Mammalian foetal blood circulation
There's two problems here. Firstly, in the mammalian foetus, the lungs are not yet functional, and the oxygen and carbon dioxide exchange takes place in the placenta. Here, oxygenated blood coming into the foetus mixes with deoxygenated blood that has already circulated. This is very inefficient, as it means much of the foetal body receives only partially oxygenated blood. In adults, the deoxygenated blood goes directly to the lungs for oxygenation before circulation to the rest of the body; the mixing does not happen because of the closed connection between the heart and the lungs.
Secondly, to make this rather circuitous circulation possible, there is a hole between the chambers in the foetal heart (the foramen ovale), and foetal blood vessels (eg the ductus arteriosus). These need to close off at birth for the transition to adult circulation. Sometimes they don't, leading to two relatively common, and sometimes fatal, birth defects -- so-called 'hole-in-the-heart' babies.
Could the designer have done it better? Sure! If the umbilical cord were inserted at the chest, rather than the belly, it would solve several of these problems, because the umbilical vein and umbilical artery could connect to the pulmonary vein and pulmonary artery. If this doesn't take a genius to work out, where does that leave the designer?
As the developing pulmonary vessels can't withstand such high pressure.