While the baby is still in its mother’s womb, it has an extra blood vessel, the ductus arteriosus (circled in yellow in the picture), that links the pulmonary artery (3) directly to the aorta (7).
This is because while the baby is in the womb, its blood is not oxygenated by its lungs, but via the placenta in its mother’s womb. For this reason, the flow of blood to the foetus’s lungs is low.
The blood that comes into the right half of the foetus’s heart (1-2) is pumped out via the pulmonary artery (3). But instead of continuing to the lungs, most of the blood flows via the duct directly out via the aorta (7). The duct thus creates a bypass for the blood away from the lungs.