The prenatal stage of any pregnancy is an extremely crucial phase. Abnormalities in the cardiac function in the fetal health during the first trimester of the pregnancy may cause miscarriage, congenital heart disease, and twin‐to‐twin transfusion syndrome. To diagnose abnormal cardiac function, it is essential to fully assess the healthy fetus in the first trimester.
Fetal cardiac function assessment also has a role in determining normal physiologic characteristics and underlying pathophysiologic characteristics of fetal conditions. Despite the size of the small heart, fetal heart evaluation is feasible through an ultrasound. A fetal ultrasound, or sonogram, is an imaging technique that uses high-frequency sound waves to produce images of a baby in the uterus. Fetal ultrasound images can help the health care provider evaluate the baby's growth and development and determine how the pregnancy is progressing. In some cases, fetal ultrasound is used to evaluate possible problems or help confirm a diagnosis.
Fetal heart rate (FHR) monitoring has been extensively used to assess fetal well-being. The process of FHR monitoring is commonly used during prenatal screening to detect possible fetal health problems that may result in neurological damage or in some cases fetal death during labor. Statistics have shown that 1 out of every 125 babies is born with some kind of congenital cardiac defect.
A broad range of ultrasound techniques has been applied for evaluation of fetal cardiac function including the fundamental image (2D image), M‐mode, conventional and tissue Doppler, 2D speckle tracking, Auto FHR, Dual Gate Doppler and 4D‐STIC.
The first fetal ultrasound is usually done during the first trimester. If the pregnancy remains uncomplicated, the next ultrasound is typically offered during the second trimester, when anatomic details are visible. If the baby's health needs to be monitored more closely, additional ultrasounds might be recommended. First-trimester ultrasound examination is done to evaluate the presence, size, number, and location of the pregnancy. Ultrasound can also be used for first-trimester sonographic genetic screening, as well as screening for any abnormalities of the uterus or cervix.
During early pregnancy (5-8 Weeks), it is very difficult to Measure Heart Rate in M mode as well as Dopper. Recent Advances in Ultrasound technology like Auto FHR allows Fetal Heart Rate to be automatically measured from B mode image with tracking of heart movement. Also, it is possible to observe fetal growth from the very early stage as this function is available in the transvaginal probe as well.
In the second or third trimester, a standard ultrasound is done to evaluate several features of the pregnancy, including fetal anatomy. This exam is typically done between weeks 18 and 20 of pregnancy. This stage is crucial as an anomaly which was missed during the first trimester can be detected clearly during this stage.
Conventional Doppler can be applied to obtain information about the blood flow through the heart. The fetal Doppler Ultrasound is commonly used for monitoring fetal heart rate and can also be used for identifying the event timings of fetal cardiac valve motions.
Assessment of cardiac function with Doppler examination of the atrioventricular (AV) valves has been performed in early pregnancy proving that early diagnosis of the problem can help with early treatment. Those studies found that the proportion of the cardiac cycle occupied by both the isovolumetric relaxation time (IVRT) and isovolumetric contraction time (ICT) reduces during the first trimester.
Advancements in Ultrasound like Dual Gate Doppler enables to observe fetal arrhythmia, diastolic function (right heart / left heart) & cardiac output from both ventricles. Dual gate Doppler generates a full FFT analysis and display from two separate sample gates allowing measurement from two different locations, during the same cardiac cycle.
In the case of Fetal Cardiac evaluation, estimation of the Regurgitation velocity is a very important parameter. But pulsed wave can't measure the velocity of more than 3 m/sec. Technology advancements in Ultrasound like CW in Convex / Linear Probe enable to observe regurgitation and measure high-velocity blood flow of stenosis area (up to 10 m/sec) which is difficult to evaluate by PW Doppler with a Convex probe.
Advances in ultrasound technology have led to improved visualization of the fetus in the first trimester. A recent article on early fetal echocardiography in first‐trimester fetuses found that a satisfactory cardiac examination was possible in 95% of cases, with the sensitivity of 70% and specificity of 98% for detection of cardiac defects.