Kate Middleton, reportedly, received a warning about having another baby because expanding their family will not be sustainable. The warning came before she and Prince William announced that they were expecting Prince Louis in 2018. 

An advocate from a children's organization, apparently, wrote Kate Middleton to reconsider getting pregnant again since it will raise plenty of issues, especially with "sustainability and equity." According to Marie Claire, the person who wrote Kate the letter was the executive director of the children's organization Having Kids, Anne Green Carter Dillard. She, allegedly, advised the Duchess of Cambridge against getting pregnant so she could set a good example to the public

Dillard's concern sprung from when Kate, reportedly, made a remark during a visit Poland and Germany that she and Prince William would just "have more babies" after they received an infant toy as a gift from the people.  Dillard said that while she knows Kate and Prince William's kids are destined to have great lives, other families who want more children do not have the same privilege. 

A few months later, the Duke and Duchess of Cambridge announced they were having another baby, following Prince George and Princess Charlotte, who were four and two years old at that time. 

Meanwhile, speculations have come to light that Kate might be pregnant with baby number four. According to reports, Prince William and Kate reportedly talked about adding one more child after spending a lot of time with their brood of three during their coronavirus isolation in Anmer Hall. 

However, in January, Kate revealed during a royal engagement that Prince William may not be inclined to have a fourth baby. When the Duchess of Cambridge was asked if she wants another child, Kate apparently laughed and said that her husband "would be a little worried." 

It may be more challenging for Kate to carry another child to term, especially at 38 years old. In all of her pregnancies, the Duchess of Cambridge suffered from extreme morning sickness that resigned her to bed rest for the first semester.

She admitted in one interview that having hyperemesis gravidarum, which happens because of hormonal imbalance, was so much torture that even Prince William felt helpless against it. Thus, she tried hypnobirthing during her labor and delivery because she said that she wanted to have control over some aspects of her pregnancy experience.