of the heart where the upper and lower chambers meet. Additionally, instead of forming two separate valves (the mitral and tricuspid valves), a single, large common atrioventricular valve forms ^[2].

Is It Linked to Genetics?

Yes, genetic factors play the biggest role in why the endocardial cushions do not fuse ^[1]. Because of this, your care team will likely recommend speaking with a genetic counselor to explore this further.

Down Syndrome (Trisomy 21)

CAVSD is very strongly associated with Down syndrome, also known as Trisomy 21 ^[4]. In fact, about half of all babies diagnosed with a complete atrioventricular septal defect also have Down syndrome ^[5]. You may be offered tests, such as Non-Invasive Prenatal Testing (NIPT) or an amniocentesis, to look for this and help you prepare emotionally and medically before birth.

Can It Happen Without a Genetic Condition?

Yes. Many babies with CAVSD have a normal number of chromosomes and do not have Down syndrome ^[6]. In these cases, the cause is often related to a combination of smaller, invisible genetic changes (like variations in genes such as GATA4 or NFATC1) ^[7][8]. In some situations, multiple small genetic factors combine in ways doctors are still trying to understand ^[9]. However, in many families, a specific genetic reason is never found ^[1].

Moving Forward Without Guilt

Receiving a diagnosis of a congenital heart defect is overwhelming, and feeling guilty is a natural response for many parents. Please remember that a baby’s heart structure is finalized in the first trimester, largely driven by microscopic genetic instructions ^[2][1]. You could not have prevented this condition. Your focus now can shift toward learning about your baby’s heart and working with your pediatric cardiology team to plan the best care possible.