Murmur is an abnormal sound heard with doctor stethoscope, this is produced due to abnormal connections in heart or blocked or stenotic valves or leaking valves.
During intra-uterine life i.e. inside mother’s womb, pattern circulation is quite different from the circulation of blood, after the birth. Soon after birth circulation of blood changes to adult pattern but how ever some pattern of fetal circulation still persist for few months to weeks
In intrauterine life right sided heart pressure is more but in adult right sided pressure is less. Soon after birth and for few weeks after birth due to high right sided pressure, pressure gradient i.e. difference between left and right side is less, murmurs which are the key for diagnosing cardiac defects may not be heard by your doctor due to less pressure gradient, and later after few weeks or months when right sided pressure decrease like adult these murmur may appear .
After Birth due to this changing pressures and hence murmurs, your doctor who sees your baby soon after birth should not be blamed for missing cardiac lesion.