Diagnosis can be done after 18 weeks gestation using an ultrasound. However, if it is not diagnosed in utero, cyanosis of the newborn (blue baby) should immediately indicate that there is a problem with the cardiovascular system.

• Most of the time, diagnosis can be done after 18 weeks gestation using an ultrasound. However, if it is not diagnosed in utero, cyanosis of the newborn (blue baby) should immediately indicate that there is a problem with the cardiovascular system.
• Prenatally, a baby with d-TGA experiences no symptoms as the lungs will not be used until after childbirth, and oxygen is provided by the mother via the placenta and umbilical cord; in order for the red blood to bypass the lungs in utero, the fetal heart has two shunts that begin to close when the newborn starts breathing; these are the foramen ovale and the ductus arteriosus. The foramen ovale is a hole in the atrial septum which allows blood from the right atrium to flow into the left atrium; after birth, the left atrium will be filled with blood returning from the lungs and the foramen ovale will close. The ductus arteriosus is a small, artery-like structure which allows blood to flow from the trunk of the pulmonary artery into the aorta; after birth, the blood in the pulmonary artery will flow into the lungs and the ductus arteriosus will close. Sometimes these shunts will fail to close after birth; these defects are called patent foramen ovale and patent ductus arteriosus, and either may occur independently, or in combination with one another, or with d-TGA or other heart and/or general defects.
• In the presence of a d-TGA, the fetus will be asymptomatic due to the presence of shunts during the intrauterine period until after birth when several changes are produced in the circulation:

The following Fetal structures become the following Infant and adult structures :

• The foramen ovale becomes the fossa ovalis
• The ductus arteriosus becomes the ligamentum arteriousus
• The ductus venosus becomes the ligamentum venosus
• The umbilical vein becomes the ligamentum teres
• The umbilical arteries becomes the medial umbilical ligaments