They have three adorable children and could even consider having more, but there may be a big reason that Kate Middleton and Prince William could decide to end their family’s growth with the three kids they have.

According to Express UK, new provisions to the Succession Act of 2013 could cause the Duke and Duchess of Cambridge to make some heartbreaking decisions in the future when it comes to the children they already have—Prince George, Princess Charlotte and Prince Louis—and as a result, it may also keep the couple from choosing to have a fourth child.

According to the website, the provision, which allowed for female heirs to take their rightful place in the line of succession based on birth order and not sex, which make things a challenge for the couple is the one which requires which heirs to seek permission from the reigning monarch before marrying. Currently, the rule states that anyone in the line of succession up to sixth-in-line for the throne must ask permission, but anyone after that no longer has that requirement.

However, while that rule currently means that Princesses Eugenie and Beatrice were not required to ask permission for their engagements, and in the future Prince Harry and Meghan Markle’s son, Archie, will also not be required to comply, it could pose a problem for Prince William and Kate, solely because of their children’s places.

When the act was being debated, MP Christopher Pincher revealed a concern that the number would pose some unfair restrictions on a monarch who has multiple children, meaning some have to comply with the rule, while others are free to do as they wish.

“Where did the number six come from? Why not three, five or 12?” he asked at the time. “If we put in place a rule which says that the monarch can and must give consent to the marrying of the six persons nearest in line to the throne, imagine a scenario where a monarch has three children, who each have two or three children.”

“The monarch will soon be in the invidious position where grandchild number four, who is fifth-in-line to the throne, must seek the consent of the monarch to marry, but grandchild number six, who is seventh-in-line to the throne, need not seek that consent,” he added. “That does not seem fair.”

Depending on when William ascends to the throne (he is currently second in line behind his father, Prince Charles), he could be forced to enact on the decision for either his own children or when he has grandchildren himself. Should Prince George marry and have five children before either his sister or brother were to marry, they would not be subject to follow the rule. Similarly, if his parents do have another child of their own, that child likely would never need to seek permission to marry at all, as it would be moved out of the six-person requirement as soon as any of its older siblings had children.

Similarly, it would mean that Prince William’s future grandkids as a whole will fall on either side of the line, and prince Louis’ children likely will not need to follow any requirements when they come of age, but Prince George and Princess Charlotte’s must.

As a result of the way the rule could affect things, it is possible the couple may officially be done having children as a result.

Kate Middleton, Prince Louis, Prince George, Prince William and Princess Charlotte
Kate Middleton carrying Prince Louis, Prince George, Prince William and Princess Charlotte after Prince Louis’ christening service at the Chapel Royal at St. James’s Palace in London on July 8, 2018. Getty Images/Dominic Lipinski