The monarch is considered to have acceded to the throne upon her/his taking of the oath as required by article 91 of the constitution.
Since 1991, Belgium practises absolute primogeniture among the descendants of King Albert II (then Prince of Liège). Descendants of earlier monarchs and princes are only eligible to succeed if male and descended from King Leopold I in male-line (i.e. according to agnatic primogeniture), meaning that descendants of all Belgian princesses not descended from Albert II are barred from the throne. There are no living princes of Belgium who are not descended from Albert II, so agnatic primogeniture de facto doesn't apply to anyone anymore and the right to succeed is effectively limited to Albert II's descendants.
A person is deprived of his or her rights to the crown if he or she marries without the consent of the monarch (or the consent of those exercising the monarch's powers). The lost right may be re-established by the monarch (or by those exercising the monarch's powers) in the event of parliamentary agreement. Should there be no eligible descendant of King Leopold I, the reigning monarch may name his or her heir presumptive with the approval of the Parliament, but if she or he doesn't name the heir presumptive, the throne would eventually become vacant.
When King Albert II's daughter Astrid married Archduke Lorenz of Austria-Este in 1984, agnatic primogeniture being in effect, she had no succession rights and therefore did not seek the consent to her marriage. Following the introduction of absolute primogeniture among her father's descendants in 1991, it was deemed that she had obtained the necessary consent and thus assumed her place in the line along with her children.
When Prince Amedeo married in 2014, it was reported that he did not ask his uncle King Philippe's permission, and had therefore lost his right to the Belgian throne. However, on November 12, 2015, a Royal Decree was published which showed that consent had been given after the marriage retroactively.
No head of a different state may become King (or Queen) of the Belgians, unless both Houses of Parliament separately agree to it, two-thirds of the Members being present in each House, and a two-thirds majority being required in each House.
Upon the King's death, both Houses of Parliament convene within ten days with no convocation and, until the next King takes the oath of office, the Ministers jointly fulfil his constitutional duties under their own responsibility.
The King only accedes to the throne by solemnly taking the following oath in front of both Houses of Parliament in joint session: "I swear to observe the Constitution and the laws of the Belgian people, to maintain national independence and territorial integrity."
If the King is prevented from reigning (French: dans l'impossibilité de régner), or if the successor of a deceased King is a minor, the Houses of Parliament jointly designate a Regent, who enters into office by taking the above oath. There can only be one Regent. 
In other words, unlike in the Commonwealth, in Belgium there is an interregnum between each monarch's demise (usually by death, twice so far by abdication: Leopold III and Albert II) and his successor's accession (by taking the constitutional oath of office).
Depuis 1991, une femme peut donc être chef d'Etat en Belgique, à condition d'être une descendante directe de l'actuel Roi Albert II.External link in
The Belgian Constitution, coordinated version 1994 including later revisions (in French)