The arrival of a healthy male heir is an essential moment for any monarchy, but it's not always a guarantee. And for the Habsburgs it became a serious problem.
The birth of Balthasar Charles was particularly welcomed by his parents. King Philip IV of Spain had married Elisabeth of France, the daughter of King Henry IV and Marie de Medici, in 1615. Elisabeth had been just thirteen years old at the time, while Philip was 16, so the pair were probably forbidden from sleeping together until they had grown up a bit. Their first child, a daughter named Maria Margaret, was born in 1621. Sadly she died two days later.